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Pull vfs name lookup updates from Al Viro: "Small namei.c patch series, mostly to simplify the rules for nameidata state. It's actually from the previous cycle - but I didn't post it for review in time... Changes visible outside of fs/namei.c: file_open_root() calling conventions change, some freed bits in LOOKUP_... space" * 'work.namei' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs: namei: make sure nd->depth is always valid teach set_nameidata() to handle setting the root as well take LOOKUP_{ROOT,ROOT_GRABBED,JUMPED} out of LOOKUP_... space switch file_open_root() to struct path
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===============
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Pathname lookup
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===============
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This write-up is based on three articles published at lwn.net:
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- <https://lwn.net/Articles/649115/> Pathname lookup in Linux
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- <https://lwn.net/Articles/649729/> RCU-walk: faster pathname lookup in Linux
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- <https://lwn.net/Articles/650786/> A walk among the symlinks
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Written by Neil Brown with help from Al Viro and Jon Corbet.
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It has subsequently been updated to reflect changes in the kernel
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including:
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- per-directory parallel name lookup.
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- ``openat2()`` resolution restriction flags.
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Introduction to pathname lookup
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===============================
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The most obvious aspect of pathname lookup, which very little
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exploration is needed to discover, is that it is complex. There are
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many rules, special cases, and implementation alternatives that all
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combine to confuse the unwary reader. Computer science has long been
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acquainted with such complexity and has tools to help manage it. One
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tool that we will make extensive use of is "divide and conquer". For
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the early parts of the analysis we will divide off symlinks - leaving
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them until the final part. Well before we get to symlinks we have
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another major division based on the VFS's approach to locking which
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will allow us to review "REF-walk" and "RCU-walk" separately. But we
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are getting ahead of ourselves. There are some important low level
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distinctions we need to clarify first.
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There are two sorts of ...
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--------------------------
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.. _openat: http://man7.org/linux/man-pages/man2/openat.2.html
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Pathnames (sometimes "file names"), used to identify objects in the
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filesystem, will be familiar to most readers. They contain two sorts
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of elements: "slashes" that are sequences of one or more "``/``"
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characters, and "components" that are sequences of one or more
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non-"``/``" characters. These form two kinds of paths. Those that
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start with slashes are "absolute" and start from the filesystem root.
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The others are "relative" and start from the current directory, or
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from some other location specified by a file descriptor given to
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"``*at()``" system calls such as `openat() <openat_>`_.
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.. _execveat: http://man7.org/linux/man-pages/man2/execveat.2.html
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It is tempting to describe the second kind as starting with a
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component, but that isn't always accurate: a pathname can lack both
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slashes and components, it can be empty, in other words. This is
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generally forbidden in POSIX, but some of those "``*at()``" system calls
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in Linux permit it when the ``AT_EMPTY_PATH`` flag is given. For
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example, if you have an open file descriptor on an executable file you
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can execute it by calling `execveat() <execveat_>`_ passing
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the file descriptor, an empty path, and the ``AT_EMPTY_PATH`` flag.
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These paths can be divided into two sections: the final component and
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everything else. The "everything else" is the easy bit. In all cases
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it must identify a directory that already exists, otherwise an error
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such as ``ENOENT`` or ``ENOTDIR`` will be reported.
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The final component is not so simple. Not only do different system
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calls interpret it quite differently (e.g. some create it, some do
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not), but it might not even exist: neither the empty pathname nor the
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pathname that is just slashes have a final component. If it does
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exist, it could be "``.``" or "``..``" which are handled quite differently
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from other components.
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.. _POSIX: https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap04.html#tag_04_12
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If a pathname ends with a slash, such as "``/tmp/foo/``" it might be
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tempting to consider that to have an empty final component. In many
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ways that would lead to correct results, but not always. In
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particular, ``mkdir()`` and ``rmdir()`` each create or remove a directory named
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by the final component, and they are required to work with pathnames
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ending in "``/``". According to POSIX_:
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A pathname that contains at least one non-<slash> character and
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that ends with one or more trailing <slash> characters shall not
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be resolved successfully unless the last pathname component before
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the trailing <slash> characters names an existing directory or a
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directory entry that is to be created for a directory immediately
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after the pathname is resolved.
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The Linux pathname walking code (mostly in ``fs/namei.c``) deals with
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all of these issues: breaking the path into components, handling the
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"everything else" quite separately from the final component, and
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checking that the trailing slash is not used where it isn't
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permitted. It also addresses the important issue of concurrent
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access.
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While one process is looking up a pathname, another might be making
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changes that affect that lookup. One fairly extreme case is that if
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"a/b" were renamed to "a/c/b" while another process were looking up
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"a/b/..", that process might successfully resolve on "a/c".
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Most races are much more subtle, and a big part of the task of
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pathname lookup is to prevent them from having damaging effects. Many
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of the possible races are seen most clearly in the context of the
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"dcache" and an understanding of that is central to understanding
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pathname lookup.
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More than just a cache
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----------------------
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The "dcache" caches information about names in each filesystem to
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make them quickly available for lookup. Each entry (known as a
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"dentry") contains three significant fields: a component name, a
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pointer to a parent dentry, and a pointer to the "inode" which
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contains further information about the object in that parent with
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the given name. The inode pointer can be ``NULL`` indicating that the
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name doesn't exist in the parent. While there can be linkage in the
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dentry of a directory to the dentries of the children, that linkage is
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not used for pathname lookup, and so will not be considered here.
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The dcache has a number of uses apart from accelerating lookup. One
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that will be particularly relevant is that it is closely integrated
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with the mount table that records which filesystem is mounted where.
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What the mount table actually stores is which dentry is mounted on top
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of which other dentry.
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When considering the dcache, we have another of our "two types"
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distinctions: there are two types of filesystems.
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Some filesystems ensure that the information in the dcache is always
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completely accurate (though not necessarily complete). This can allow
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the VFS to determine if a particular file does or doesn't exist
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without checking with the filesystem, and means that the VFS can
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protect the filesystem against certain races and other problems.
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These are typically "local" filesystems such as ext3, XFS, and Btrfs.
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Other filesystems don't provide that guarantee because they cannot.
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These are typically filesystems that are shared across a network,
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whether remote filesystems like NFS and 9P, or cluster filesystems
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like ocfs2 or cephfs. These filesystems allow the VFS to revalidate
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cached information, and must provide their own protection against
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awkward races. The VFS can detect these filesystems by the
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``DCACHE_OP_REVALIDATE`` flag being set in the dentry.
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REF-walk: simple concurrency management with refcounts and spinlocks
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--------------------------------------------------------------------
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With all of those divisions carefully classified, we can now start
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looking at the actual process of walking along a path. In particular
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we will start with the handling of the "everything else" part of a
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pathname, and focus on the "REF-walk" approach to concurrency
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management. This code is found in the ``link_path_walk()`` function, if
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you ignore all the places that only run when "``LOOKUP_RCU``"
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(indicating the use of RCU-walk) is set.
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.. _Meet the Lockers: https://lwn.net/Articles/453685/
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REF-walk is fairly heavy-handed with locks and reference counts. Not
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as heavy-handed as in the old "big kernel lock" days, but certainly not
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afraid of taking a lock when one is needed. It uses a variety of
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different concurrency controls. A background understanding of the
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various primitives is assumed, or can be gleaned from elsewhere such
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as in `Meet the Lockers`_.
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The locking mechanisms used by REF-walk include:
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dentry->d_lockref
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~~~~~~~~~~~~~~~~~
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This uses the lockref primitive to provide both a spinlock and a
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reference count. The special-sauce of this primitive is that the
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conceptual sequence "lock; inc_ref; unlock;" can often be performed
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with a single atomic memory operation.
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Holding a reference on a dentry ensures that the dentry won't suddenly
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be freed and used for something else, so the values in various fields
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will behave as expected. It also protects the ``->d_inode`` reference
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to the inode to some extent.
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The association between a dentry and its inode is fairly permanent.
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For example, when a file is renamed, the dentry and inode move
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together to the new location. When a file is created the dentry will
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initially be negative (i.e. ``d_inode`` is ``NULL``), and will be assigned
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to the new inode as part of the act of creation.
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When a file is deleted, this can be reflected in the cache either by
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setting ``d_inode`` to ``NULL``, or by removing it from the hash table
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(described shortly) used to look up the name in the parent directory.
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If the dentry is still in use the second option is used as it is
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perfectly legal to keep using an open file after it has been deleted
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and having the dentry around helps. If the dentry is not otherwise in
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use (i.e. if the refcount in ``d_lockref`` is one), only then will
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``d_inode`` be set to ``NULL``. Doing it this way is more efficient for a
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very common case.
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So as long as a counted reference is held to a dentry, a non-``NULL`` ``->d_inode``
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value will never be changed.
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dentry->d_lock
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~~~~~~~~~~~~~~
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``d_lock`` is a synonym for the spinlock that is part of ``d_lockref`` above.
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For our purposes, holding this lock protects against the dentry being
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renamed or unlinked. In particular, its parent (``d_parent``), and its
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name (``d_name``) cannot be changed, and it cannot be removed from the
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dentry hash table.
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When looking for a name in a directory, REF-walk takes ``d_lock`` on
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each candidate dentry that it finds in the hash table and then checks
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that the parent and name are correct. So it doesn't lock the parent
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while searching in the cache; it only locks children.
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When looking for the parent for a given name (to handle "``..``"),
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REF-walk can take ``d_lock`` to get a stable reference to ``d_parent``,
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but it first tries a more lightweight approach. As seen in
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``dget_parent()``, if a reference can be claimed on the parent, and if
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subsequently ``d_parent`` can be seen to have not changed, then there is
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no need to actually take the lock on the child.
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rename_lock
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~~~~~~~~~~~
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Looking up a given name in a given directory involves computing a hash
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from the two values (the name and the dentry of the directory),
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accessing that slot in a hash table, and searching the linked list
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that is found there.
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When a dentry is renamed, the name and the parent dentry can both
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change so the hash will almost certainly change too. This would move the
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dentry to a different chain in the hash table. If a filename search
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happened to be looking at a dentry that was moved in this way,
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it might end up continuing the search down the wrong chain,
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and so miss out on part of the correct chain.
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The name-lookup process (``d_lookup()``) does *not* try to prevent this
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from happening, but only to detect when it happens.
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``rename_lock`` is a seqlock that is updated whenever any dentry is
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renamed. If ``d_lookup`` finds that a rename happened while it
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unsuccessfully scanned a chain in the hash table, it simply tries
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again.
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``rename_lock`` is also used to detect and defend against potential attacks
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against ``LOOKUP_BENEATH`` and ``LOOKUP_IN_ROOT`` when resolving ".." (where
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the parent directory is moved outside the root, bypassing the ``path_equal()``
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check). If ``rename_lock`` is updated during the lookup and the path encounters
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a "..", a potential attack occurred and ``handle_dots()`` will bail out with
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``-EAGAIN``.
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inode->i_rwsem
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~~~~~~~~~~~~~~
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``i_rwsem`` is a read/write semaphore that serializes all changes to a particular
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directory. This ensures that, for example, an ``unlink()`` and a ``rename()``
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cannot both happen at the same time. It also keeps the directory
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stable while the filesystem is asked to look up a name that is not
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currently in the dcache or, optionally, when the list of entries in a
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directory is being retrieved with ``readdir()``.
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This has a complementary role to that of ``d_lock``: ``i_rwsem`` on a
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directory protects all of the names in that directory, while ``d_lock``
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on a name protects just one name in a directory. Most changes to the
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dcache hold ``i_rwsem`` on the relevant directory inode and briefly take
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``d_lock`` on one or more the dentries while the change happens. One
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exception is when idle dentries are removed from the dcache due to
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memory pressure. This uses ``d_lock``, but ``i_rwsem`` plays no role.
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The semaphore affects pathname lookup in two distinct ways. Firstly it
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prevents changes during lookup of a name in a directory. ``walk_component()`` uses
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``lookup_fast()`` first which, in turn, checks to see if the name is in the cache,
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using only ``d_lock`` locking. If the name isn't found, then ``walk_component()``
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falls back to ``lookup_slow()`` which takes a shared lock on ``i_rwsem``, checks again that
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the name isn't in the cache, and then calls in to the filesystem to get a
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definitive answer. A new dentry will be added to the cache regardless of
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the result.
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Secondly, when pathname lookup reaches the final component, it will
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sometimes need to take an exclusive lock on ``i_rwsem`` before performing the last lookup so
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that the required exclusion can be achieved. How path lookup chooses
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to take, or not take, ``i_rwsem`` is one of the
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issues addressed in a subsequent section.
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If two threads attempt to look up the same name at the same time - a
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name that is not yet in the dcache - the shared lock on ``i_rwsem`` will
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not prevent them both adding new dentries with the same name. As this
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would result in confusion an extra level of interlocking is used,
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based around a secondary hash table (``in_lookup_hashtable``) and a
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per-dentry flag bit (``DCACHE_PAR_LOOKUP``).
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To add a new dentry to the cache while only holding a shared lock on
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``i_rwsem``, a thread must call ``d_alloc_parallel()``. This allocates a
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dentry, stores the required name and parent in it, checks if there
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is already a matching dentry in the primary or secondary hash
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tables, and if not, stores the newly allocated dentry in the secondary
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hash table, with ``DCACHE_PAR_LOOKUP`` set.
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If a matching dentry was found in the primary hash table then that is
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returned and the caller can know that it lost a race with some other
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thread adding the entry. If no matching dentry is found in either
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cache, the newly allocated dentry is returned and the caller can
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detect this from the presence of ``DCACHE_PAR_LOOKUP``. In this case it
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knows that it has won any race and now is responsible for asking the
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filesystem to perform the lookup and find the matching inode. When
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the lookup is complete, it must call ``d_lookup_done()`` which clears
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the flag and does some other house keeping, including removing the
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dentry from the secondary hash table - it will normally have been
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added to the primary hash table already. Note that a ``struct
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waitqueue_head`` is passed to ``d_alloc_parallel()``, and
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``d_lookup_done()`` must be called while this ``waitqueue_head`` is still
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in scope.
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If a matching dentry is found in the secondary hash table,
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``d_alloc_parallel()`` has a little more work to do. It first waits for
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``DCACHE_PAR_LOOKUP`` to be cleared, using a wait_queue that was passed
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to the instance of ``d_alloc_parallel()`` that won the race and that
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will be woken by the call to ``d_lookup_done()``. It then checks to see
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if the dentry has now been added to the primary hash table. If it
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has, the dentry is returned and the caller just sees that it lost any
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race. If it hasn't been added to the primary hash table, the most
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likely explanation is that some other dentry was added instead using
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``d_splice_alias()``. In any case, ``d_alloc_parallel()`` repeats all the
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look ups from the start and will normally return something from the
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primary hash table.
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mnt->mnt_count
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~~~~~~~~~~~~~~
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``mnt_count`` is a per-CPU reference counter on "``mount``" structures.
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Per-CPU here means that incrementing the count is cheap as it only
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uses CPU-local memory, but checking if the count is zero is expensive as
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it needs to check with every CPU. Taking a ``mnt_count`` reference
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prevents the mount structure from disappearing as the result of regular
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unmount operations, but does not prevent a "lazy" unmount. So holding
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``mnt_count`` doesn't ensure that the mount remains in the namespace and,
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in particular, doesn't stabilize the link to the mounted-on dentry. It
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does, however, ensure that the ``mount`` data structure remains coherent,
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and it provides a reference to the root dentry of the mounted
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filesystem. So a reference through ``->mnt_count`` provides a stable
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reference to the mounted dentry, but not the mounted-on dentry.
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mount_lock
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~~~~~~~~~~
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``mount_lock`` is a global seqlock, a bit like ``rename_lock``. It can be used to
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check if any change has been made to any mount points.
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While walking down the tree (away from the root) this lock is used when
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crossing a mount point to check that the crossing was safe. That is,
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the value in the seqlock is read, then the code finds the mount that
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is mounted on the current directory, if there is one, and increments
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the ``mnt_count``. Finally the value in ``mount_lock`` is checked against
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the old value. If there is no change, then the crossing was safe. If there
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was a change, the ``mnt_count`` is decremented and the whole process is
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retried.
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When walking up the tree (towards the root) by following a ".." link,
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a little more care is needed. In this case the seqlock (which
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contains both a counter and a spinlock) is fully locked to prevent
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any changes to any mount points while stepping up. This locking is
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needed to stabilize the link to the mounted-on dentry, which the
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refcount on the mount itself doesn't ensure.
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``mount_lock`` is also used to detect and defend against potential attacks
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against ``LOOKUP_BENEATH`` and ``LOOKUP_IN_ROOT`` when resolving ".." (where
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the parent directory is moved outside the root, bypassing the ``path_equal()``
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check). If ``mount_lock`` is updated during the lookup and the path encounters
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a "..", a potential attack occurred and ``handle_dots()`` will bail out with
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``-EAGAIN``.
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RCU
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~~~
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Finally the global (but extremely lightweight) RCU read lock is held
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from time to time to ensure certain data structures don't get freed
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unexpectedly.
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In particular it is held while scanning chains in the dcache hash
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table, and the mount point hash table.
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Bringing it together with ``struct nameidata``
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----------------------------------------------
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.. _First edition Unix: https://minnie.tuhs.org/cgi-bin/utree.pl?file=V1/u2.s
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Throughout the process of walking a path, the current status is stored
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in a ``struct nameidata``, "namei" being the traditional name - dating
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all the way back to `First Edition Unix`_ - of the function that
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converts a "name" to an "inode". ``struct nameidata`` contains (among
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other fields):
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``struct path path``
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~~~~~~~~~~~~~~~~~~~~
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A ``path`` contains a ``struct vfsmount`` (which is
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embedded in a ``struct mount``) and a ``struct dentry``. Together these
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record the current status of the walk. They start out referring to the
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starting point (the current working directory, the root directory, or some other
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directory identified by a file descriptor), and are updated on each
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step. A reference through ``d_lockref`` and ``mnt_count`` is always
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held.
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``struct qstr last``
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~~~~~~~~~~~~~~~~~~~~
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This is a string together with a length (i.e. *not* ``nul`` terminated)
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that is the "next" component in the pathname.
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``int last_type``
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~~~~~~~~~~~~~~~~~
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This is one of ``LAST_NORM``, ``LAST_ROOT``, ``LAST_DOT`` or ``LAST_DOTDOT``.
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The ``last`` field is only valid if the type is ``LAST_NORM``.
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``struct path root``
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~~~~~~~~~~~~~~~~~~~~
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|
|
This is used to hold a reference to the effective root of the
|
|
filesystem. Often that reference won't be needed, so this field is
|
|
only assigned the first time it is used, or when a non-standard root
|
|
is requested. Keeping a reference in the ``nameidata`` ensures that
|
|
only one root is in effect for the entire path walk, even if it races
|
|
with a ``chroot()`` system call.
|
|
|
|
It should be noted that in the case of ``LOOKUP_IN_ROOT`` or
|
|
``LOOKUP_BENEATH``, the effective root becomes the directory file descriptor
|
|
passed to ``openat2()`` (which exposes these ``LOOKUP_`` flags).
|
|
|
|
The root is needed when either of two conditions holds: (1) either the
|
|
pathname or a symbolic link starts with a "'/'", or (2) a "``..``"
|
|
component is being handled, since "``..``" from the root must always stay
|
|
at the root. The value used is usually the current root directory of
|
|
the calling process. An alternate root can be provided as when
|
|
``sysctl()`` calls ``file_open_root()``, and when NFSv4 or Btrfs call
|
|
``mount_subtree()``. In each case a pathname is being looked up in a very
|
|
specific part of the filesystem, and the lookup must not be allowed to
|
|
escape that subtree. It works a bit like a local ``chroot()``.
|
|
|
|
Ignoring the handling of symbolic links, we can now describe the
|
|
"``link_path_walk()``" function, which handles the lookup of everything
|
|
except the final component as:
|
|
|
|
Given a path (``name``) and a nameidata structure (``nd``), check that the
|
|
current directory has execute permission and then advance ``name``
|
|
over one component while updating ``last_type`` and ``last``. If that
|
|
was the final component, then return, otherwise call
|
|
``walk_component()`` and repeat from the top.
|
|
|
|
``walk_component()`` is even easier. If the component is ``LAST_DOTS``,
|
|
it calls ``handle_dots()`` which does the necessary locking as already
|
|
described. If it finds a ``LAST_NORM`` component it first calls
|
|
"``lookup_fast()``" which only looks in the dcache, but will ask the
|
|
filesystem to revalidate the result if it is that sort of filesystem.
|
|
If that doesn't get a good result, it calls "``lookup_slow()``" which
|
|
takes ``i_rwsem``, rechecks the cache, and then asks the filesystem
|
|
to find a definitive answer.
|
|
|
|
As the last step of walk_component(), step_into() will be called either
|
|
directly from walk_component() or from handle_dots(). It calls
|
|
handle_mounts(), to check and handle mount points, in which a new
|
|
``struct path`` is created containing a counted reference to the new dentry and
|
|
a reference to the new ``vfsmount`` which is only counted if it is
|
|
different from the previous ``vfsmount``. Then if there is
|
|
a symbolic link, step_into() calls pick_link() to deal with it,
|
|
otherwise it installs the new ``struct path`` in the ``struct nameidata``, and
|
|
drops the unneeded references.
|
|
|
|
This "hand-over-hand" sequencing of getting a reference to the new
|
|
dentry before dropping the reference to the previous dentry may
|
|
seem obvious, but is worth pointing out so that we will recognize its
|
|
analogue in the "RCU-walk" version.
|
|
|
|
Handling the final component
|
|
----------------------------
|
|
|
|
``link_path_walk()`` only walks as far as setting ``nd->last`` and
|
|
``nd->last_type`` to refer to the final component of the path. It does
|
|
not call ``walk_component()`` that last time. Handling that final
|
|
component remains for the caller to sort out. Those callers are
|
|
path_lookupat(), path_parentat() and
|
|
path_openat() each of which handles the differing requirements of
|
|
different system calls.
|
|
|
|
``path_parentat()`` is clearly the simplest - it just wraps a little bit
|
|
of housekeeping around ``link_path_walk()`` and returns the parent
|
|
directory and final component to the caller. The caller will be either
|
|
aiming to create a name (via ``filename_create()``) or remove or rename
|
|
a name (in which case ``user_path_parent()`` is used). They will use
|
|
``i_rwsem`` to exclude other changes while they validate and then
|
|
perform their operation.
|
|
|
|
``path_lookupat()`` is nearly as simple - it is used when an existing
|
|
object is wanted such as by ``stat()`` or ``chmod()``. It essentially just
|
|
calls ``walk_component()`` on the final component through a call to
|
|
``lookup_last()``. ``path_lookupat()`` returns just the final dentry.
|
|
It is worth noting that when flag ``LOOKUP_MOUNTPOINT`` is set,
|
|
path_lookupat() will unset LOOKUP_JUMPED in nameidata so that in the
|
|
subsequent path traversal d_weak_revalidate() won't be called.
|
|
This is important when unmounting a filesystem that is inaccessible, such as
|
|
one provided by a dead NFS server.
|
|
|
|
Finally ``path_openat()`` is used for the ``open()`` system call; it
|
|
contains, in support functions starting with "open_last_lookups()", all the
|
|
complexity needed to handle the different subtleties of O_CREAT (with
|
|
or without O_EXCL), final "``/``" characters, and trailing symbolic
|
|
links. We will revisit this in the final part of this series, which
|
|
focuses on those symbolic links. "open_last_lookups()" will sometimes, but
|
|
not always, take ``i_rwsem``, depending on what it finds.
|
|
|
|
Each of these, or the functions which call them, need to be alert to
|
|
the possibility that the final component is not ``LAST_NORM``. If the
|
|
goal of the lookup is to create something, then any value for
|
|
``last_type`` other than ``LAST_NORM`` will result in an error. For
|
|
example if ``path_parentat()`` reports ``LAST_DOTDOT``, then the caller
|
|
won't try to create that name. They also check for trailing slashes
|
|
by testing ``last.name[last.len]``. If there is any character beyond
|
|
the final component, it must be a trailing slash.
|
|
|
|
Revalidation and automounts
|
|
---------------------------
|
|
|
|
Apart from symbolic links, there are only two parts of the "REF-walk"
|
|
process not yet covered. One is the handling of stale cache entries
|
|
and the other is automounts.
|
|
|
|
On filesystems that require it, the lookup routines will call the
|
|
``->d_revalidate()`` dentry method to ensure that the cached information
|
|
is current. This will often confirm validity or update a few details
|
|
from a server. In some cases it may find that there has been change
|
|
further up the path and that something that was thought to be valid
|
|
previously isn't really. When this happens the lookup of the whole
|
|
path is aborted and retried with the "``LOOKUP_REVAL``" flag set. This
|
|
forces revalidation to be more thorough. We will see more details of
|
|
this retry process in the next article.
|
|
|
|
Automount points are locations in the filesystem where an attempt to
|
|
lookup a name can trigger changes to how that lookup should be
|
|
handled, in particular by mounting a filesystem there. These are
|
|
covered in greater detail in autofs.txt in the Linux documentation
|
|
tree, but a few notes specifically related to path lookup are in order
|
|
here.
|
|
|
|
The Linux VFS has a concept of "managed" dentries. There are three
|
|
potentially interesting things about these dentries corresponding
|
|
to three different flags that might be set in ``dentry->d_flags``:
|
|
|
|
``DCACHE_MANAGE_TRANSIT``
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
If this flag has been set, then the filesystem has requested that the
|
|
``d_manage()`` dentry operation be called before handling any possible
|
|
mount point. This can perform two particular services:
|
|
|
|
It can block to avoid races. If an automount point is being
|
|
unmounted, the ``d_manage()`` function will usually wait for that
|
|
process to complete before letting the new lookup proceed and possibly
|
|
trigger a new automount.
|
|
|
|
It can selectively allow only some processes to transit through a
|
|
mount point. When a server process is managing automounts, it may
|
|
need to access a directory without triggering normal automount
|
|
processing. That server process can identify itself to the ``autofs``
|
|
filesystem, which will then give it a special pass through
|
|
``d_manage()`` by returning ``-EISDIR``.
|
|
|
|
``DCACHE_MOUNTED``
|
|
~~~~~~~~~~~~~~~~~~
|
|
|
|
This flag is set on every dentry that is mounted on. As Linux
|
|
supports multiple filesystem namespaces, it is possible that the
|
|
dentry may not be mounted on in *this* namespace, just in some
|
|
other. So this flag is seen as a hint, not a promise.
|
|
|
|
If this flag is set, and ``d_manage()`` didn't return ``-EISDIR``,
|
|
``lookup_mnt()`` is called to examine the mount hash table (honoring the
|
|
``mount_lock`` described earlier) and possibly return a new ``vfsmount``
|
|
and a new ``dentry`` (both with counted references).
|
|
|
|
``DCACHE_NEED_AUTOMOUNT``
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
If ``d_manage()`` allowed us to get this far, and ``lookup_mnt()`` didn't
|
|
find a mount point, then this flag causes the ``d_automount()`` dentry
|
|
operation to be called.
|
|
|
|
The ``d_automount()`` operation can be arbitrarily complex and may
|
|
communicate with server processes etc. but it should ultimately either
|
|
report that there was an error, that there was nothing to mount, or
|
|
should provide an updated ``struct path`` with new ``dentry`` and ``vfsmount``.
|
|
|
|
In the latter case, ``finish_automount()`` will be called to safely
|
|
install the new mount point into the mount table.
|
|
|
|
There is no new locking of import here and it is important that no
|
|
locks (only counted references) are held over this processing due to
|
|
the very real possibility of extended delays.
|
|
This will become more important next time when we examine RCU-walk
|
|
which is particularly sensitive to delays.
|
|
|
|
RCU-walk - faster pathname lookup in Linux
|
|
==========================================
|
|
|
|
RCU-walk is another algorithm for performing pathname lookup in Linux.
|
|
It is in many ways similar to REF-walk and the two share quite a bit
|
|
of code. The significant difference in RCU-walk is how it allows for
|
|
the possibility of concurrent access.
|
|
|
|
We noted that REF-walk is complex because there are numerous details
|
|
and special cases. RCU-walk reduces this complexity by simply
|
|
refusing to handle a number of cases -- it instead falls back to
|
|
REF-walk. The difficulty with RCU-walk comes from a different
|
|
direction: unfamiliarity. The locking rules when depending on RCU are
|
|
quite different from traditional locking, so we will spend a little extra
|
|
time when we come to those.
|
|
|
|
Clear demarcation of roles
|
|
--------------------------
|
|
|
|
The easiest way to manage concurrency is to forcibly stop any other
|
|
thread from changing the data structures that a given thread is
|
|
looking at. In cases where no other thread would even think of
|
|
changing the data and lots of different threads want to read at the
|
|
same time, this can be very costly. Even when using locks that permit
|
|
multiple concurrent readers, the simple act of updating the count of
|
|
the number of current readers can impose an unwanted cost. So the
|
|
goal when reading a shared data structure that no other process is
|
|
changing is to avoid writing anything to memory at all. Take no
|
|
locks, increment no counts, leave no footprints.
|
|
|
|
The REF-walk mechanism already described certainly doesn't follow this
|
|
principle, but then it is really designed to work when there may well
|
|
be other threads modifying the data. RCU-walk, in contrast, is
|
|
designed for the common situation where there are lots of frequent
|
|
readers and only occasional writers. This may not be common in all
|
|
parts of the filesystem tree, but in many parts it will be. For the
|
|
other parts it is important that RCU-walk can quickly fall back to
|
|
using REF-walk.
|
|
|
|
Pathname lookup always starts in RCU-walk mode but only remains there
|
|
as long as what it is looking for is in the cache and is stable. It
|
|
dances lightly down the cached filesystem image, leaving no footprints
|
|
and carefully watching where it is, to be sure it doesn't trip. If it
|
|
notices that something has changed or is changing, or if something
|
|
isn't in the cache, then it tries to stop gracefully and switch to
|
|
REF-walk.
|
|
|
|
This stopping requires getting a counted reference on the current
|
|
``vfsmount`` and ``dentry``, and ensuring that these are still valid -
|
|
that a path walk with REF-walk would have found the same entries.
|
|
This is an invariant that RCU-walk must guarantee. It can only make
|
|
decisions, such as selecting the next step, that are decisions which
|
|
REF-walk could also have made if it were walking down the tree at the
|
|
same time. If the graceful stop succeeds, the rest of the path is
|
|
processed with the reliable, if slightly sluggish, REF-walk. If
|
|
RCU-walk finds it cannot stop gracefully, it simply gives up and
|
|
restarts from the top with REF-walk.
|
|
|
|
This pattern of "try RCU-walk, if that fails try REF-walk" can be
|
|
clearly seen in functions like filename_lookup(),
|
|
filename_parentat(),
|
|
do_filp_open(), and do_file_open_root(). These four
|
|
correspond roughly to the three ``path_*()`` functions we met earlier,
|
|
each of which calls ``link_path_walk()``. The ``path_*()`` functions are
|
|
called using different mode flags until a mode is found which works.
|
|
They are first called with ``LOOKUP_RCU`` set to request "RCU-walk". If
|
|
that fails with the error ``ECHILD`` they are called again with no
|
|
special flag to request "REF-walk". If either of those report the
|
|
error ``ESTALE`` a final attempt is made with ``LOOKUP_REVAL`` set (and no
|
|
``LOOKUP_RCU``) to ensure that entries found in the cache are forcibly
|
|
revalidated - normally entries are only revalidated if the filesystem
|
|
determines that they are too old to trust.
|
|
|
|
The ``LOOKUP_RCU`` attempt may drop that flag internally and switch to
|
|
REF-walk, but will never then try to switch back to RCU-walk. Places
|
|
that trip up RCU-walk are much more likely to be near the leaves and
|
|
so it is very unlikely that there will be much, if any, benefit from
|
|
switching back.
|
|
|
|
RCU and seqlocks: fast and light
|
|
--------------------------------
|
|
|
|
RCU is, unsurprisingly, critical to RCU-walk mode. The
|
|
``rcu_read_lock()`` is held for the entire time that RCU-walk is walking
|
|
down a path. The particular guarantee it provides is that the key
|
|
data structures - dentries, inodes, super_blocks, and mounts - will
|
|
not be freed while the lock is held. They might be unlinked or
|
|
invalidated in one way or another, but the memory will not be
|
|
repurposed so values in various fields will still be meaningful. This
|
|
is the only guarantee that RCU provides; everything else is done using
|
|
seqlocks.
|
|
|
|
As we saw above, REF-walk holds a counted reference to the current
|
|
dentry and the current vfsmount, and does not release those references
|
|
before taking references to the "next" dentry or vfsmount. It also
|
|
sometimes takes the ``d_lock`` spinlock. These references and locks are
|
|
taken to prevent certain changes from happening. RCU-walk must not
|
|
take those references or locks and so cannot prevent such changes.
|
|
Instead, it checks to see if a change has been made, and aborts or
|
|
retries if it has.
|
|
|
|
To preserve the invariant mentioned above (that RCU-walk may only make
|
|
decisions that REF-walk could have made), it must make the checks at
|
|
or near the same places that REF-walk holds the references. So, when
|
|
REF-walk increments a reference count or takes a spinlock, RCU-walk
|
|
samples the status of a seqlock using ``read_seqcount_begin()`` or a
|
|
similar function. When REF-walk decrements the count or drops the
|
|
lock, RCU-walk checks if the sampled status is still valid using
|
|
``read_seqcount_retry()`` or similar.
|
|
|
|
However, there is a little bit more to seqlocks than that. If
|
|
RCU-walk accesses two different fields in a seqlock-protected
|
|
structure, or accesses the same field twice, there is no a priori
|
|
guarantee of any consistency between those accesses. When consistency
|
|
is needed - which it usually is - RCU-walk must take a copy and then
|
|
use ``read_seqcount_retry()`` to validate that copy.
|
|
|
|
``read_seqcount_retry()`` not only checks the sequence number, but also
|
|
imposes a memory barrier so that no memory-read instruction from
|
|
*before* the call can be delayed until *after* the call, either by the
|
|
CPU or by the compiler. A simple example of this can be seen in
|
|
``slow_dentry_cmp()`` which, for filesystems which do not use simple
|
|
byte-wise name equality, calls into the filesystem to compare a name
|
|
against a dentry. The length and name pointer are copied into local
|
|
variables, then ``read_seqcount_retry()`` is called to confirm the two
|
|
are consistent, and only then is ``->d_compare()`` called. When
|
|
standard filename comparison is used, ``dentry_cmp()`` is called
|
|
instead. Notably it does *not* use ``read_seqcount_retry()``, but
|
|
instead has a large comment explaining why the consistency guarantee
|
|
isn't necessary. A subsequent ``read_seqcount_retry()`` will be
|
|
sufficient to catch any problem that could occur at this point.
|
|
|
|
With that little refresher on seqlocks out of the way we can look at
|
|
the bigger picture of how RCU-walk uses seqlocks.
|
|
|
|
``mount_lock`` and ``nd->m_seq``
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
We already met the ``mount_lock`` seqlock when REF-walk used it to
|
|
ensure that crossing a mount point is performed safely. RCU-walk uses
|
|
it for that too, but for quite a bit more.
|
|
|
|
Instead of taking a counted reference to each ``vfsmount`` as it
|
|
descends the tree, RCU-walk samples the state of ``mount_lock`` at the
|
|
start of the walk and stores this initial sequence number in the
|
|
``struct nameidata`` in the ``m_seq`` field. This one lock and one
|
|
sequence number are used to validate all accesses to all ``vfsmounts``,
|
|
and all mount point crossings. As changes to the mount table are
|
|
relatively rare, it is reasonable to fall back on REF-walk any time
|
|
that any "mount" or "unmount" happens.
|
|
|
|
``m_seq`` is checked (using ``read_seqretry()``) at the end of an RCU-walk
|
|
sequence, whether switching to REF-walk for the rest of the path or
|
|
when the end of the path is reached. It is also checked when stepping
|
|
down over a mount point (in ``__follow_mount_rcu()``) or up (in
|
|
``follow_dotdot_rcu()``). If it is ever found to have changed, the
|
|
whole RCU-walk sequence is aborted and the path is processed again by
|
|
REF-walk.
|
|
|
|
If RCU-walk finds that ``mount_lock`` hasn't changed then it can be sure
|
|
that, had REF-walk taken counted references on each vfsmount, the
|
|
results would have been the same. This ensures the invariant holds,
|
|
at least for vfsmount structures.
|
|
|
|
``dentry->d_seq`` and ``nd->seq``
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
In place of taking a count or lock on ``d_reflock``, RCU-walk samples
|
|
the per-dentry ``d_seq`` seqlock, and stores the sequence number in the
|
|
``seq`` field of the nameidata structure, so ``nd->seq`` should always be
|
|
the current sequence number of ``nd->dentry``. This number needs to be
|
|
revalidated after copying, and before using, the name, parent, or
|
|
inode of the dentry.
|
|
|
|
The handling of the name we have already looked at, and the parent is
|
|
only accessed in ``follow_dotdot_rcu()`` which fairly trivially follows
|
|
the required pattern, though it does so for three different cases.
|
|
|
|
When not at a mount point, ``d_parent`` is followed and its ``d_seq`` is
|
|
collected. When we are at a mount point, we instead follow the
|
|
``mnt->mnt_mountpoint`` link to get a new dentry and collect its
|
|
``d_seq``. Then, after finally finding a ``d_parent`` to follow, we must
|
|
check if we have landed on a mount point and, if so, must find that
|
|
mount point and follow the ``mnt->mnt_root`` link. This would imply a
|
|
somewhat unusual, but certainly possible, circumstance where the
|
|
starting point of the path lookup was in part of the filesystem that
|
|
was mounted on, and so not visible from the root.
|
|
|
|
The inode pointer, stored in ``->d_inode``, is a little more
|
|
interesting. The inode will always need to be accessed at least
|
|
twice, once to determine if it is NULL and once to verify access
|
|
permissions. Symlink handling requires a validated inode pointer too.
|
|
Rather than revalidating on each access, a copy is made on the first
|
|
access and it is stored in the ``inode`` field of ``nameidata`` from where
|
|
it can be safely accessed without further validation.
|
|
|
|
``lookup_fast()`` is the only lookup routine that is used in RCU-mode,
|
|
``lookup_slow()`` being too slow and requiring locks. It is in
|
|
``lookup_fast()`` that we find the important "hand over hand" tracking
|
|
of the current dentry.
|
|
|
|
The current ``dentry`` and current ``seq`` number are passed to
|
|
``__d_lookup_rcu()`` which, on success, returns a new ``dentry`` and a
|
|
new ``seq`` number. ``lookup_fast()`` then copies the inode pointer and
|
|
revalidates the new ``seq`` number. It then validates the old ``dentry``
|
|
with the old ``seq`` number one last time and only then continues. This
|
|
process of getting the ``seq`` number of the new dentry and then
|
|
checking the ``seq`` number of the old exactly mirrors the process of
|
|
getting a counted reference to the new dentry before dropping that for
|
|
the old dentry which we saw in REF-walk.
|
|
|
|
No ``inode->i_rwsem`` or even ``rename_lock``
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
A semaphore is a fairly heavyweight lock that can only be taken when it is
|
|
permissible to sleep. As ``rcu_read_lock()`` forbids sleeping,
|
|
``inode->i_rwsem`` plays no role in RCU-walk. If some other thread does
|
|
take ``i_rwsem`` and modifies the directory in a way that RCU-walk needs
|
|
to notice, the result will be either that RCU-walk fails to find the
|
|
dentry that it is looking for, or it will find a dentry which
|
|
``read_seqretry()`` won't validate. In either case it will drop down to
|
|
REF-walk mode which can take whatever locks are needed.
|
|
|
|
Though ``rename_lock`` could be used by RCU-walk as it doesn't require
|
|
any sleeping, RCU-walk doesn't bother. REF-walk uses ``rename_lock`` to
|
|
protect against the possibility of hash chains in the dcache changing
|
|
while they are being searched. This can result in failing to find
|
|
something that actually is there. When RCU-walk fails to find
|
|
something in the dentry cache, whether it is really there or not, it
|
|
already drops down to REF-walk and tries again with appropriate
|
|
locking. This neatly handles all cases, so adding extra checks on
|
|
rename_lock would bring no significant value.
|
|
|
|
``unlazy walk()`` and ``complete_walk()``
|
|
-----------------------------------------
|
|
|
|
That "dropping down to REF-walk" typically involves a call to
|
|
``unlazy_walk()``, so named because "RCU-walk" is also sometimes
|
|
referred to as "lazy walk". ``unlazy_walk()`` is called when
|
|
following the path down to the current vfsmount/dentry pair seems to
|
|
have proceeded successfully, but the next step is problematic. This
|
|
can happen if the next name cannot be found in the dcache, if
|
|
permission checking or name revalidation couldn't be achieved while
|
|
the ``rcu_read_lock()`` is held (which forbids sleeping), if an
|
|
automount point is found, or in a couple of cases involving symlinks.
|
|
It is also called from ``complete_walk()`` when the lookup has reached
|
|
the final component, or the very end of the path, depending on which
|
|
particular flavor of lookup is used.
|
|
|
|
Other reasons for dropping out of RCU-walk that do not trigger a call
|
|
to ``unlazy_walk()`` are when some inconsistency is found that cannot be
|
|
handled immediately, such as ``mount_lock`` or one of the ``d_seq``
|
|
seqlocks reporting a change. In these cases the relevant function
|
|
will return ``-ECHILD`` which will percolate up until it triggers a new
|
|
attempt from the top using REF-walk.
|
|
|
|
For those cases where ``unlazy_walk()`` is an option, it essentially
|
|
takes a reference on each of the pointers that it holds (vfsmount,
|
|
dentry, and possibly some symbolic links) and then verifies that the
|
|
relevant seqlocks have not been changed. If there have been changes,
|
|
it, too, aborts with ``-ECHILD``, otherwise the transition to REF-walk
|
|
has been a success and the lookup process continues.
|
|
|
|
Taking a reference on those pointers is not quite as simple as just
|
|
incrementing a counter. That works to take a second reference if you
|
|
already have one (often indirectly through another object), but it
|
|
isn't sufficient if you don't actually have a counted reference at
|
|
all. For ``dentry->d_lockref``, it is safe to increment the reference
|
|
counter to get a reference unless it has been explicitly marked as
|
|
"dead" which involves setting the counter to ``-128``.
|
|
``lockref_get_not_dead()`` achieves this.
|
|
|
|
For ``mnt->mnt_count`` it is safe to take a reference as long as
|
|
``mount_lock`` is then used to validate the reference. If that
|
|
validation fails, it may *not* be safe to just drop that reference in
|
|
the standard way of calling ``mnt_put()`` - an unmount may have
|
|
progressed too far. So the code in ``legitimize_mnt()``, when it
|
|
finds that the reference it got might not be safe, checks the
|
|
``MNT_SYNC_UMOUNT`` flag to determine if a simple ``mnt_put()`` is
|
|
correct, or if it should just decrement the count and pretend none of
|
|
this ever happened.
|
|
|
|
Taking care in filesystems
|
|
--------------------------
|
|
|
|
RCU-walk depends almost entirely on cached information and often will
|
|
not call into the filesystem at all. However there are two places,
|
|
besides the already-mentioned component-name comparison, where the
|
|
file system might be included in RCU-walk, and it must know to be
|
|
careful.
|
|
|
|
If the filesystem has non-standard permission-checking requirements -
|
|
such as a networked filesystem which may need to check with the server
|
|
- the ``i_op->permission`` interface might be called during RCU-walk.
|
|
In this case an extra "``MAY_NOT_BLOCK``" flag is passed so that it
|
|
knows not to sleep, but to return ``-ECHILD`` if it cannot complete
|
|
promptly. ``i_op->permission`` is given the inode pointer, not the
|
|
dentry, so it doesn't need to worry about further consistency checks.
|
|
However if it accesses any other filesystem data structures, it must
|
|
ensure they are safe to be accessed with only the ``rcu_read_lock()``
|
|
held. This typically means they must be freed using ``kfree_rcu()`` or
|
|
similar.
|
|
|
|
.. _READ_ONCE: https://lwn.net/Articles/624126/
|
|
|
|
If the filesystem may need to revalidate dcache entries, then
|
|
``d_op->d_revalidate`` may be called in RCU-walk too. This interface
|
|
*is* passed the dentry but does not have access to the ``inode`` or the
|
|
``seq`` number from the ``nameidata``, so it needs to be extra careful
|
|
when accessing fields in the dentry. This "extra care" typically
|
|
involves using `READ_ONCE() <READ_ONCE_>`_ to access fields, and verifying the
|
|
result is not NULL before using it. This pattern can be seen in
|
|
``nfs_lookup_revalidate()``.
|
|
|
|
A pair of patterns
|
|
------------------
|
|
|
|
In various places in the details of REF-walk and RCU-walk, and also in
|
|
the big picture, there are a couple of related patterns that are worth
|
|
being aware of.
|
|
|
|
The first is "try quickly and check, if that fails try slowly". We
|
|
can see that in the high-level approach of first trying RCU-walk and
|
|
then trying REF-walk, and in places where ``unlazy_walk()`` is used to
|
|
switch to REF-walk for the rest of the path. We also saw it earlier
|
|
in ``dget_parent()`` when following a "``..``" link. It tries a quick way
|
|
to get a reference, then falls back to taking locks if needed.
|
|
|
|
The second pattern is "try quickly and check, if that fails try
|
|
again - repeatedly". This is seen with the use of ``rename_lock`` and
|
|
``mount_lock`` in REF-walk. RCU-walk doesn't make use of this pattern -
|
|
if anything goes wrong it is much safer to just abort and try a more
|
|
sedate approach.
|
|
|
|
The emphasis here is "try quickly and check". It should probably be
|
|
"try quickly *and carefully*, then check". The fact that checking is
|
|
needed is a reminder that the system is dynamic and only a limited
|
|
number of things are safe at all. The most likely cause of errors in
|
|
this whole process is assuming something is safe when in reality it
|
|
isn't. Careful consideration of what exactly guarantees the safety of
|
|
each access is sometimes necessary.
|
|
|
|
A walk among the symlinks
|
|
=========================
|
|
|
|
There are several basic issues that we will examine to understand the
|
|
handling of symbolic links: the symlink stack, together with cache
|
|
lifetimes, will help us understand the overall recursive handling of
|
|
symlinks and lead to the special care needed for the final component.
|
|
Then a consideration of access-time updates and summary of the various
|
|
flags controlling lookup will finish the story.
|
|
|
|
The symlink stack
|
|
-----------------
|
|
|
|
There are only two sorts of filesystem objects that can usefully
|
|
appear in a path prior to the final component: directories and symlinks.
|
|
Handling directories is quite straightforward: the new directory
|
|
simply becomes the starting point at which to interpret the next
|
|
component on the path. Handling symbolic links requires a bit more
|
|
work.
|
|
|
|
Conceptually, symbolic links could be handled by editing the path. If
|
|
a component name refers to a symbolic link, then that component is
|
|
replaced by the body of the link and, if that body starts with a '/',
|
|
then all preceding parts of the path are discarded. This is what the
|
|
"``readlink -f``" command does, though it also edits out "``.``" and
|
|
"``..``" components.
|
|
|
|
Directly editing the path string is not really necessary when looking
|
|
up a path, and discarding early components is pointless as they aren't
|
|
looked at anyway. Keeping track of all remaining components is
|
|
important, but they can of course be kept separately; there is no need
|
|
to concatenate them. As one symlink may easily refer to another,
|
|
which in turn can refer to a third, we may need to keep the remaining
|
|
components of several paths, each to be processed when the preceding
|
|
ones are completed. These path remnants are kept on a stack of
|
|
limited size.
|
|
|
|
There are two reasons for placing limits on how many symlinks can
|
|
occur in a single path lookup. The most obvious is to avoid loops.
|
|
If a symlink referred to itself either directly or through
|
|
intermediaries, then following the symlink can never complete
|
|
successfully - the error ``ELOOP`` must be returned. Loops can be
|
|
detected without imposing limits, but limits are the simplest solution
|
|
and, given the second reason for restriction, quite sufficient.
|
|
|
|
.. _outlined recently: http://thread.gmane.org/gmane.linux.kernel/1934390/focus=1934550
|
|
|
|
The second reason was `outlined recently`_ by Linus:
|
|
|
|
Because it's a latency and DoS issue too. We need to react well to
|
|
true loops, but also to "very deep" non-loops. It's not about memory
|
|
use, it's about users triggering unreasonable CPU resources.
|
|
|
|
Linux imposes a limit on the length of any pathname: ``PATH_MAX``, which
|
|
is 4096. There are a number of reasons for this limit; not letting the
|
|
kernel spend too much time on just one path is one of them. With
|
|
symbolic links you can effectively generate much longer paths so some
|
|
sort of limit is needed for the same reason. Linux imposes a limit of
|
|
at most 40 (MAXSYMLINKS) symlinks in any one path lookup. It previously imposed
|
|
a further limit of eight on the maximum depth of recursion, but that was
|
|
raised to 40 when a separate stack was implemented, so there is now
|
|
just the one limit.
|
|
|
|
The ``nameidata`` structure that we met in an earlier article contains a
|
|
small stack that can be used to store the remaining part of up to two
|
|
symlinks. In many cases this will be sufficient. If it isn't, a
|
|
separate stack is allocated with room for 40 symlinks. Pathname
|
|
lookup will never exceed that stack as, once the 40th symlink is
|
|
detected, an error is returned.
|
|
|
|
It might seem that the name remnants are all that needs to be stored on
|
|
this stack, but we need a bit more. To see that, we need to move on to
|
|
cache lifetimes.
|
|
|
|
Storage and lifetime of cached symlinks
|
|
---------------------------------------
|
|
|
|
Like other filesystem resources, such as inodes and directory
|
|
entries, symlinks are cached by Linux to avoid repeated costly access
|
|
to external storage. It is particularly important for RCU-walk to be
|
|
able to find and temporarily hold onto these cached entries, so that
|
|
it doesn't need to drop down into REF-walk.
|
|
|
|
.. _object-oriented design pattern: https://lwn.net/Articles/446317/
|
|
|
|
While each filesystem is free to make its own choice, symlinks are
|
|
typically stored in one of two places. Short symlinks are often
|
|
stored directly in the inode. When a filesystem allocates a ``struct
|
|
inode`` it typically allocates extra space to store private data (a
|
|
common `object-oriented design pattern`_ in the kernel). This will
|
|
sometimes include space for a symlink. The other common location is
|
|
in the page cache, which normally stores the content of files. The
|
|
pathname in a symlink can be seen as the content of that symlink and
|
|
can easily be stored in the page cache just like file content.
|
|
|
|
When neither of these is suitable, the next most likely scenario is
|
|
that the filesystem will allocate some temporary memory and copy or
|
|
construct the symlink content into that memory whenever it is needed.
|
|
|
|
When the symlink is stored in the inode, it has the same lifetime as
|
|
the inode which, itself, is protected by RCU or by a counted reference
|
|
on the dentry. This means that the mechanisms that pathname lookup
|
|
uses to access the dcache and icache (inode cache) safely are quite
|
|
sufficient for accessing some cached symlinks safely. In these cases,
|
|
the ``i_link`` pointer in the inode is set to point to wherever the
|
|
symlink is stored and it can be accessed directly whenever needed.
|
|
|
|
When the symlink is stored in the page cache or elsewhere, the
|
|
situation is not so straightforward. A reference on a dentry or even
|
|
on an inode does not imply any reference on cached pages of that
|
|
inode, and even an ``rcu_read_lock()`` is not sufficient to ensure that
|
|
a page will not disappear. So for these symlinks the pathname lookup
|
|
code needs to ask the filesystem to provide a stable reference and,
|
|
significantly, needs to release that reference when it is finished
|
|
with it.
|
|
|
|
Taking a reference to a cache page is often possible even in RCU-walk
|
|
mode. It does require making changes to memory, which is best avoided,
|
|
but that isn't necessarily a big cost and it is better than dropping
|
|
out of RCU-walk mode completely. Even filesystems that allocate
|
|
space to copy the symlink into can use ``GFP_ATOMIC`` to often successfully
|
|
allocate memory without the need to drop out of RCU-walk. If a
|
|
filesystem cannot successfully get a reference in RCU-walk mode, it
|
|
must return ``-ECHILD`` and ``unlazy_walk()`` will be called to return to
|
|
REF-walk mode in which the filesystem is allowed to sleep.
|
|
|
|
The place for all this to happen is the ``i_op->get_link()`` inode
|
|
method. This is called both in RCU-walk and REF-walk. In RCU-walk the
|
|
``dentry*`` argument is NULL, ``->get_link()`` can return -ECHILD to drop out of
|
|
RCU-walk. Much like the ``i_op->permission()`` method we
|
|
looked at previously, ``->get_link()`` would need to be careful that
|
|
all the data structures it references are safe to be accessed while
|
|
holding no counted reference, only the RCU lock. A callback
|
|
``struct delayed_called`` will be passed to ``->get_link()``:
|
|
file systems can set their own put_link function and argument through
|
|
set_delayed_call(). Later on, when VFS wants to put link, it will call
|
|
do_delayed_call() to invoke that callback function with the argument.
|
|
|
|
In order for the reference to each symlink to be dropped when the walk completes,
|
|
whether in RCU-walk or REF-walk, the symlink stack needs to contain,
|
|
along with the path remnants:
|
|
|
|
- the ``struct path`` to provide a reference to the previous path
|
|
- the ``const char *`` to provide a reference to the to previous name
|
|
- the ``seq`` to allow the path to be safely switched from RCU-walk to REF-walk
|
|
- the ``struct delayed_call`` for later invocation.
|
|
|
|
This means that each entry in the symlink stack needs to hold five
|
|
pointers and an integer instead of just one pointer (the path
|
|
remnant). On a 64-bit system, this is about 40 bytes per entry;
|
|
with 40 entries it adds up to 1600 bytes total, which is less than
|
|
half a page. So it might seem like a lot, but is by no means
|
|
excessive.
|
|
|
|
Note that, in a given stack frame, the path remnant (``name``) is not
|
|
part of the symlink that the other fields refer to. It is the remnant
|
|
to be followed once that symlink has been fully parsed.
|
|
|
|
Following the symlink
|
|
---------------------
|
|
|
|
The main loop in ``link_path_walk()`` iterates seamlessly over all
|
|
components in the path and all of the non-final symlinks. As symlinks
|
|
are processed, the ``name`` pointer is adjusted to point to a new
|
|
symlink, or is restored from the stack, so that much of the loop
|
|
doesn't need to notice. Getting this ``name`` variable on and off the
|
|
stack is very straightforward; pushing and popping the references is
|
|
a little more complex.
|
|
|
|
When a symlink is found, walk_component() calls pick_link() via step_into()
|
|
which returns the link from the filesystem.
|
|
Providing that operation is successful, the old path ``name`` is placed on the
|
|
stack, and the new value is used as the ``name`` for a while. When the end of
|
|
the path is found (i.e. ``*name`` is ``'\0'``) the old ``name`` is restored
|
|
off the stack and path walking continues.
|
|
|
|
Pushing and popping the reference pointers (inode, cookie, etc.) is more
|
|
complex in part because of the desire to handle tail recursion. When
|
|
the last component of a symlink itself points to a symlink, we
|
|
want to pop the symlink-just-completed off the stack before pushing
|
|
the symlink-just-found to avoid leaving empty path remnants that would
|
|
just get in the way.
|
|
|
|
It is most convenient to push the new symlink references onto the
|
|
stack in ``walk_component()`` immediately when the symlink is found;
|
|
``walk_component()`` is also the last piece of code that needs to look at the
|
|
old symlink as it walks that last component. So it is quite
|
|
convenient for ``walk_component()`` to release the old symlink and pop
|
|
the references just before pushing the reference information for the
|
|
new symlink. It is guided in this by three flags: ``WALK_NOFOLLOW`` which
|
|
forbids it from following a symlink if it finds one, ``WALK_MORE``
|
|
which indicates that it is yet too early to release the
|
|
current symlink, and ``WALK_TRAILING`` which indicates that it is on the final
|
|
component of the lookup, so we will check userspace flag ``LOOKUP_FOLLOW`` to
|
|
decide whether follow it when it is a symlink and call ``may_follow_link()`` to
|
|
check if we have privilege to follow it.
|
|
|
|
Symlinks with no final component
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
A pair of special-case symlinks deserve a little further explanation.
|
|
Both result in a new ``struct path`` (with mount and dentry) being set
|
|
up in the ``nameidata``, and result in pick_link() returning ``NULL``.
|
|
|
|
The more obvious case is a symlink to "``/``". All symlinks starting
|
|
with "``/``" are detected in pick_link() which resets the ``nameidata``
|
|
to point to the effective filesystem root. If the symlink only
|
|
contains "``/``" then there is nothing more to do, no components at all,
|
|
so ``NULL`` is returned to indicate that the symlink can be released and
|
|
the stack frame discarded.
|
|
|
|
The other case involves things in ``/proc`` that look like symlinks but
|
|
aren't really (and are therefore commonly referred to as "magic-links")::
|
|
|
|
$ ls -l /proc/self/fd/1
|
|
lrwx------ 1 neilb neilb 64 Jun 13 10:19 /proc/self/fd/1 -> /dev/pts/4
|
|
|
|
Every open file descriptor in any process is represented in ``/proc`` by
|
|
something that looks like a symlink. It is really a reference to the
|
|
target file, not just the name of it. When you ``readlink`` these
|
|
objects you get a name that might refer to the same file - unless it
|
|
has been unlinked or mounted over. When ``walk_component()`` follows
|
|
one of these, the ``->get_link()`` method in "procfs" doesn't return
|
|
a string name, but instead calls nd_jump_link() which updates the
|
|
``nameidata`` in place to point to that target. ``->get_link()`` then
|
|
returns ``NULL``. Again there is no final component and pick_link()
|
|
returns ``NULL``.
|
|
|
|
Following the symlink in the final component
|
|
--------------------------------------------
|
|
|
|
All this leads to ``link_path_walk()`` walking down every component, and
|
|
following all symbolic links it finds, until it reaches the final
|
|
component. This is just returned in the ``last`` field of ``nameidata``.
|
|
For some callers, this is all they need; they want to create that
|
|
``last`` name if it doesn't exist or give an error if it does. Other
|
|
callers will want to follow a symlink if one is found, and possibly
|
|
apply special handling to the last component of that symlink, rather
|
|
than just the last component of the original file name. These callers
|
|
potentially need to call ``link_path_walk()`` again and again on
|
|
successive symlinks until one is found that doesn't point to another
|
|
symlink.
|
|
|
|
This case is handled by relevant callers of link_path_walk(), such as
|
|
path_lookupat(), path_openat() using a loop that calls link_path_walk(),
|
|
and then handles the final component by calling open_last_lookups() or
|
|
lookup_last(). If it is a symlink that needs to be followed,
|
|
open_last_lookups() or lookup_last() will set things up properly and
|
|
return the path so that the loop repeats, calling
|
|
link_path_walk() again. This could loop as many as 40 times if the last
|
|
component of each symlink is another symlink.
|
|
|
|
Of the various functions that examine the final component,
|
|
open_last_lookups() is the most interesting as it works in tandem
|
|
with do_open() for opening a file. Part of open_last_lookups() runs
|
|
with ``i_rwsem`` held and this part is in a separate function: lookup_open().
|
|
|
|
Explaining open_last_lookups() and do_open() completely is beyond the scope
|
|
of this article, but a few highlights should help those interested in exploring
|
|
the code.
|
|
|
|
1. Rather than just finding the target file, do_open() is used after
|
|
open_last_lookup() to open
|
|
it. If the file was found in the dcache, then ``vfs_open()`` is used for
|
|
this. If not, then ``lookup_open()`` will either call ``atomic_open()`` (if
|
|
the filesystem provides it) to combine the final lookup with the open, or
|
|
will perform the separate ``i_op->lookup()`` and ``i_op->create()`` steps
|
|
directly. In the later case the actual "open" of this newly found or
|
|
created file will be performed by vfs_open(), just as if the name
|
|
were found in the dcache.
|
|
|
|
2. vfs_open() can fail with ``-EOPENSTALE`` if the cached information
|
|
wasn't quite current enough. If it's in RCU-walk ``-ECHILD`` will be returned
|
|
otherwise ``-ESTALE`` is returned. When ``-ESTALE`` is returned, the caller may
|
|
retry with ``LOOKUP_REVAL`` flag set.
|
|
|
|
3. An open with O_CREAT **does** follow a symlink in the final component,
|
|
unlike other creation system calls (like ``mkdir``). So the sequence::
|
|
|
|
ln -s bar /tmp/foo
|
|
echo hello > /tmp/foo
|
|
|
|
will create a file called ``/tmp/bar``. This is not permitted if
|
|
``O_EXCL`` is set but otherwise is handled for an O_CREAT open much
|
|
like for a non-creating open: lookup_last() or open_last_lookup()
|
|
returns a non ``NULL`` value, and link_path_walk() gets called and the
|
|
open process continues on the symlink that was found.
|
|
|
|
Updating the access time
|
|
------------------------
|
|
|
|
We previously said of RCU-walk that it would "take no locks, increment
|
|
no counts, leave no footprints." We have since seen that some
|
|
"footprints" can be needed when handling symlinks as a counted
|
|
reference (or even a memory allocation) may be needed. But these
|
|
footprints are best kept to a minimum.
|
|
|
|
One other place where walking down a symlink can involve leaving
|
|
footprints in a way that doesn't affect directories is in updating access times.
|
|
In Unix (and Linux) every filesystem object has a "last accessed
|
|
time", or "``atime``". Passing through a directory to access a file
|
|
within is not considered to be an access for the purposes of
|
|
``atime``; only listing the contents of a directory can update its ``atime``.
|
|
Symlinks are different it seems. Both reading a symlink (with ``readlink()``)
|
|
and looking up a symlink on the way to some other destination can
|
|
update the atime on that symlink.
|
|
|
|
.. _clearest statement: https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap04.html#tag_04_08
|
|
|
|
It is not clear why this is the case; POSIX has little to say on the
|
|
subject. The `clearest statement`_ is that, if a particular implementation
|
|
updates a timestamp in a place not specified by POSIX, this must be
|
|
documented "except that any changes caused by pathname resolution need
|
|
not be documented". This seems to imply that POSIX doesn't really
|
|
care about access-time updates during pathname lookup.
|
|
|
|
.. _Linux 1.3.87: https://git.kernel.org/cgit/linux/kernel/git/history/history.git/diff/fs/ext2/symlink.c?id=f806c6db77b8eaa6e00dcfb6b567706feae8dbb8
|
|
|
|
An examination of history shows that prior to `Linux 1.3.87`_, the ext2
|
|
filesystem, at least, didn't update atime when following a link.
|
|
Unfortunately we have no record of why that behavior was changed.
|
|
|
|
In any case, access time must now be updated and that operation can be
|
|
quite complex. Trying to stay in RCU-walk while doing it is best
|
|
avoided. Fortunately it is often permitted to skip the ``atime``
|
|
update. Because ``atime`` updates cause performance problems in various
|
|
areas, Linux supports the ``relatime`` mount option, which generally
|
|
limits the updates of ``atime`` to once per day on files that aren't
|
|
being changed (and symlinks never change once created). Even without
|
|
``relatime``, many filesystems record ``atime`` with a one-second
|
|
granularity, so only one update per second is required.
|
|
|
|
It is easy to test if an ``atime`` update is needed while in RCU-walk
|
|
mode and, if it isn't, the update can be skipped and RCU-walk mode
|
|
continues. Only when an ``atime`` update is actually required does the
|
|
path walk drop down to REF-walk. All of this is handled in the
|
|
``get_link()`` function.
|
|
|
|
A few flags
|
|
-----------
|
|
|
|
A suitable way to wrap up this tour of pathname walking is to list
|
|
the various flags that can be stored in the ``nameidata`` to guide the
|
|
lookup process. Many of these are only meaningful on the final
|
|
component, others reflect the current state of the pathname lookup, and some
|
|
apply restrictions to all path components encountered in the path lookup.
|
|
|
|
And then there is ``LOOKUP_EMPTY``, which doesn't fit conceptually with
|
|
the others. If this is not set, an empty pathname causes an error
|
|
very early on. If it is set, empty pathnames are not considered to be
|
|
an error.
|
|
|
|
Global state flags
|
|
~~~~~~~~~~~~~~~~~~
|
|
|
|
We have already met two global state flags: ``LOOKUP_RCU`` and
|
|
``LOOKUP_REVAL``. These select between one of three overall approaches
|
|
to lookup: RCU-walk, REF-walk, and REF-walk with forced revalidation.
|
|
|
|
``LOOKUP_PARENT`` indicates that the final component hasn't been reached
|
|
yet. This is primarily used to tell the audit subsystem the full
|
|
context of a particular access being audited.
|
|
|
|
``ND_ROOT_PRESET`` indicates that the ``root`` field in the ``nameidata`` was
|
|
provided by the caller, so it shouldn't be released when it is no
|
|
longer needed.
|
|
|
|
``ND_JUMPED`` means that the current dentry was chosen not because
|
|
it had the right name but for some other reason. This happens when
|
|
following "``..``", following a symlink to ``/``, crossing a mount point
|
|
or accessing a "``/proc/$PID/fd/$FD``" symlink (also known as a "magic
|
|
link"). In this case the filesystem has not been asked to revalidate the
|
|
name (with ``d_revalidate()``). In such cases the inode may still need
|
|
to be revalidated, so ``d_op->d_weak_revalidate()`` is called if
|
|
``ND_JUMPED`` is set when the look completes - which may be at the
|
|
final component or, when creating, unlinking, or renaming, at the penultimate component.
|
|
|
|
Resolution-restriction flags
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
In order to allow userspace to protect itself against certain race conditions
|
|
and attack scenarios involving changing path components, a series of flags are
|
|
available which apply restrictions to all path components encountered during
|
|
path lookup. These flags are exposed through ``openat2()``'s ``resolve`` field.
|
|
|
|
``LOOKUP_NO_SYMLINKS`` blocks all symlink traversals (including magic-links).
|
|
This is distinctly different from ``LOOKUP_FOLLOW``, because the latter only
|
|
relates to restricting the following of trailing symlinks.
|
|
|
|
``LOOKUP_NO_MAGICLINKS`` blocks all magic-link traversals. Filesystems must
|
|
ensure that they return errors from ``nd_jump_link()``, because that is how
|
|
``LOOKUP_NO_MAGICLINKS`` and other magic-link restrictions are implemented.
|
|
|
|
``LOOKUP_NO_XDEV`` blocks all ``vfsmount`` traversals (this includes both
|
|
bind-mounts and ordinary mounts). Note that the ``vfsmount`` which contains the
|
|
lookup is determined by the first mountpoint the path lookup reaches --
|
|
absolute paths start with the ``vfsmount`` of ``/``, and relative paths start
|
|
with the ``dfd``'s ``vfsmount``. Magic-links are only permitted if the
|
|
``vfsmount`` of the path is unchanged.
|
|
|
|
``LOOKUP_BENEATH`` blocks any path components which resolve outside the
|
|
starting point of the resolution. This is done by blocking ``nd_jump_root()``
|
|
as well as blocking ".." if it would jump outside the starting point.
|
|
``rename_lock`` and ``mount_lock`` are used to detect attacks against the
|
|
resolution of "..". Magic-links are also blocked.
|
|
|
|
``LOOKUP_IN_ROOT`` resolves all path components as though the starting point
|
|
were the filesystem root. ``nd_jump_root()`` brings the resolution back to
|
|
the starting point, and ".." at the starting point will act as a no-op. As with
|
|
``LOOKUP_BENEATH``, ``rename_lock`` and ``mount_lock`` are used to detect
|
|
attacks against ".." resolution. Magic-links are also blocked.
|
|
|
|
Final-component flags
|
|
~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
Some of these flags are only set when the final component is being
|
|
considered. Others are only checked for when considering that final
|
|
component.
|
|
|
|
``LOOKUP_AUTOMOUNT`` ensures that, if the final component is an automount
|
|
point, then the mount is triggered. Some operations would trigger it
|
|
anyway, but operations like ``stat()`` deliberately don't. ``statfs()``
|
|
needs to trigger the mount but otherwise behaves a lot like ``stat()``, so
|
|
it sets ``LOOKUP_AUTOMOUNT``, as does "``quotactl()``" and the handling of
|
|
"``mount --bind``".
|
|
|
|
``LOOKUP_FOLLOW`` has a similar function to ``LOOKUP_AUTOMOUNT`` but for
|
|
symlinks. Some system calls set or clear it implicitly, while
|
|
others have API flags such as ``AT_SYMLINK_FOLLOW`` and
|
|
``UMOUNT_NOFOLLOW`` to control it. Its effect is similar to
|
|
``WALK_GET`` that we already met, but it is used in a different way.
|
|
|
|
``LOOKUP_DIRECTORY`` insists that the final component is a directory.
|
|
Various callers set this and it is also set when the final component
|
|
is found to be followed by a slash.
|
|
|
|
Finally ``LOOKUP_OPEN``, ``LOOKUP_CREATE``, ``LOOKUP_EXCL``, and
|
|
``LOOKUP_RENAME_TARGET`` are not used directly by the VFS but are made
|
|
available to the filesystem and particularly the ``->d_revalidate()``
|
|
method. A filesystem can choose not to bother revalidating too hard
|
|
if it knows that it will be asked to open or create the file soon.
|
|
These flags were previously useful for ``->lookup()`` too but with the
|
|
introduction of ``->atomic_open()`` they are less relevant there.
|
|
|
|
End of the road
|
|
---------------
|
|
|
|
Despite its complexity, all this pathname lookup code appears to be
|
|
in good shape - various parts are certainly easier to understand now
|
|
than even a couple of releases ago. But that doesn't mean it is
|
|
"finished". As already mentioned, RCU-walk currently only follows
|
|
symlinks that are stored in the inode so, while it handles many ext4
|
|
symlinks, it doesn't help with NFS, XFS, or Btrfs. That support
|
|
is not likely to be long delayed.
|