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Merge tag 'idmapped-mounts-v5.12' of git://git.kernel.org/pub/scm/linux/kernel/git/brauner/linux
Pull idmapped mounts from Christian Brauner:
"This introduces idmapped mounts which has been in the making for some
time. Simply put, different mounts can expose the same file or
directory with different ownership. This initial implementation comes
with ports for fat, ext4 and with Christoph's port for xfs with more
filesystems being actively worked on by independent people and
maintainers.
Idmapping mounts handle a wide range of long standing use-cases. Here
are just a few:
- Idmapped mounts make it possible to easily share files between
multiple users or multiple machines especially in complex
scenarios. For example, idmapped mounts will be used in the
implementation of portable home directories in
systemd-homed.service(8) where they allow users to move their home
directory to an external storage device and use it on multiple
computers where they are assigned different uids and gids. This
effectively makes it possible to assign random uids and gids at
login time.
- It is possible to share files from the host with unprivileged
containers without having to change ownership permanently through
chown(2).
- It is possible to idmap a container's rootfs and without having to
mangle every file. For example, Chromebooks use it to share the
user's Download folder with their unprivileged containers in their
Linux subsystem.
- It is possible to share files between containers with
non-overlapping idmappings.
- Filesystem that lack a proper concept of ownership such as fat can
use idmapped mounts to implement discretionary access (DAC)
permission checking.
- They allow users to efficiently changing ownership on a per-mount
basis without having to (recursively) chown(2) all files. In
contrast to chown (2) changing ownership of large sets of files is
instantenous with idmapped mounts. This is especially useful when
ownership of a whole root filesystem of a virtual machine or
container is changed. With idmapped mounts a single syscall
mount_setattr syscall will be sufficient to change the ownership of
all files.
- Idmapped mounts always take the current ownership into account as
idmappings specify what a given uid or gid is supposed to be mapped
to. This contrasts with the chown(2) syscall which cannot by itself
take the current ownership of the files it changes into account. It
simply changes the ownership to the specified uid and gid. This is
especially problematic when recursively chown(2)ing a large set of
files which is commong with the aforementioned portable home
directory and container and vm scenario.
- Idmapped mounts allow to change ownership locally, restricting it
to specific mounts, and temporarily as the ownership changes only
apply as long as the mount exists.
Several userspace projects have either already put up patches and
pull-requests for this feature or will do so should you decide to pull
this:
- systemd: In a wide variety of scenarios but especially right away
in their implementation of portable home directories.
https://systemd.io/HOME_DIRECTORY/
- container runtimes: containerd, runC, LXD:To share data between
host and unprivileged containers, unprivileged and privileged
containers, etc. The pull request for idmapped mounts support in
containerd, the default Kubernetes runtime is already up for quite
a while now: https://github.com/containerd/containerd/pull/4734
- The virtio-fs developers and several users have expressed interest
in using this feature with virtual machines once virtio-fs is
ported.
- ChromeOS: Sharing host-directories with unprivileged containers.
I've tightly synced with all those projects and all of those listed
here have also expressed their need/desire for this feature on the
mailing list. For more info on how people use this there's a bunch of
talks about this too. Here's just two recent ones:
https://www.cncf.io/wp-content/uploads/2020/12/Rootless-Containers-in-Gitpod.pdf
https://fosdem.org/2021/schedule/event/containers_idmap/
This comes with an extensive xfstests suite covering both ext4 and
xfs:
https://git.kernel.org/brauner/xfstests-dev/h/idmapped_mounts
It covers truncation, creation, opening, xattrs, vfscaps, setid
execution, setgid inheritance and more both with idmapped and
non-idmapped mounts. It already helped to discover an unrelated xfs
setgid inheritance bug which has since been fixed in mainline. It will
be sent for inclusion with the xfstests project should you decide to
merge this.
In order to support per-mount idmappings vfsmounts are marked with
user namespaces. The idmapping of the user namespace will be used to
map the ids of vfs objects when they are accessed through that mount.
By default all vfsmounts are marked with the initial user namespace.
The initial user namespace is used to indicate that a mount is not
idmapped. All operations behave as before and this is verified in the
testsuite.
Based on prior discussions we want to attach the whole user namespace
and not just a dedicated idmapping struct. This allows us to reuse all
the helpers that already exist for dealing with idmappings instead of
introducing a whole new range of helpers. In addition, if we decide in
the future that we are confident enough to enable unprivileged users
to setup idmapped mounts the permission checking can take into account
whether the caller is privileged in the user namespace the mount is
currently marked with.
The user namespace the mount will be marked with can be specified by
passing a file descriptor refering to the user namespace as an
argument to the new mount_setattr() syscall together with the new
MOUNT_ATTR_IDMAP flag. The system call follows the openat2() pattern
of extensibility.
The following conditions must be met in order to create an idmapped
mount:
- The caller must currently have the CAP_SYS_ADMIN capability in the
user namespace the underlying filesystem has been mounted in.
- The underlying filesystem must support idmapped mounts.
- The mount must not already be idmapped. This also implies that the
idmapping of a mount cannot be altered once it has been idmapped.
- The mount must be a detached/anonymous mount, i.e. it must have
been created by calling open_tree() with the OPEN_TREE_CLONE flag
and it must not already have been visible in the filesystem.
The last two points guarantee easier semantics for userspace and the
kernel and make the implementation significantly simpler.
By default vfsmounts are marked with the initial user namespace and no
behavioral or performance changes are observed.
The manpage with a detailed description can be found here:
1d7b902e28
In order to support idmapped mounts, filesystems need to be changed
and mark themselves with the FS_ALLOW_IDMAP flag in fs_flags. The
patches to convert individual filesystem are not very large or
complicated overall as can be seen from the included fat, ext4, and
xfs ports. Patches for other filesystems are actively worked on and
will be sent out separately. The xfstestsuite can be used to verify
that port has been done correctly.
The mount_setattr() syscall is motivated independent of the idmapped
mounts patches and it's been around since July 2019. One of the most
valuable features of the new mount api is the ability to perform
mounts based on file descriptors only.
Together with the lookup restrictions available in the openat2()
RESOLVE_* flag namespace which we added in v5.6 this is the first time
we are close to hardened and race-free (e.g. symlinks) mounting and
path resolution.
While userspace has started porting to the new mount api to mount
proper filesystems and create new bind-mounts it is currently not
possible to change mount options of an already existing bind mount in
the new mount api since the mount_setattr() syscall is missing.
With the addition of the mount_setattr() syscall we remove this last
restriction and userspace can now fully port to the new mount api,
covering every use-case the old mount api could. We also add the
crucial ability to recursively change mount options for a whole mount
tree, both removing and adding mount options at the same time. This
syscall has been requested multiple times by various people and
projects.
There is a simple tool available at
https://github.com/brauner/mount-idmapped
that allows to create idmapped mounts so people can play with this
patch series. I'll add support for the regular mount binary should you
decide to pull this in the following weeks:
Here's an example to a simple idmapped mount of another user's home
directory:
u1001@f2-vm:/$ sudo ./mount --idmap both:1000:1001:1 /home/ubuntu/ /mnt
u1001@f2-vm:/$ ls -al /home/ubuntu/
total 28
drwxr-xr-x 2 ubuntu ubuntu 4096 Oct 28 22:07 .
drwxr-xr-x 4 root root 4096 Oct 28 04:00 ..
-rw------- 1 ubuntu ubuntu 3154 Oct 28 22:12 .bash_history
-rw-r--r-- 1 ubuntu ubuntu 220 Feb 25 2020 .bash_logout
-rw-r--r-- 1 ubuntu ubuntu 3771 Feb 25 2020 .bashrc
-rw-r--r-- 1 ubuntu ubuntu 807 Feb 25 2020 .profile
-rw-r--r-- 1 ubuntu ubuntu 0 Oct 16 16:11 .sudo_as_admin_successful
-rw------- 1 ubuntu ubuntu 1144 Oct 28 00:43 .viminfo
u1001@f2-vm:/$ ls -al /mnt/
total 28
drwxr-xr-x 2 u1001 u1001 4096 Oct 28 22:07 .
drwxr-xr-x 29 root root 4096 Oct 28 22:01 ..
-rw------- 1 u1001 u1001 3154 Oct 28 22:12 .bash_history
-rw-r--r-- 1 u1001 u1001 220 Feb 25 2020 .bash_logout
-rw-r--r-- 1 u1001 u1001 3771 Feb 25 2020 .bashrc
-rw-r--r-- 1 u1001 u1001 807 Feb 25 2020 .profile
-rw-r--r-- 1 u1001 u1001 0 Oct 16 16:11 .sudo_as_admin_successful
-rw------- 1 u1001 u1001 1144 Oct 28 00:43 .viminfo
u1001@f2-vm:/$ touch /mnt/my-file
u1001@f2-vm:/$ setfacl -m u:1001:rwx /mnt/my-file
u1001@f2-vm:/$ sudo setcap -n 1001 cap_net_raw+ep /mnt/my-file
u1001@f2-vm:/$ ls -al /mnt/my-file
-rw-rwxr--+ 1 u1001 u1001 0 Oct 28 22:14 /mnt/my-file
u1001@f2-vm:/$ ls -al /home/ubuntu/my-file
-rw-rwxr--+ 1 ubuntu ubuntu 0 Oct 28 22:14 /home/ubuntu/my-file
u1001@f2-vm:/$ getfacl /mnt/my-file
getfacl: Removing leading '/' from absolute path names
# file: mnt/my-file
# owner: u1001
# group: u1001
user::rw-
user:u1001:rwx
group::rw-
mask::rwx
other::r--
u1001@f2-vm:/$ getfacl /home/ubuntu/my-file
getfacl: Removing leading '/' from absolute path names
# file: home/ubuntu/my-file
# owner: ubuntu
# group: ubuntu
user::rw-
user:ubuntu:rwx
group::rw-
mask::rwx
other::r--"
* tag 'idmapped-mounts-v5.12' of git://git.kernel.org/pub/scm/linux/kernel/git/brauner/linux: (41 commits)
xfs: remove the possibly unused mp variable in xfs_file_compat_ioctl
xfs: support idmapped mounts
ext4: support idmapped mounts
fat: handle idmapped mounts
tests: add mount_setattr() selftests
fs: introduce MOUNT_ATTR_IDMAP
fs: add mount_setattr()
fs: add attr_flags_to_mnt_flags helper
fs: split out functions to hold writers
namespace: only take read lock in do_reconfigure_mnt()
mount: make {lock,unlock}_mount_hash() static
namespace: take lock_mount_hash() directly when changing flags
nfs: do not export idmapped mounts
overlayfs: do not mount on top of idmapped mounts
ecryptfs: do not mount on top of idmapped mounts
ima: handle idmapped mounts
apparmor: handle idmapped mounts
fs: make helpers idmap mount aware
exec: handle idmapped mounts
would_dump: handle idmapped mounts
...
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ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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=========================================
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Overview of the Linux Virtual File System
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=========================================
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Original author: Richard Gooch <rgooch@atnf.csiro.au>
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- Copyright (C) 1999 Richard Gooch
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- Copyright (C) 2005 Pekka Enberg
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Introduction
|
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============
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The Virtual File System (also known as the Virtual Filesystem Switch) is
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the software layer in the kernel that provides the filesystem interface
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to userspace programs. It also provides an abstraction within the
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kernel which allows different filesystem implementations to coexist.
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VFS system calls open(2), stat(2), read(2), write(2), chmod(2) and so on
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are called from a process context. Filesystem locking is described in
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the document Documentation/filesystems/locking.rst.
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Directory Entry Cache (dcache)
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------------------------------
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The VFS implements the open(2), stat(2), chmod(2), and similar system
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calls. The pathname argument that is passed to them is used by the VFS
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to search through the directory entry cache (also known as the dentry
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cache or dcache). This provides a very fast look-up mechanism to
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translate a pathname (filename) into a specific dentry. Dentries live
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in RAM and are never saved to disc: they exist only for performance.
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The dentry cache is meant to be a view into your entire filespace. As
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most computers cannot fit all dentries in the RAM at the same time, some
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bits of the cache are missing. In order to resolve your pathname into a
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dentry, the VFS may have to resort to creating dentries along the way,
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and then loading the inode. This is done by looking up the inode.
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The Inode Object
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----------------
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An individual dentry usually has a pointer to an inode. Inodes are
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filesystem objects such as regular files, directories, FIFOs and other
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beasts. They live either on the disc (for block device filesystems) or
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in the memory (for pseudo filesystems). Inodes that live on the disc
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are copied into the memory when required and changes to the inode are
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written back to disc. A single inode can be pointed to by multiple
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dentries (hard links, for example, do this).
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To look up an inode requires that the VFS calls the lookup() method of
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the parent directory inode. This method is installed by the specific
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filesystem implementation that the inode lives in. Once the VFS has the
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required dentry (and hence the inode), we can do all those boring things
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like open(2) the file, or stat(2) it to peek at the inode data. The
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stat(2) operation is fairly simple: once the VFS has the dentry, it
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peeks at the inode data and passes some of it back to userspace.
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The File Object
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---------------
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Opening a file requires another operation: allocation of a file
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structure (this is the kernel-side implementation of file descriptors).
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The freshly allocated file structure is initialized with a pointer to
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the dentry and a set of file operation member functions. These are
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taken from the inode data. The open() file method is then called so the
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specific filesystem implementation can do its work. You can see that
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this is another switch performed by the VFS. The file structure is
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placed into the file descriptor table for the process.
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Reading, writing and closing files (and other assorted VFS operations)
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is done by using the userspace file descriptor to grab the appropriate
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file structure, and then calling the required file structure method to
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do whatever is required. For as long as the file is open, it keeps the
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dentry in use, which in turn means that the VFS inode is still in use.
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Registering and Mounting a Filesystem
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=====================================
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To register and unregister a filesystem, use the following API
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functions:
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.. code-block:: c
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#include <linux/fs.h>
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extern int register_filesystem(struct file_system_type *);
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extern int unregister_filesystem(struct file_system_type *);
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The passed struct file_system_type describes your filesystem. When a
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request is made to mount a filesystem onto a directory in your
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namespace, the VFS will call the appropriate mount() method for the
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specific filesystem. New vfsmount referring to the tree returned by
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->mount() will be attached to the mountpoint, so that when pathname
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resolution reaches the mountpoint it will jump into the root of that
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vfsmount.
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You can see all filesystems that are registered to the kernel in the
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file /proc/filesystems.
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struct file_system_type
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-----------------------
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This describes the filesystem. As of kernel 2.6.39, the following
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members are defined:
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.. code-block:: c
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struct file_system_type {
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const char *name;
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int fs_flags;
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struct dentry *(*mount) (struct file_system_type *, int,
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const char *, void *);
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void (*kill_sb) (struct super_block *);
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struct module *owner;
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struct file_system_type * next;
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struct list_head fs_supers;
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struct lock_class_key s_lock_key;
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struct lock_class_key s_umount_key;
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};
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``name``
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the name of the filesystem type, such as "ext2", "iso9660",
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"msdos" and so on
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``fs_flags``
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various flags (i.e. FS_REQUIRES_DEV, FS_NO_DCACHE, etc.)
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``mount``
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the method to call when a new instance of this filesystem should
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be mounted
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``kill_sb``
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the method to call when an instance of this filesystem should be
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shut down
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``owner``
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for internal VFS use: you should initialize this to THIS_MODULE
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in most cases.
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``next``
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for internal VFS use: you should initialize this to NULL
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s_lock_key, s_umount_key: lockdep-specific
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The mount() method has the following arguments:
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``struct file_system_type *fs_type``
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describes the filesystem, partly initialized by the specific
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filesystem code
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``int flags``
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mount flags
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``const char *dev_name``
|
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the device name we are mounting.
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``void *data``
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arbitrary mount options, usually comes as an ASCII string (see
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"Mount Options" section)
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The mount() method must return the root dentry of the tree requested by
|
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caller. An active reference to its superblock must be grabbed and the
|
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superblock must be locked. On failure it should return ERR_PTR(error).
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The arguments match those of mount(2) and their interpretation depends
|
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on filesystem type. E.g. for block filesystems, dev_name is interpreted
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as block device name, that device is opened and if it contains a
|
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suitable filesystem image the method creates and initializes struct
|
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super_block accordingly, returning its root dentry to caller.
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->mount() may choose to return a subtree of existing filesystem - it
|
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doesn't have to create a new one. The main result from the caller's
|
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point of view is a reference to dentry at the root of (sub)tree to be
|
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attached; creation of new superblock is a common side effect.
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The most interesting member of the superblock structure that the mount()
|
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method fills in is the "s_op" field. This is a pointer to a "struct
|
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super_operations" which describes the next level of the filesystem
|
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implementation.
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Usually, a filesystem uses one of the generic mount() implementations
|
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and provides a fill_super() callback instead. The generic variants are:
|
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``mount_bdev``
|
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mount a filesystem residing on a block device
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``mount_nodev``
|
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mount a filesystem that is not backed by a device
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``mount_single``
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mount a filesystem which shares the instance between all mounts
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|
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A fill_super() callback implementation has the following arguments:
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``struct super_block *sb``
|
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the superblock structure. The callback must initialize this
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properly.
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``void *data``
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arbitrary mount options, usually comes as an ASCII string (see
|
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"Mount Options" section)
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``int silent``
|
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whether or not to be silent on error
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|
|
|
|
The Superblock Object
|
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=====================
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A superblock object represents a mounted filesystem.
|
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|
|
|
|
struct super_operations
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-----------------------
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|
|
This describes how the VFS can manipulate the superblock of your
|
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filesystem. As of kernel 2.6.22, the following members are defined:
|
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|
|
.. code-block:: c
|
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|
|
struct super_operations {
|
|
struct inode *(*alloc_inode)(struct super_block *sb);
|
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void (*destroy_inode)(struct inode *);
|
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|
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void (*dirty_inode) (struct inode *, int flags);
|
|
int (*write_inode) (struct inode *, int);
|
|
void (*drop_inode) (struct inode *);
|
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void (*delete_inode) (struct inode *);
|
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void (*put_super) (struct super_block *);
|
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int (*sync_fs)(struct super_block *sb, int wait);
|
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int (*freeze_fs) (struct super_block *);
|
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int (*unfreeze_fs) (struct super_block *);
|
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int (*statfs) (struct dentry *, struct kstatfs *);
|
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int (*remount_fs) (struct super_block *, int *, char *);
|
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void (*clear_inode) (struct inode *);
|
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void (*umount_begin) (struct super_block *);
|
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int (*show_options)(struct seq_file *, struct dentry *);
|
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|
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ssize_t (*quota_read)(struct super_block *, int, char *, size_t, loff_t);
|
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ssize_t (*quota_write)(struct super_block *, int, const char *, size_t, loff_t);
|
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int (*nr_cached_objects)(struct super_block *);
|
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void (*free_cached_objects)(struct super_block *, int);
|
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};
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|
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All methods are called without any locks being held, unless otherwise
|
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noted. This means that most methods can block safely. All methods are
|
|
only called from a process context (i.e. not from an interrupt handler
|
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or bottom half).
|
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|
|
``alloc_inode``
|
|
this method is called by alloc_inode() to allocate memory for
|
|
struct inode and initialize it. If this function is not
|
|
defined, a simple 'struct inode' is allocated. Normally
|
|
alloc_inode will be used to allocate a larger structure which
|
|
contains a 'struct inode' embedded within it.
|
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|
|
``destroy_inode``
|
|
this method is called by destroy_inode() to release resources
|
|
allocated for struct inode. It is only required if
|
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->alloc_inode was defined and simply undoes anything done by
|
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->alloc_inode.
|
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|
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``dirty_inode``
|
|
this method is called by the VFS when an inode is marked dirty.
|
|
This is specifically for the inode itself being marked dirty,
|
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not its data. If the update needs to be persisted by fdatasync(),
|
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then I_DIRTY_DATASYNC will be set in the flags argument.
|
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|
|
``write_inode``
|
|
this method is called when the VFS needs to write an inode to
|
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disc. The second parameter indicates whether the write should
|
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be synchronous or not, not all filesystems check this flag.
|
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|
|
``drop_inode``
|
|
called when the last access to the inode is dropped, with the
|
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inode->i_lock spinlock held.
|
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|
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This method should be either NULL (normal UNIX filesystem
|
|
semantics) or "generic_delete_inode" (for filesystems that do
|
|
not want to cache inodes - causing "delete_inode" to always be
|
|
called regardless of the value of i_nlink)
|
|
|
|
The "generic_delete_inode()" behavior is equivalent to the old
|
|
practice of using "force_delete" in the put_inode() case, but
|
|
does not have the races that the "force_delete()" approach had.
|
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|
|
``delete_inode``
|
|
called when the VFS wants to delete an inode
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|
|
``put_super``
|
|
called when the VFS wishes to free the superblock
|
|
(i.e. unmount). This is called with the superblock lock held
|
|
|
|
``sync_fs``
|
|
called when VFS is writing out all dirty data associated with a
|
|
superblock. The second parameter indicates whether the method
|
|
should wait until the write out has been completed. Optional.
|
|
|
|
``freeze_fs``
|
|
called when VFS is locking a filesystem and forcing it into a
|
|
consistent state. This method is currently used by the Logical
|
|
Volume Manager (LVM).
|
|
|
|
``unfreeze_fs``
|
|
called when VFS is unlocking a filesystem and making it writable
|
|
again.
|
|
|
|
``statfs``
|
|
called when the VFS needs to get filesystem statistics.
|
|
|
|
``remount_fs``
|
|
called when the filesystem is remounted. This is called with
|
|
the kernel lock held
|
|
|
|
``clear_inode``
|
|
called then the VFS clears the inode. Optional
|
|
|
|
``umount_begin``
|
|
called when the VFS is unmounting a filesystem.
|
|
|
|
``show_options``
|
|
called by the VFS to show mount options for /proc/<pid>/mounts.
|
|
(see "Mount Options" section)
|
|
|
|
``quota_read``
|
|
called by the VFS to read from filesystem quota file.
|
|
|
|
``quota_write``
|
|
called by the VFS to write to filesystem quota file.
|
|
|
|
``nr_cached_objects``
|
|
called by the sb cache shrinking function for the filesystem to
|
|
return the number of freeable cached objects it contains.
|
|
Optional.
|
|
|
|
``free_cache_objects``
|
|
called by the sb cache shrinking function for the filesystem to
|
|
scan the number of objects indicated to try to free them.
|
|
Optional, but any filesystem implementing this method needs to
|
|
also implement ->nr_cached_objects for it to be called
|
|
correctly.
|
|
|
|
We can't do anything with any errors that the filesystem might
|
|
encountered, hence the void return type. This will never be
|
|
called if the VM is trying to reclaim under GFP_NOFS conditions,
|
|
hence this method does not need to handle that situation itself.
|
|
|
|
Implementations must include conditional reschedule calls inside
|
|
any scanning loop that is done. This allows the VFS to
|
|
determine appropriate scan batch sizes without having to worry
|
|
about whether implementations will cause holdoff problems due to
|
|
large scan batch sizes.
|
|
|
|
Whoever sets up the inode is responsible for filling in the "i_op"
|
|
field. This is a pointer to a "struct inode_operations" which describes
|
|
the methods that can be performed on individual inodes.
|
|
|
|
|
|
struct xattr_handlers
|
|
---------------------
|
|
|
|
On filesystems that support extended attributes (xattrs), the s_xattr
|
|
superblock field points to a NULL-terminated array of xattr handlers.
|
|
Extended attributes are name:value pairs.
|
|
|
|
``name``
|
|
Indicates that the handler matches attributes with the specified
|
|
name (such as "system.posix_acl_access"); the prefix field must
|
|
be NULL.
|
|
|
|
``prefix``
|
|
Indicates that the handler matches all attributes with the
|
|
specified name prefix (such as "user."); the name field must be
|
|
NULL.
|
|
|
|
``list``
|
|
Determine if attributes matching this xattr handler should be
|
|
listed for a particular dentry. Used by some listxattr
|
|
implementations like generic_listxattr.
|
|
|
|
``get``
|
|
Called by the VFS to get the value of a particular extended
|
|
attribute. This method is called by the getxattr(2) system
|
|
call.
|
|
|
|
``set``
|
|
Called by the VFS to set the value of a particular extended
|
|
attribute. When the new value is NULL, called to remove a
|
|
particular extended attribute. This method is called by the
|
|
setxattr(2) and removexattr(2) system calls.
|
|
|
|
When none of the xattr handlers of a filesystem match the specified
|
|
attribute name or when a filesystem doesn't support extended attributes,
|
|
the various ``*xattr(2)`` system calls return -EOPNOTSUPP.
|
|
|
|
|
|
The Inode Object
|
|
================
|
|
|
|
An inode object represents an object within the filesystem.
|
|
|
|
|
|
struct inode_operations
|
|
-----------------------
|
|
|
|
This describes how the VFS can manipulate an inode in your filesystem.
|
|
As of kernel 2.6.22, the following members are defined:
|
|
|
|
.. code-block:: c
|
|
|
|
struct inode_operations {
|
|
int (*create) (struct user_namespace *, struct inode *,struct dentry *, umode_t, bool);
|
|
struct dentry * (*lookup) (struct inode *,struct dentry *, unsigned int);
|
|
int (*link) (struct dentry *,struct inode *,struct dentry *);
|
|
int (*unlink) (struct inode *,struct dentry *);
|
|
int (*symlink) (struct user_namespace *, struct inode *,struct dentry *,const char *);
|
|
int (*mkdir) (struct user_namespace *, struct inode *,struct dentry *,umode_t);
|
|
int (*rmdir) (struct inode *,struct dentry *);
|
|
int (*mknod) (struct user_namespace *, struct inode *,struct dentry *,umode_t,dev_t);
|
|
int (*rename) (struct user_namespace *, struct inode *, struct dentry *,
|
|
struct inode *, struct dentry *, unsigned int);
|
|
int (*readlink) (struct dentry *, char __user *,int);
|
|
const char *(*get_link) (struct dentry *, struct inode *,
|
|
struct delayed_call *);
|
|
int (*permission) (struct user_namespace *, struct inode *, int);
|
|
int (*get_acl)(struct inode *, int);
|
|
int (*setattr) (struct user_namespace *, struct dentry *, struct iattr *);
|
|
int (*getattr) (struct user_namespace *, const struct path *, struct kstat *, u32, unsigned int);
|
|
ssize_t (*listxattr) (struct dentry *, char *, size_t);
|
|
void (*update_time)(struct inode *, struct timespec *, int);
|
|
int (*atomic_open)(struct inode *, struct dentry *, struct file *,
|
|
unsigned open_flag, umode_t create_mode);
|
|
int (*tmpfile) (struct user_namespace *, struct inode *, struct dentry *, umode_t);
|
|
int (*set_acl)(struct user_namespace *, struct inode *, struct posix_acl *, int);
|
|
};
|
|
|
|
Again, all methods are called without any locks being held, unless
|
|
otherwise noted.
|
|
|
|
``create``
|
|
called by the open(2) and creat(2) system calls. Only required
|
|
if you want to support regular files. The dentry you get should
|
|
not have an inode (i.e. it should be a negative dentry). Here
|
|
you will probably call d_instantiate() with the dentry and the
|
|
newly created inode
|
|
|
|
``lookup``
|
|
called when the VFS needs to look up an inode in a parent
|
|
directory. The name to look for is found in the dentry. This
|
|
method must call d_add() to insert the found inode into the
|
|
dentry. The "i_count" field in the inode structure should be
|
|
incremented. If the named inode does not exist a NULL inode
|
|
should be inserted into the dentry (this is called a negative
|
|
dentry). Returning an error code from this routine must only be
|
|
done on a real error, otherwise creating inodes with system
|
|
calls like create(2), mknod(2), mkdir(2) and so on will fail.
|
|
If you wish to overload the dentry methods then you should
|
|
initialise the "d_dop" field in the dentry; this is a pointer to
|
|
a struct "dentry_operations". This method is called with the
|
|
directory inode semaphore held
|
|
|
|
``link``
|
|
called by the link(2) system call. Only required if you want to
|
|
support hard links. You will probably need to call
|
|
d_instantiate() just as you would in the create() method
|
|
|
|
``unlink``
|
|
called by the unlink(2) system call. Only required if you want
|
|
to support deleting inodes
|
|
|
|
``symlink``
|
|
called by the symlink(2) system call. Only required if you want
|
|
to support symlinks. You will probably need to call
|
|
d_instantiate() just as you would in the create() method
|
|
|
|
``mkdir``
|
|
called by the mkdir(2) system call. Only required if you want
|
|
to support creating subdirectories. You will probably need to
|
|
call d_instantiate() just as you would in the create() method
|
|
|
|
``rmdir``
|
|
called by the rmdir(2) system call. Only required if you want
|
|
to support deleting subdirectories
|
|
|
|
``mknod``
|
|
called by the mknod(2) system call to create a device (char,
|
|
block) inode or a named pipe (FIFO) or socket. Only required if
|
|
you want to support creating these types of inodes. You will
|
|
probably need to call d_instantiate() just as you would in the
|
|
create() method
|
|
|
|
``rename``
|
|
called by the rename(2) system call to rename the object to have
|
|
the parent and name given by the second inode and dentry.
|
|
|
|
The filesystem must return -EINVAL for any unsupported or
|
|
unknown flags. Currently the following flags are implemented:
|
|
(1) RENAME_NOREPLACE: this flag indicates that if the target of
|
|
the rename exists the rename should fail with -EEXIST instead of
|
|
replacing the target. The VFS already checks for existence, so
|
|
for local filesystems the RENAME_NOREPLACE implementation is
|
|
equivalent to plain rename.
|
|
(2) RENAME_EXCHANGE: exchange source and target. Both must
|
|
exist; this is checked by the VFS. Unlike plain rename, source
|
|
and target may be of different type.
|
|
|
|
``get_link``
|
|
called by the VFS to follow a symbolic link to the inode it
|
|
points to. Only required if you want to support symbolic links.
|
|
This method returns the symlink body to traverse (and possibly
|
|
resets the current position with nd_jump_link()). If the body
|
|
won't go away until the inode is gone, nothing else is needed;
|
|
if it needs to be otherwise pinned, arrange for its release by
|
|
having get_link(..., ..., done) do set_delayed_call(done,
|
|
destructor, argument). In that case destructor(argument) will
|
|
be called once VFS is done with the body you've returned. May
|
|
be called in RCU mode; that is indicated by NULL dentry
|
|
argument. If request can't be handled without leaving RCU mode,
|
|
have it return ERR_PTR(-ECHILD).
|
|
|
|
If the filesystem stores the symlink target in ->i_link, the
|
|
VFS may use it directly without calling ->get_link(); however,
|
|
->get_link() must still be provided. ->i_link must not be
|
|
freed until after an RCU grace period. Writing to ->i_link
|
|
post-iget() time requires a 'release' memory barrier.
|
|
|
|
``readlink``
|
|
this is now just an override for use by readlink(2) for the
|
|
cases when ->get_link uses nd_jump_link() or object is not in
|
|
fact a symlink. Normally filesystems should only implement
|
|
->get_link for symlinks and readlink(2) will automatically use
|
|
that.
|
|
|
|
``permission``
|
|
called by the VFS to check for access rights on a POSIX-like
|
|
filesystem.
|
|
|
|
May be called in rcu-walk mode (mask & MAY_NOT_BLOCK). If in
|
|
rcu-walk mode, the filesystem must check the permission without
|
|
blocking or storing to the inode.
|
|
|
|
If a situation is encountered that rcu-walk cannot handle,
|
|
return
|
|
-ECHILD and it will be called again in ref-walk mode.
|
|
|
|
``setattr``
|
|
called by the VFS to set attributes for a file. This method is
|
|
called by chmod(2) and related system calls.
|
|
|
|
``getattr``
|
|
called by the VFS to get attributes of a file. This method is
|
|
called by stat(2) and related system calls.
|
|
|
|
``listxattr``
|
|
called by the VFS to list all extended attributes for a given
|
|
file. This method is called by the listxattr(2) system call.
|
|
|
|
``update_time``
|
|
called by the VFS to update a specific time or the i_version of
|
|
an inode. If this is not defined the VFS will update the inode
|
|
itself and call mark_inode_dirty_sync.
|
|
|
|
``atomic_open``
|
|
called on the last component of an open. Using this optional
|
|
method the filesystem can look up, possibly create and open the
|
|
file in one atomic operation. If it wants to leave actual
|
|
opening to the caller (e.g. if the file turned out to be a
|
|
symlink, device, or just something filesystem won't do atomic
|
|
open for), it may signal this by returning finish_no_open(file,
|
|
dentry). This method is only called if the last component is
|
|
negative or needs lookup. Cached positive dentries are still
|
|
handled by f_op->open(). If the file was created, FMODE_CREATED
|
|
flag should be set in file->f_mode. In case of O_EXCL the
|
|
method must only succeed if the file didn't exist and hence
|
|
FMODE_CREATED shall always be set on success.
|
|
|
|
``tmpfile``
|
|
called in the end of O_TMPFILE open(). Optional, equivalent to
|
|
atomically creating, opening and unlinking a file in given
|
|
directory.
|
|
|
|
|
|
The Address Space Object
|
|
========================
|
|
|
|
The address space object is used to group and manage pages in the page
|
|
cache. It can be used to keep track of the pages in a file (or anything
|
|
else) and also track the mapping of sections of the file into process
|
|
address spaces.
|
|
|
|
There are a number of distinct yet related services that an
|
|
address-space can provide. These include communicating memory pressure,
|
|
page lookup by address, and keeping track of pages tagged as Dirty or
|
|
Writeback.
|
|
|
|
The first can be used independently to the others. The VM can try to
|
|
either write dirty pages in order to clean them, or release clean pages
|
|
in order to reuse them. To do this it can call the ->writepage method
|
|
on dirty pages, and ->releasepage on clean pages with PagePrivate set.
|
|
Clean pages without PagePrivate and with no external references will be
|
|
released without notice being given to the address_space.
|
|
|
|
To achieve this functionality, pages need to be placed on an LRU with
|
|
lru_cache_add and mark_page_active needs to be called whenever the page
|
|
is used.
|
|
|
|
Pages are normally kept in a radix tree index by ->index. This tree
|
|
maintains information about the PG_Dirty and PG_Writeback status of each
|
|
page, so that pages with either of these flags can be found quickly.
|
|
|
|
The Dirty tag is primarily used by mpage_writepages - the default
|
|
->writepages method. It uses the tag to find dirty pages to call
|
|
->writepage on. If mpage_writepages is not used (i.e. the address
|
|
provides its own ->writepages) , the PAGECACHE_TAG_DIRTY tag is almost
|
|
unused. write_inode_now and sync_inode do use it (through
|
|
__sync_single_inode) to check if ->writepages has been successful in
|
|
writing out the whole address_space.
|
|
|
|
The Writeback tag is used by filemap*wait* and sync_page* functions, via
|
|
filemap_fdatawait_range, to wait for all writeback to complete.
|
|
|
|
An address_space handler may attach extra information to a page,
|
|
typically using the 'private' field in the 'struct page'. If such
|
|
information is attached, the PG_Private flag should be set. This will
|
|
cause various VM routines to make extra calls into the address_space
|
|
handler to deal with that data.
|
|
|
|
An address space acts as an intermediate between storage and
|
|
application. Data is read into the address space a whole page at a
|
|
time, and provided to the application either by copying of the page, or
|
|
by memory-mapping the page. Data is written into the address space by
|
|
the application, and then written-back to storage typically in whole
|
|
pages, however the address_space has finer control of write sizes.
|
|
|
|
The read process essentially only requires 'readpage'. The write
|
|
process is more complicated and uses write_begin/write_end or
|
|
set_page_dirty to write data into the address_space, and writepage and
|
|
writepages to writeback data to storage.
|
|
|
|
Adding and removing pages to/from an address_space is protected by the
|
|
inode's i_mutex.
|
|
|
|
When data is written to a page, the PG_Dirty flag should be set. It
|
|
typically remains set until writepage asks for it to be written. This
|
|
should clear PG_Dirty and set PG_Writeback. It can be actually written
|
|
at any point after PG_Dirty is clear. Once it is known to be safe,
|
|
PG_Writeback is cleared.
|
|
|
|
Writeback makes use of a writeback_control structure to direct the
|
|
operations. This gives the writepage and writepages operations some
|
|
information about the nature of and reason for the writeback request,
|
|
and the constraints under which it is being done. It is also used to
|
|
return information back to the caller about the result of a writepage or
|
|
writepages request.
|
|
|
|
|
|
Handling errors during writeback
|
|
--------------------------------
|
|
|
|
Most applications that do buffered I/O will periodically call a file
|
|
synchronization call (fsync, fdatasync, msync or sync_file_range) to
|
|
ensure that data written has made it to the backing store. When there
|
|
is an error during writeback, they expect that error to be reported when
|
|
a file sync request is made. After an error has been reported on one
|
|
request, subsequent requests on the same file descriptor should return
|
|
0, unless further writeback errors have occurred since the previous file
|
|
syncronization.
|
|
|
|
Ideally, the kernel would report errors only on file descriptions on
|
|
which writes were done that subsequently failed to be written back. The
|
|
generic pagecache infrastructure does not track the file descriptions
|
|
that have dirtied each individual page however, so determining which
|
|
file descriptors should get back an error is not possible.
|
|
|
|
Instead, the generic writeback error tracking infrastructure in the
|
|
kernel settles for reporting errors to fsync on all file descriptions
|
|
that were open at the time that the error occurred. In a situation with
|
|
multiple writers, all of them will get back an error on a subsequent
|
|
fsync, even if all of the writes done through that particular file
|
|
descriptor succeeded (or even if there were no writes on that file
|
|
descriptor at all).
|
|
|
|
Filesystems that wish to use this infrastructure should call
|
|
mapping_set_error to record the error in the address_space when it
|
|
occurs. Then, after writing back data from the pagecache in their
|
|
file->fsync operation, they should call file_check_and_advance_wb_err to
|
|
ensure that the struct file's error cursor has advanced to the correct
|
|
point in the stream of errors emitted by the backing device(s).
|
|
|
|
|
|
struct address_space_operations
|
|
-------------------------------
|
|
|
|
This describes how the VFS can manipulate mapping of a file to page
|
|
cache in your filesystem. The following members are defined:
|
|
|
|
.. code-block:: c
|
|
|
|
struct address_space_operations {
|
|
int (*writepage)(struct page *page, struct writeback_control *wbc);
|
|
int (*readpage)(struct file *, struct page *);
|
|
int (*writepages)(struct address_space *, struct writeback_control *);
|
|
int (*set_page_dirty)(struct page *page);
|
|
void (*readahead)(struct readahead_control *);
|
|
int (*readpages)(struct file *filp, struct address_space *mapping,
|
|
struct list_head *pages, unsigned nr_pages);
|
|
int (*write_begin)(struct file *, struct address_space *mapping,
|
|
loff_t pos, unsigned len, unsigned flags,
|
|
struct page **pagep, void **fsdata);
|
|
int (*write_end)(struct file *, struct address_space *mapping,
|
|
loff_t pos, unsigned len, unsigned copied,
|
|
struct page *page, void *fsdata);
|
|
sector_t (*bmap)(struct address_space *, sector_t);
|
|
void (*invalidatepage) (struct page *, unsigned int, unsigned int);
|
|
int (*releasepage) (struct page *, int);
|
|
void (*freepage)(struct page *);
|
|
ssize_t (*direct_IO)(struct kiocb *, struct iov_iter *iter);
|
|
/* isolate a page for migration */
|
|
bool (*isolate_page) (struct page *, isolate_mode_t);
|
|
/* migrate the contents of a page to the specified target */
|
|
int (*migratepage) (struct page *, struct page *);
|
|
/* put migration-failed page back to right list */
|
|
void (*putback_page) (struct page *);
|
|
int (*launder_page) (struct page *);
|
|
|
|
int (*is_partially_uptodate) (struct page *, unsigned long,
|
|
unsigned long);
|
|
void (*is_dirty_writeback) (struct page *, bool *, bool *);
|
|
int (*error_remove_page) (struct mapping *mapping, struct page *page);
|
|
int (*swap_activate)(struct file *);
|
|
int (*swap_deactivate)(struct file *);
|
|
};
|
|
|
|
``writepage``
|
|
called by the VM to write a dirty page to backing store. This
|
|
may happen for data integrity reasons (i.e. 'sync'), or to free
|
|
up memory (flush). The difference can be seen in
|
|
wbc->sync_mode. The PG_Dirty flag has been cleared and
|
|
PageLocked is true. writepage should start writeout, should set
|
|
PG_Writeback, and should make sure the page is unlocked, either
|
|
synchronously or asynchronously when the write operation
|
|
completes.
|
|
|
|
If wbc->sync_mode is WB_SYNC_NONE, ->writepage doesn't have to
|
|
try too hard if there are problems, and may choose to write out
|
|
other pages from the mapping if that is easier (e.g. due to
|
|
internal dependencies). If it chooses not to start writeout, it
|
|
should return AOP_WRITEPAGE_ACTIVATE so that the VM will not
|
|
keep calling ->writepage on that page.
|
|
|
|
See the file "Locking" for more details.
|
|
|
|
``readpage``
|
|
called by the VM to read a page from backing store. The page
|
|
will be Locked when readpage is called, and should be unlocked
|
|
and marked uptodate once the read completes. If ->readpage
|
|
discovers that it needs to unlock the page for some reason, it
|
|
can do so, and then return AOP_TRUNCATED_PAGE. In this case,
|
|
the page will be relocated, relocked and if that all succeeds,
|
|
->readpage will be called again.
|
|
|
|
``writepages``
|
|
called by the VM to write out pages associated with the
|
|
address_space object. If wbc->sync_mode is WB_SYNC_ALL, then
|
|
the writeback_control will specify a range of pages that must be
|
|
written out. If it is WB_SYNC_NONE, then a nr_to_write is
|
|
given and that many pages should be written if possible. If no
|
|
->writepages is given, then mpage_writepages is used instead.
|
|
This will choose pages from the address space that are tagged as
|
|
DIRTY and will pass them to ->writepage.
|
|
|
|
``set_page_dirty``
|
|
called by the VM to set a page dirty. This is particularly
|
|
needed if an address space attaches private data to a page, and
|
|
that data needs to be updated when a page is dirtied. This is
|
|
called, for example, when a memory mapped page gets modified.
|
|
If defined, it should set the PageDirty flag, and the
|
|
PAGECACHE_TAG_DIRTY tag in the radix tree.
|
|
|
|
``readahead``
|
|
Called by the VM to read pages associated with the address_space
|
|
object. The pages are consecutive in the page cache and are
|
|
locked. The implementation should decrement the page refcount
|
|
after starting I/O on each page. Usually the page will be
|
|
unlocked by the I/O completion handler. If the filesystem decides
|
|
to stop attempting I/O before reaching the end of the readahead
|
|
window, it can simply return. The caller will decrement the page
|
|
refcount and unlock the remaining pages for you. Set PageUptodate
|
|
if the I/O completes successfully. Setting PageError on any page
|
|
will be ignored; simply unlock the page if an I/O error occurs.
|
|
|
|
``readpages``
|
|
called by the VM to read pages associated with the address_space
|
|
object. This is essentially just a vector version of readpage.
|
|
Instead of just one page, several pages are requested.
|
|
readpages is only used for read-ahead, so read errors are
|
|
ignored. If anything goes wrong, feel free to give up.
|
|
This interface is deprecated and will be removed by the end of
|
|
2020; implement readahead instead.
|
|
|
|
``write_begin``
|
|
Called by the generic buffered write code to ask the filesystem
|
|
to prepare to write len bytes at the given offset in the file.
|
|
The address_space should check that the write will be able to
|
|
complete, by allocating space if necessary and doing any other
|
|
internal housekeeping. If the write will update parts of any
|
|
basic-blocks on storage, then those blocks should be pre-read
|
|
(if they haven't been read already) so that the updated blocks
|
|
can be written out properly.
|
|
|
|
The filesystem must return the locked pagecache page for the
|
|
specified offset, in ``*pagep``, for the caller to write into.
|
|
|
|
It must be able to cope with short writes (where the length
|
|
passed to write_begin is greater than the number of bytes copied
|
|
into the page).
|
|
|
|
flags is a field for AOP_FLAG_xxx flags, described in
|
|
include/linux/fs.h.
|
|
|
|
A void * may be returned in fsdata, which then gets passed into
|
|
write_end.
|
|
|
|
Returns 0 on success; < 0 on failure (which is the error code),
|
|
in which case write_end is not called.
|
|
|
|
``write_end``
|
|
After a successful write_begin, and data copy, write_end must be
|
|
called. len is the original len passed to write_begin, and
|
|
copied is the amount that was able to be copied.
|
|
|
|
The filesystem must take care of unlocking the page and
|
|
releasing it refcount, and updating i_size.
|
|
|
|
Returns < 0 on failure, otherwise the number of bytes (<=
|
|
'copied') that were able to be copied into pagecache.
|
|
|
|
``bmap``
|
|
called by the VFS to map a logical block offset within object to
|
|
physical block number. This method is used by the FIBMAP ioctl
|
|
and for working with swap-files. To be able to swap to a file,
|
|
the file must have a stable mapping to a block device. The swap
|
|
system does not go through the filesystem but instead uses bmap
|
|
to find out where the blocks in the file are and uses those
|
|
addresses directly.
|
|
|
|
``invalidatepage``
|
|
If a page has PagePrivate set, then invalidatepage will be
|
|
called when part or all of the page is to be removed from the
|
|
address space. This generally corresponds to either a
|
|
truncation, punch hole or a complete invalidation of the address
|
|
space (in the latter case 'offset' will always be 0 and 'length'
|
|
will be PAGE_SIZE). Any private data associated with the page
|
|
should be updated to reflect this truncation. If offset is 0
|
|
and length is PAGE_SIZE, then the private data should be
|
|
released, because the page must be able to be completely
|
|
discarded. This may be done by calling the ->releasepage
|
|
function, but in this case the release MUST succeed.
|
|
|
|
``releasepage``
|
|
releasepage is called on PagePrivate pages to indicate that the
|
|
page should be freed if possible. ->releasepage should remove
|
|
any private data from the page and clear the PagePrivate flag.
|
|
If releasepage() fails for some reason, it must indicate failure
|
|
with a 0 return value. releasepage() is used in two distinct
|
|
though related cases. The first is when the VM finds a clean
|
|
page with no active users and wants to make it a free page. If
|
|
->releasepage succeeds, the page will be removed from the
|
|
address_space and become free.
|
|
|
|
The second case is when a request has been made to invalidate
|
|
some or all pages in an address_space. This can happen through
|
|
the fadvise(POSIX_FADV_DONTNEED) system call or by the
|
|
filesystem explicitly requesting it as nfs and 9fs do (when they
|
|
believe the cache may be out of date with storage) by calling
|
|
invalidate_inode_pages2(). If the filesystem makes such a call,
|
|
and needs to be certain that all pages are invalidated, then its
|
|
releasepage will need to ensure this. Possibly it can clear the
|
|
PageUptodate bit if it cannot free private data yet.
|
|
|
|
``freepage``
|
|
freepage is called once the page is no longer visible in the
|
|
page cache in order to allow the cleanup of any private data.
|
|
Since it may be called by the memory reclaimer, it should not
|
|
assume that the original address_space mapping still exists, and
|
|
it should not block.
|
|
|
|
``direct_IO``
|
|
called by the generic read/write routines to perform direct_IO -
|
|
that is IO requests which bypass the page cache and transfer
|
|
data directly between the storage and the application's address
|
|
space.
|
|
|
|
``isolate_page``
|
|
Called by the VM when isolating a movable non-lru page. If page
|
|
is successfully isolated, VM marks the page as PG_isolated via
|
|
__SetPageIsolated.
|
|
|
|
``migrate_page``
|
|
This is used to compact the physical memory usage. If the VM
|
|
wants to relocate a page (maybe off a memory card that is
|
|
signalling imminent failure) it will pass a new page and an old
|
|
page to this function. migrate_page should transfer any private
|
|
data across and update any references that it has to the page.
|
|
|
|
``putback_page``
|
|
Called by the VM when isolated page's migration fails.
|
|
|
|
``launder_page``
|
|
Called before freeing a page - it writes back the dirty page.
|
|
To prevent redirtying the page, it is kept locked during the
|
|
whole operation.
|
|
|
|
``is_partially_uptodate``
|
|
Called by the VM when reading a file through the pagecache when
|
|
the underlying blocksize != pagesize. If the required block is
|
|
up to date then the read can complete without needing the IO to
|
|
bring the whole page up to date.
|
|
|
|
``is_dirty_writeback``
|
|
Called by the VM when attempting to reclaim a page. The VM uses
|
|
dirty and writeback information to determine if it needs to
|
|
stall to allow flushers a chance to complete some IO.
|
|
Ordinarily it can use PageDirty and PageWriteback but some
|
|
filesystems have more complex state (unstable pages in NFS
|
|
prevent reclaim) or do not set those flags due to locking
|
|
problems. This callback allows a filesystem to indicate to the
|
|
VM if a page should be treated as dirty or writeback for the
|
|
purposes of stalling.
|
|
|
|
``error_remove_page``
|
|
normally set to generic_error_remove_page if truncation is ok
|
|
for this address space. Used for memory failure handling.
|
|
Setting this implies you deal with pages going away under you,
|
|
unless you have them locked or reference counts increased.
|
|
|
|
``swap_activate``
|
|
Called when swapon is used on a file to allocate space if
|
|
necessary and pin the block lookup information in memory. A
|
|
return value of zero indicates success, in which case this file
|
|
can be used to back swapspace.
|
|
|
|
``swap_deactivate``
|
|
Called during swapoff on files where swap_activate was
|
|
successful.
|
|
|
|
|
|
The File Object
|
|
===============
|
|
|
|
A file object represents a file opened by a process. This is also known
|
|
as an "open file description" in POSIX parlance.
|
|
|
|
|
|
struct file_operations
|
|
----------------------
|
|
|
|
This describes how the VFS can manipulate an open file. As of kernel
|
|
4.18, the following members are defined:
|
|
|
|
.. code-block:: c
|
|
|
|
struct file_operations {
|
|
struct module *owner;
|
|
loff_t (*llseek) (struct file *, loff_t, int);
|
|
ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);
|
|
ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);
|
|
ssize_t (*read_iter) (struct kiocb *, struct iov_iter *);
|
|
ssize_t (*write_iter) (struct kiocb *, struct iov_iter *);
|
|
int (*iopoll)(struct kiocb *kiocb, bool spin);
|
|
int (*iterate) (struct file *, struct dir_context *);
|
|
int (*iterate_shared) (struct file *, struct dir_context *);
|
|
__poll_t (*poll) (struct file *, struct poll_table_struct *);
|
|
long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);
|
|
long (*compat_ioctl) (struct file *, unsigned int, unsigned long);
|
|
int (*mmap) (struct file *, struct vm_area_struct *);
|
|
int (*open) (struct inode *, struct file *);
|
|
int (*flush) (struct file *, fl_owner_t id);
|
|
int (*release) (struct inode *, struct file *);
|
|
int (*fsync) (struct file *, loff_t, loff_t, int datasync);
|
|
int (*fasync) (int, struct file *, int);
|
|
int (*lock) (struct file *, int, struct file_lock *);
|
|
ssize_t (*sendpage) (struct file *, struct page *, int, size_t, loff_t *, int);
|
|
unsigned long (*get_unmapped_area)(struct file *, unsigned long, unsigned long, unsigned long, unsigned long);
|
|
int (*check_flags)(int);
|
|
int (*flock) (struct file *, int, struct file_lock *);
|
|
ssize_t (*splice_write)(struct pipe_inode_info *, struct file *, loff_t *, size_t, unsigned int);
|
|
ssize_t (*splice_read)(struct file *, loff_t *, struct pipe_inode_info *, size_t, unsigned int);
|
|
int (*setlease)(struct file *, long, struct file_lock **, void **);
|
|
long (*fallocate)(struct file *file, int mode, loff_t offset,
|
|
loff_t len);
|
|
void (*show_fdinfo)(struct seq_file *m, struct file *f);
|
|
#ifndef CONFIG_MMU
|
|
unsigned (*mmap_capabilities)(struct file *);
|
|
#endif
|
|
ssize_t (*copy_file_range)(struct file *, loff_t, struct file *, loff_t, size_t, unsigned int);
|
|
loff_t (*remap_file_range)(struct file *file_in, loff_t pos_in,
|
|
struct file *file_out, loff_t pos_out,
|
|
loff_t len, unsigned int remap_flags);
|
|
int (*fadvise)(struct file *, loff_t, loff_t, int);
|
|
};
|
|
|
|
Again, all methods are called without any locks being held, unless
|
|
otherwise noted.
|
|
|
|
``llseek``
|
|
called when the VFS needs to move the file position index
|
|
|
|
``read``
|
|
called by read(2) and related system calls
|
|
|
|
``read_iter``
|
|
possibly asynchronous read with iov_iter as destination
|
|
|
|
``write``
|
|
called by write(2) and related system calls
|
|
|
|
``write_iter``
|
|
possibly asynchronous write with iov_iter as source
|
|
|
|
``iopoll``
|
|
called when aio wants to poll for completions on HIPRI iocbs
|
|
|
|
``iterate``
|
|
called when the VFS needs to read the directory contents
|
|
|
|
``iterate_shared``
|
|
called when the VFS needs to read the directory contents when
|
|
filesystem supports concurrent dir iterators
|
|
|
|
``poll``
|
|
called by the VFS when a process wants to check if there is
|
|
activity on this file and (optionally) go to sleep until there
|
|
is activity. Called by the select(2) and poll(2) system calls
|
|
|
|
``unlocked_ioctl``
|
|
called by the ioctl(2) system call.
|
|
|
|
``compat_ioctl``
|
|
called by the ioctl(2) system call when 32 bit system calls are
|
|
used on 64 bit kernels.
|
|
|
|
``mmap``
|
|
called by the mmap(2) system call
|
|
|
|
``open``
|
|
called by the VFS when an inode should be opened. When the VFS
|
|
opens a file, it creates a new "struct file". It then calls the
|
|
open method for the newly allocated file structure. You might
|
|
think that the open method really belongs in "struct
|
|
inode_operations", and you may be right. I think it's done the
|
|
way it is because it makes filesystems simpler to implement.
|
|
The open() method is a good place to initialize the
|
|
"private_data" member in the file structure if you want to point
|
|
to a device structure
|
|
|
|
``flush``
|
|
called by the close(2) system call to flush a file
|
|
|
|
``release``
|
|
called when the last reference to an open file is closed
|
|
|
|
``fsync``
|
|
called by the fsync(2) system call. Also see the section above
|
|
entitled "Handling errors during writeback".
|
|
|
|
``fasync``
|
|
called by the fcntl(2) system call when asynchronous
|
|
(non-blocking) mode is enabled for a file
|
|
|
|
``lock``
|
|
called by the fcntl(2) system call for F_GETLK, F_SETLK, and
|
|
F_SETLKW commands
|
|
|
|
``get_unmapped_area``
|
|
called by the mmap(2) system call
|
|
|
|
``check_flags``
|
|
called by the fcntl(2) system call for F_SETFL command
|
|
|
|
``flock``
|
|
called by the flock(2) system call
|
|
|
|
``splice_write``
|
|
called by the VFS to splice data from a pipe to a file. This
|
|
method is used by the splice(2) system call
|
|
|
|
``splice_read``
|
|
called by the VFS to splice data from file to a pipe. This
|
|
method is used by the splice(2) system call
|
|
|
|
``setlease``
|
|
called by the VFS to set or release a file lock lease. setlease
|
|
implementations should call generic_setlease to record or remove
|
|
the lease in the inode after setting it.
|
|
|
|
``fallocate``
|
|
called by the VFS to preallocate blocks or punch a hole.
|
|
|
|
``copy_file_range``
|
|
called by the copy_file_range(2) system call.
|
|
|
|
``remap_file_range``
|
|
called by the ioctl(2) system call for FICLONERANGE and FICLONE
|
|
and FIDEDUPERANGE commands to remap file ranges. An
|
|
implementation should remap len bytes at pos_in of the source
|
|
file into the dest file at pos_out. Implementations must handle
|
|
callers passing in len == 0; this means "remap to the end of the
|
|
source file". The return value should the number of bytes
|
|
remapped, or the usual negative error code if errors occurred
|
|
before any bytes were remapped. The remap_flags parameter
|
|
accepts REMAP_FILE_* flags. If REMAP_FILE_DEDUP is set then the
|
|
implementation must only remap if the requested file ranges have
|
|
identical contents. If REMAP_FILE_CAN_SHORTEN is set, the caller is
|
|
ok with the implementation shortening the request length to
|
|
satisfy alignment or EOF requirements (or any other reason).
|
|
|
|
``fadvise``
|
|
possibly called by the fadvise64() system call.
|
|
|
|
Note that the file operations are implemented by the specific
|
|
filesystem in which the inode resides. When opening a device node
|
|
(character or block special) most filesystems will call special
|
|
support routines in the VFS which will locate the required device
|
|
driver information. These support routines replace the filesystem file
|
|
operations with those for the device driver, and then proceed to call
|
|
the new open() method for the file. This is how opening a device file
|
|
in the filesystem eventually ends up calling the device driver open()
|
|
method.
|
|
|
|
|
|
Directory Entry Cache (dcache)
|
|
==============================
|
|
|
|
|
|
struct dentry_operations
|
|
------------------------
|
|
|
|
This describes how a filesystem can overload the standard dentry
|
|
operations. Dentries and the dcache are the domain of the VFS and the
|
|
individual filesystem implementations. Device drivers have no business
|
|
here. These methods may be set to NULL, as they are either optional or
|
|
the VFS uses a default. As of kernel 2.6.22, the following members are
|
|
defined:
|
|
|
|
.. code-block:: c
|
|
|
|
struct dentry_operations {
|
|
int (*d_revalidate)(struct dentry *, unsigned int);
|
|
int (*d_weak_revalidate)(struct dentry *, unsigned int);
|
|
int (*d_hash)(const struct dentry *, struct qstr *);
|
|
int (*d_compare)(const struct dentry *,
|
|
unsigned int, const char *, const struct qstr *);
|
|
int (*d_delete)(const struct dentry *);
|
|
int (*d_init)(struct dentry *);
|
|
void (*d_release)(struct dentry *);
|
|
void (*d_iput)(struct dentry *, struct inode *);
|
|
char *(*d_dname)(struct dentry *, char *, int);
|
|
struct vfsmount *(*d_automount)(struct path *);
|
|
int (*d_manage)(const struct path *, bool);
|
|
struct dentry *(*d_real)(struct dentry *, const struct inode *);
|
|
};
|
|
|
|
``d_revalidate``
|
|
called when the VFS needs to revalidate a dentry. This is
|
|
called whenever a name look-up finds a dentry in the dcache.
|
|
Most local filesystems leave this as NULL, because all their
|
|
dentries in the dcache are valid. Network filesystems are
|
|
different since things can change on the server without the
|
|
client necessarily being aware of it.
|
|
|
|
This function should return a positive value if the dentry is
|
|
still valid, and zero or a negative error code if it isn't.
|
|
|
|
d_revalidate may be called in rcu-walk mode (flags &
|
|
LOOKUP_RCU). If in rcu-walk mode, the filesystem must
|
|
revalidate the dentry without blocking or storing to the dentry,
|
|
d_parent and d_inode should not be used without care (because
|
|
they can change and, in d_inode case, even become NULL under
|
|
us).
|
|
|
|
If a situation is encountered that rcu-walk cannot handle,
|
|
return
|
|
-ECHILD and it will be called again in ref-walk mode.
|
|
|
|
``_weak_revalidate``
|
|
called when the VFS needs to revalidate a "jumped" dentry. This
|
|
is called when a path-walk ends at dentry that was not acquired
|
|
by doing a lookup in the parent directory. This includes "/",
|
|
"." and "..", as well as procfs-style symlinks and mountpoint
|
|
traversal.
|
|
|
|
In this case, we are less concerned with whether the dentry is
|
|
still fully correct, but rather that the inode is still valid.
|
|
As with d_revalidate, most local filesystems will set this to
|
|
NULL since their dcache entries are always valid.
|
|
|
|
This function has the same return code semantics as
|
|
d_revalidate.
|
|
|
|
d_weak_revalidate is only called after leaving rcu-walk mode.
|
|
|
|
``d_hash``
|
|
called when the VFS adds a dentry to the hash table. The first
|
|
dentry passed to d_hash is the parent directory that the name is
|
|
to be hashed into.
|
|
|
|
Same locking and synchronisation rules as d_compare regarding
|
|
what is safe to dereference etc.
|
|
|
|
``d_compare``
|
|
called to compare a dentry name with a given name. The first
|
|
dentry is the parent of the dentry to be compared, the second is
|
|
the child dentry. len and name string are properties of the
|
|
dentry to be compared. qstr is the name to compare it with.
|
|
|
|
Must be constant and idempotent, and should not take locks if
|
|
possible, and should not or store into the dentry. Should not
|
|
dereference pointers outside the dentry without lots of care
|
|
(eg. d_parent, d_inode, d_name should not be used).
|
|
|
|
However, our vfsmount is pinned, and RCU held, so the dentries
|
|
and inodes won't disappear, neither will our sb or filesystem
|
|
module. ->d_sb may be used.
|
|
|
|
It is a tricky calling convention because it needs to be called
|
|
under "rcu-walk", ie. without any locks or references on things.
|
|
|
|
``d_delete``
|
|
called when the last reference to a dentry is dropped and the
|
|
dcache is deciding whether or not to cache it. Return 1 to
|
|
delete immediately, or 0 to cache the dentry. Default is NULL
|
|
which means to always cache a reachable dentry. d_delete must
|
|
be constant and idempotent.
|
|
|
|
``d_init``
|
|
called when a dentry is allocated
|
|
|
|
``d_release``
|
|
called when a dentry is really deallocated
|
|
|
|
``d_iput``
|
|
called when a dentry loses its inode (just prior to its being
|
|
deallocated). The default when this is NULL is that the VFS
|
|
calls iput(). If you define this method, you must call iput()
|
|
yourself
|
|
|
|
``d_dname``
|
|
called when the pathname of a dentry should be generated.
|
|
Useful for some pseudo filesystems (sockfs, pipefs, ...) to
|
|
delay pathname generation. (Instead of doing it when dentry is
|
|
created, it's done only when the path is needed.). Real
|
|
filesystems probably dont want to use it, because their dentries
|
|
are present in global dcache hash, so their hash should be an
|
|
invariant. As no lock is held, d_dname() should not try to
|
|
modify the dentry itself, unless appropriate SMP safety is used.
|
|
CAUTION : d_path() logic is quite tricky. The correct way to
|
|
return for example "Hello" is to put it at the end of the
|
|
buffer, and returns a pointer to the first char.
|
|
dynamic_dname() helper function is provided to take care of
|
|
this.
|
|
|
|
Example :
|
|
|
|
.. code-block:: c
|
|
|
|
static char *pipefs_dname(struct dentry *dent, char *buffer, int buflen)
|
|
{
|
|
return dynamic_dname(dentry, buffer, buflen, "pipe:[%lu]",
|
|
dentry->d_inode->i_ino);
|
|
}
|
|
|
|
``d_automount``
|
|
called when an automount dentry is to be traversed (optional).
|
|
This should create a new VFS mount record and return the record
|
|
to the caller. The caller is supplied with a path parameter
|
|
giving the automount directory to describe the automount target
|
|
and the parent VFS mount record to provide inheritable mount
|
|
parameters. NULL should be returned if someone else managed to
|
|
make the automount first. If the vfsmount creation failed, then
|
|
an error code should be returned. If -EISDIR is returned, then
|
|
the directory will be treated as an ordinary directory and
|
|
returned to pathwalk to continue walking.
|
|
|
|
If a vfsmount is returned, the caller will attempt to mount it
|
|
on the mountpoint and will remove the vfsmount from its
|
|
expiration list in the case of failure. The vfsmount should be
|
|
returned with 2 refs on it to prevent automatic expiration - the
|
|
caller will clean up the additional ref.
|
|
|
|
This function is only used if DCACHE_NEED_AUTOMOUNT is set on
|
|
the dentry. This is set by __d_instantiate() if S_AUTOMOUNT is
|
|
set on the inode being added.
|
|
|
|
``d_manage``
|
|
called to allow the filesystem to manage the transition from a
|
|
dentry (optional). This allows autofs, for example, to hold up
|
|
clients waiting to explore behind a 'mountpoint' while letting
|
|
the daemon go past and construct the subtree there. 0 should be
|
|
returned to let the calling process continue. -EISDIR can be
|
|
returned to tell pathwalk to use this directory as an ordinary
|
|
directory and to ignore anything mounted on it and not to check
|
|
the automount flag. Any other error code will abort pathwalk
|
|
completely.
|
|
|
|
If the 'rcu_walk' parameter is true, then the caller is doing a
|
|
pathwalk in RCU-walk mode. Sleeping is not permitted in this
|
|
mode, and the caller can be asked to leave it and call again by
|
|
returning -ECHILD. -EISDIR may also be returned to tell
|
|
pathwalk to ignore d_automount or any mounts.
|
|
|
|
This function is only used if DCACHE_MANAGE_TRANSIT is set on
|
|
the dentry being transited from.
|
|
|
|
``d_real``
|
|
overlay/union type filesystems implement this method to return
|
|
one of the underlying dentries hidden by the overlay. It is
|
|
used in two different modes:
|
|
|
|
Called from file_dentry() it returns the real dentry matching
|
|
the inode argument. The real dentry may be from a lower layer
|
|
already copied up, but still referenced from the file. This
|
|
mode is selected with a non-NULL inode argument.
|
|
|
|
With NULL inode the topmost real underlying dentry is returned.
|
|
|
|
Each dentry has a pointer to its parent dentry, as well as a hash list
|
|
of child dentries. Child dentries are basically like files in a
|
|
directory.
|
|
|
|
|
|
Directory Entry Cache API
|
|
--------------------------
|
|
|
|
There are a number of functions defined which permit a filesystem to
|
|
manipulate dentries:
|
|
|
|
``dget``
|
|
open a new handle for an existing dentry (this just increments
|
|
the usage count)
|
|
|
|
``dput``
|
|
close a handle for a dentry (decrements the usage count). If
|
|
the usage count drops to 0, and the dentry is still in its
|
|
parent's hash, the "d_delete" method is called to check whether
|
|
it should be cached. If it should not be cached, or if the
|
|
dentry is not hashed, it is deleted. Otherwise cached dentries
|
|
are put into an LRU list to be reclaimed on memory shortage.
|
|
|
|
``d_drop``
|
|
this unhashes a dentry from its parents hash list. A subsequent
|
|
call to dput() will deallocate the dentry if its usage count
|
|
drops to 0
|
|
|
|
``d_delete``
|
|
delete a dentry. If there are no other open references to the
|
|
dentry then the dentry is turned into a negative dentry (the
|
|
d_iput() method is called). If there are other references, then
|
|
d_drop() is called instead
|
|
|
|
``d_add``
|
|
add a dentry to its parents hash list and then calls
|
|
d_instantiate()
|
|
|
|
``d_instantiate``
|
|
add a dentry to the alias hash list for the inode and updates
|
|
the "d_inode" member. The "i_count" member in the inode
|
|
structure should be set/incremented. If the inode pointer is
|
|
NULL, the dentry is called a "negative dentry". This function
|
|
is commonly called when an inode is created for an existing
|
|
negative dentry
|
|
|
|
``d_lookup``
|
|
look up a dentry given its parent and path name component It
|
|
looks up the child of that given name from the dcache hash
|
|
table. If it is found, the reference count is incremented and
|
|
the dentry is returned. The caller must use dput() to free the
|
|
dentry when it finishes using it.
|
|
|
|
|
|
Mount Options
|
|
=============
|
|
|
|
|
|
Parsing options
|
|
---------------
|
|
|
|
On mount and remount the filesystem is passed a string containing a
|
|
comma separated list of mount options. The options can have either of
|
|
these forms:
|
|
|
|
option
|
|
option=value
|
|
|
|
The <linux/parser.h> header defines an API that helps parse these
|
|
options. There are plenty of examples on how to use it in existing
|
|
filesystems.
|
|
|
|
|
|
Showing options
|
|
---------------
|
|
|
|
If a filesystem accepts mount options, it must define show_options() to
|
|
show all the currently active options. The rules are:
|
|
|
|
- options MUST be shown which are not default or their values differ
|
|
from the default
|
|
|
|
- options MAY be shown which are enabled by default or have their
|
|
default value
|
|
|
|
Options used only internally between a mount helper and the kernel (such
|
|
as file descriptors), or which only have an effect during the mounting
|
|
(such as ones controlling the creation of a journal) are exempt from the
|
|
above rules.
|
|
|
|
The underlying reason for the above rules is to make sure, that a mount
|
|
can be accurately replicated (e.g. umounting and mounting again) based
|
|
on the information found in /proc/mounts.
|
|
|
|
|
|
Resources
|
|
=========
|
|
|
|
(Note some of these resources are not up-to-date with the latest kernel
|
|
version.)
|
|
|
|
Creating Linux virtual filesystems. 2002
|
|
<https://lwn.net/Articles/13325/>
|
|
|
|
The Linux Virtual File-system Layer by Neil Brown. 1999
|
|
<http://www.cse.unsw.edu.au/~neilb/oss/linux-commentary/vfs.html>
|
|
|
|
A tour of the Linux VFS by Michael K. Johnson. 1996
|
|
<https://www.tldp.org/LDP/khg/HyperNews/get/fs/vfstour.html>
|
|
|
|
A small trail through the Linux kernel by Andries Brouwer. 2001
|
|
<https://www.win.tue.nl/~aeb/linux/vfs/trail.html>
|