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![Dave Hansen](/assets/img/avatar_default.png)
Why do we need r/o bind mounts? This feature allows a read-only view into a read-write filesystem. In the process of doing that, it also provides infrastructure for keeping track of the number of writers to any given mount. This has a number of uses. It allows chroots to have parts of filesystems writable. It will be useful for containers in the future because users may have root inside a container, but should not be allowed to write to somefilesystems. This also replaces patches that vserver has had out of the tree for several years. It allows security enhancement by making sure that parts of your filesystem read-only (such as when you don't trust your FTP server), when you don't want to have entire new filesystems mounted, or when you want atime selectively updated. I've been using the following script to test that the feature is working as desired. It takes a directory and makes a regular bind and a r/o bind mount of it. It then performs some normal filesystem operations on the three directories, including ones that are expected to fail, like creating a file on the r/o mount. This patch: Some filesystems forego the vfs and may_open() and create their own 'struct file's. This patch creates a couple of helper functions which can be used by these filesystems, and will provide a unified place which the r/o bind mount code may patch. Also, rename an existing, static-scope init_file() to a less generic name. Signed-off-by: Dave Hansen <haveblue@us.ibm.com> Cc: Christoph Hellwig <hch@lst.de> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
362 lines
8.6 KiB
C
362 lines
8.6 KiB
C
/*
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* linux/fs/file_table.c
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*
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* Copyright (C) 1991, 1992 Linus Torvalds
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* Copyright (C) 1997 David S. Miller (davem@caip.rutgers.edu)
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*/
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#include <linux/string.h>
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#include <linux/slab.h>
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#include <linux/file.h>
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#include <linux/init.h>
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#include <linux/module.h>
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#include <linux/fs.h>
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#include <linux/security.h>
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#include <linux/eventpoll.h>
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#include <linux/rcupdate.h>
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#include <linux/mount.h>
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#include <linux/capability.h>
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#include <linux/cdev.h>
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#include <linux/fsnotify.h>
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#include <linux/sysctl.h>
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#include <linux/percpu_counter.h>
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#include <asm/atomic.h>
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/* sysctl tunables... */
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struct files_stat_struct files_stat = {
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.max_files = NR_FILE
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};
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/* public. Not pretty! */
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__cacheline_aligned_in_smp DEFINE_SPINLOCK(files_lock);
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static struct percpu_counter nr_files __cacheline_aligned_in_smp;
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static inline void file_free_rcu(struct rcu_head *head)
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{
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struct file *f = container_of(head, struct file, f_u.fu_rcuhead);
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kmem_cache_free(filp_cachep, f);
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}
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static inline void file_free(struct file *f)
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{
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percpu_counter_dec(&nr_files);
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call_rcu(&f->f_u.fu_rcuhead, file_free_rcu);
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}
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/*
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* Return the total number of open files in the system
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*/
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static int get_nr_files(void)
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{
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return percpu_counter_read_positive(&nr_files);
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}
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/*
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* Return the maximum number of open files in the system
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*/
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int get_max_files(void)
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{
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return files_stat.max_files;
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}
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EXPORT_SYMBOL_GPL(get_max_files);
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/*
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* Handle nr_files sysctl
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*/
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#if defined(CONFIG_SYSCTL) && defined(CONFIG_PROC_FS)
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int proc_nr_files(ctl_table *table, int write, struct file *filp,
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void __user *buffer, size_t *lenp, loff_t *ppos)
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{
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files_stat.nr_files = get_nr_files();
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return proc_dointvec(table, write, filp, buffer, lenp, ppos);
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}
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#else
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int proc_nr_files(ctl_table *table, int write, struct file *filp,
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void __user *buffer, size_t *lenp, loff_t *ppos)
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{
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return -ENOSYS;
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}
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#endif
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/* Find an unused file structure and return a pointer to it.
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* Returns NULL, if there are no more free file structures or
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* we run out of memory.
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*/
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struct file *get_empty_filp(void)
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{
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struct task_struct *tsk;
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static int old_max;
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struct file * f;
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/*
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* Privileged users can go above max_files
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*/
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if (get_nr_files() >= files_stat.max_files && !capable(CAP_SYS_ADMIN)) {
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/*
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* percpu_counters are inaccurate. Do an expensive check before
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* we go and fail.
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*/
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if (percpu_counter_sum_positive(&nr_files) >= files_stat.max_files)
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goto over;
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}
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f = kmem_cache_zalloc(filp_cachep, GFP_KERNEL);
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if (f == NULL)
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goto fail;
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percpu_counter_inc(&nr_files);
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if (security_file_alloc(f))
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goto fail_sec;
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tsk = current;
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INIT_LIST_HEAD(&f->f_u.fu_list);
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atomic_set(&f->f_count, 1);
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rwlock_init(&f->f_owner.lock);
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f->f_uid = tsk->fsuid;
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f->f_gid = tsk->fsgid;
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eventpoll_init_file(f);
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/* f->f_version: 0 */
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return f;
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over:
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/* Ran out of filps - report that */
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if (get_nr_files() > old_max) {
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printk(KERN_INFO "VFS: file-max limit %d reached\n",
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get_max_files());
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old_max = get_nr_files();
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}
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goto fail;
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fail_sec:
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file_free(f);
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fail:
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return NULL;
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}
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EXPORT_SYMBOL(get_empty_filp);
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/**
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* alloc_file - allocate and initialize a 'struct file'
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* @mnt: the vfsmount on which the file will reside
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* @dentry: the dentry representing the new file
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* @mode: the mode with which the new file will be opened
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* @fop: the 'struct file_operations' for the new file
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*
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* Use this instead of get_empty_filp() to get a new
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* 'struct file'. Do so because of the same initialization
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* pitfalls reasons listed for init_file(). This is a
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* preferred interface to using init_file().
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*
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* If all the callers of init_file() are eliminated, its
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* code should be moved into this function.
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*/
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struct file *alloc_file(struct vfsmount *mnt, struct dentry *dentry,
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mode_t mode, const struct file_operations *fop)
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{
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struct file *file;
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struct path;
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file = get_empty_filp();
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if (!file)
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return NULL;
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init_file(file, mnt, dentry, mode, fop);
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return file;
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}
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EXPORT_SYMBOL(alloc_file);
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/**
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* init_file - initialize a 'struct file'
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* @file: the already allocated 'struct file' to initialized
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* @mnt: the vfsmount on which the file resides
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* @dentry: the dentry representing this file
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* @mode: the mode the file is opened with
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* @fop: the 'struct file_operations' for this file
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*
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* Use this instead of setting the members directly. Doing so
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* avoids making mistakes like forgetting the mntget() or
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* forgetting to take a write on the mnt.
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*
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* Note: This is a crappy interface. It is here to make
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* merging with the existing users of get_empty_filp()
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* who have complex failure logic easier. All users
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* of this should be moving to alloc_file().
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*/
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int init_file(struct file *file, struct vfsmount *mnt, struct dentry *dentry,
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mode_t mode, const struct file_operations *fop)
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{
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int error = 0;
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file->f_path.dentry = dentry;
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file->f_path.mnt = mntget(mnt);
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file->f_mapping = dentry->d_inode->i_mapping;
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file->f_mode = mode;
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file->f_op = fop;
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return error;
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}
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EXPORT_SYMBOL(init_file);
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void fastcall fput(struct file *file)
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{
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if (atomic_dec_and_test(&file->f_count))
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__fput(file);
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}
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EXPORT_SYMBOL(fput);
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/* __fput is called from task context when aio completion releases the last
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* last use of a struct file *. Do not use otherwise.
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*/
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void fastcall __fput(struct file *file)
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{
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struct dentry *dentry = file->f_path.dentry;
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struct vfsmount *mnt = file->f_path.mnt;
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struct inode *inode = dentry->d_inode;
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might_sleep();
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fsnotify_close(file);
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/*
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* The function eventpoll_release() should be the first called
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* in the file cleanup chain.
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*/
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eventpoll_release(file);
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locks_remove_flock(file);
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if (file->f_op && file->f_op->release)
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file->f_op->release(inode, file);
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security_file_free(file);
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if (unlikely(S_ISCHR(inode->i_mode) && inode->i_cdev != NULL))
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cdev_put(inode->i_cdev);
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fops_put(file->f_op);
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if (file->f_mode & FMODE_WRITE)
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put_write_access(inode);
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put_pid(file->f_owner.pid);
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file_kill(file);
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file->f_path.dentry = NULL;
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file->f_path.mnt = NULL;
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file_free(file);
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dput(dentry);
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mntput(mnt);
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}
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struct file fastcall *fget(unsigned int fd)
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{
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struct file *file;
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struct files_struct *files = current->files;
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rcu_read_lock();
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file = fcheck_files(files, fd);
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if (file) {
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if (!atomic_inc_not_zero(&file->f_count)) {
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/* File object ref couldn't be taken */
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rcu_read_unlock();
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return NULL;
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}
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}
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rcu_read_unlock();
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return file;
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}
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EXPORT_SYMBOL(fget);
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/*
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* Lightweight file lookup - no refcnt increment if fd table isn't shared.
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* You can use this only if it is guranteed that the current task already
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* holds a refcnt to that file. That check has to be done at fget() only
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* and a flag is returned to be passed to the corresponding fput_light().
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* There must not be a cloning between an fget_light/fput_light pair.
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*/
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struct file fastcall *fget_light(unsigned int fd, int *fput_needed)
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{
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struct file *file;
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struct files_struct *files = current->files;
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*fput_needed = 0;
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if (likely((atomic_read(&files->count) == 1))) {
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file = fcheck_files(files, fd);
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} else {
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rcu_read_lock();
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file = fcheck_files(files, fd);
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if (file) {
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if (atomic_inc_not_zero(&file->f_count))
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*fput_needed = 1;
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else
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/* Didn't get the reference, someone's freed */
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file = NULL;
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}
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rcu_read_unlock();
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}
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return file;
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}
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void put_filp(struct file *file)
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{
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if (atomic_dec_and_test(&file->f_count)) {
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security_file_free(file);
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file_kill(file);
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file_free(file);
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}
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}
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void file_move(struct file *file, struct list_head *list)
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{
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if (!list)
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return;
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file_list_lock();
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list_move(&file->f_u.fu_list, list);
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file_list_unlock();
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}
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void file_kill(struct file *file)
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{
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if (!list_empty(&file->f_u.fu_list)) {
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file_list_lock();
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list_del_init(&file->f_u.fu_list);
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file_list_unlock();
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}
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}
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int fs_may_remount_ro(struct super_block *sb)
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{
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struct list_head *p;
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/* Check that no files are currently opened for writing. */
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file_list_lock();
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list_for_each(p, &sb->s_files) {
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struct file *file = list_entry(p, struct file, f_u.fu_list);
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struct inode *inode = file->f_path.dentry->d_inode;
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/* File with pending delete? */
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if (inode->i_nlink == 0)
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goto too_bad;
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/* Writeable file? */
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if (S_ISREG(inode->i_mode) && (file->f_mode & FMODE_WRITE))
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goto too_bad;
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}
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file_list_unlock();
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return 1; /* Tis' cool bro. */
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too_bad:
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file_list_unlock();
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return 0;
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}
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void __init files_init(unsigned long mempages)
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{
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int n;
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/* One file with associated inode and dcache is very roughly 1K.
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* Per default don't use more than 10% of our memory for files.
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*/
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n = (mempages * (PAGE_SIZE / 1024)) / 10;
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files_stat.max_files = n;
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if (files_stat.max_files < NR_FILE)
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files_stat.max_files = NR_FILE;
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files_defer_init();
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percpu_counter_init(&nr_files, 0);
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}
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