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[PATCH] r/o bind mounts: stub functions

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This patch adds two function mnt_want_write() and mnt_drop_write().  These are
used like a lock pair around and fs operations that might cause a write to the
filesystem.

Before these can become useful, we must first cover each place in the VFS
where writes are performed with a want/drop pair.  When that is complete, we
can actually introduce code that will safely check the counts before allowing
r/w<->r/o transitions to occur.

Acked-by: Serge Hallyn <serue@us.ibm.com>
Acked-by: Al Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Dave Hansen <haveblue@us.ibm.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
This commit is contained in:
Dave Hansen 2008-02-15 14:37:30 -08:00 committed by Al Viro
parent a70e65df88
commit 8366025eb8
2 changed files with 57 additions and 0 deletions
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54
fs/namespace.c
View file

@ -80,6 +80,60 @@ struct vfsmount *alloc_vfsmnt(const char *name)
return mnt;
}
/*
* Most r/o checks on a fs are for operations that take
* discrete amounts of time, like a write() or unlink().
* We must keep track of when those operations start
* (for permission checks) and when they end, so that
* we can determine when writes are able to occur to
* a filesystem.
*/
/**
* mnt_want_write - get write access to a mount
* @mnt: the mount on which to take a write
*
* This tells the low-level filesystem that a write is
* about to be performed to it, and makes sure that
* writes are allowed before returning success. When
* the write operation is finished, mnt_drop_write()
* must be called. This is effectively a refcount.
*/
int mnt_want_write(struct vfsmount *mnt)
{
if (__mnt_is_readonly(mnt))
return -EROFS;
return 0;
}
EXPORT_SYMBOL_GPL(mnt_want_write);
/**
* mnt_drop_write - give up write access to a mount
* @mnt: the mount on which to give up write access
*
* Tells the low-level filesystem that we are done
* performing writes to it. Must be matched with
* mnt_want_write() call above.
*/
void mnt_drop_write(struct vfsmount *mnt)
{
}
EXPORT_SYMBOL_GPL(mnt_drop_write);
/*
* __mnt_is_readonly: check whether a mount is read-only
* @mnt: the mount to check for its write status
*
* This shouldn't be used directly ouside of the VFS.
* It does not guarantee that the filesystem will stay
* r/w, just that it is right *now*. This can not and
* should not be used in place of IS_RDONLY(inode).
*/
int __mnt_is_readonly(struct vfsmount *mnt)
{
return (mnt->mnt_sb->s_flags & MS_RDONLY);
}
EXPORT_SYMBOL_GPL(__mnt_is_readonly);
int simple_set_mnt(struct vfsmount *mnt, struct super_block *sb)
{
mnt->mnt_sb = sb;

3
include/linux/mount.h
View file

@ -71,9 +71,12 @@ static inline struct vfsmount *mntget(struct vfsmount *mnt)
return mnt;
}
extern int mnt_want_write(struct vfsmount *mnt);
extern void mnt_drop_write(struct vfsmount *mnt);
extern void mntput_no_expire(struct vfsmount *mnt);
extern void mnt_pin(struct vfsmount *mnt);
extern void mnt_unpin(struct vfsmount *mnt);
extern int __mnt_is_readonly(struct vfsmount *mnt);
static inline void mntput(struct vfsmount *mnt)
{