Commit 8366025e authored by Dave Hansen's avatar Dave Hansen Committed by Al Viro
Browse files

[PATCH] r/o bind mounts: stub functions 



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: default avatarSerge Hallyn <serue@us.ibm.com>
Acked-by: default avatarAl Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: default avatarChristoph Hellwig <hch@infradead.org>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarDave Hansen <haveblue@us.ibm.com>
Signed-off-by: default avatarAl Viro <viro@zeniv.linux.org.uk>
parent a70e65df

fs/namespace.c

+54 −0
Original line number Diff line number Diff line
@@ -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;

include/linux/mount.h

+3 −0
Original line number Diff line number Diff line
@@ -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)

{

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