From 8366025eb80dfa0d8d94b286d53027081c280ef1 Mon Sep 17 00:00:00 2001 From: Dave Hansen Date: Fri, 15 Feb 2008 14:37:30 -0800 Subject: [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: Serge Hallyn Acked-by: Al Viro Signed-off-by: Christoph Hellwig Signed-off-by: Andrew Morton Signed-off-by: Dave Hansen Signed-off-by: Al Viro --- fs/namespace.c | 54 ++++++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 54 insertions(+) (limited to 'fs/namespace.c') diff --git a/fs/namespace.c b/fs/namespace.c index 94f026ec990..066b393578c 100644 --- a/fs/namespace.c +++ b/fs/namespace.c @@ -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; -- cgit v1.2.3