[PATCH 2/2] vfs: force reval on dentry of bind mounted files on FS_REVAL_DOT filesystems

[Jeff Layton <jlayton@redhat.com>]

In the case of a bind mounted file, the path walking code will assume
that the cached dentry that was bind mounted is valid. This is a problem
problem for NFSv4 in a way that's similar to LAST_BIND symlinks.

Fix this by revalidating the dentry if FS_FOLLOW_DOT is set and
__follow_mount returns true.

Note that in the non-open codepath, we cannot return an error to the
lookup if the revalidation fails. Doing so will leave a bind mount in
a state such that we can't unmount it. In that case we'll just have to
settle for d_invalidating it (which should mostly turn out to be a
d_drop in this case) and returning success.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
---
 fs/namei.c |   11 ++++++++++-
 1 files changed, 10 insertions(+), 1 deletions(-)

diff --git a/fs/namei.c b/fs/namei.c
index 339789e..0d55b6f 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -851,7 +851,13 @@ static int do_lookup(struct nameidata *nd, struct qstr *name,
 done:
 	path->mnt = mnt;
 	path->dentry = dentry;
-	__follow_mount(path);
+
+	/*
+	 * cannot return the error returned by force_reval_path as that can
+	 * make it impossible to unmount a bind mounted dentry if it's stale.
+	 */
+	if (__follow_mount(path))
+		force_reval_path(path, nd);
 	return 0;
 
 need_lookup:
@@ -1840,6 +1846,9 @@ do_last:
 		error = -ELOOP;
 		if (flag & O_NOFOLLOW)
 			goto exit_dput;
+		error = force_reval_path(&path, &nd);
+		if (error)
+			goto exit_dput;
 	}
 
 	error = -ENOENT;
-- 
1.5.5.6