make sure that mntns_install() doesn't end up with referral for root

new flag: LOOKUP_DOWN.  If the starting point is overmounted, cross
into whatever's mounted on top, triggering referrals et.al.

Use that instead of follow_down_one() loop in mntns_install(), handle
errors properly.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 38 insertions, 0 deletions

diff --git a/fs/namei.c b/fs/namei.c
index 60c0a78ebca7..646db9cf2579 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -2252,6 +2252,35 @@ static inline int lookup_last(struct nameidata *nd)
 	return walk_component(nd, 0);
 }
 
+static int handle_lookup_down(struct nameidata *nd)
+{
+	struct path path = nd->path;
+	struct inode *inode = nd->inode;
+	unsigned seq = nd->seq;
+	int err;
+
+	if (nd->flags & LOOKUP_RCU) {
+		/*
+		 * don't bother with unlazy_walk on failure - we are
+		 * at the very beginning of walk, so we lose nothing
+		 * if we simply redo everything in non-RCU mode
+		 */
+		if (unlikely(!__follow_mount_rcu(nd, &path, &inode, &seq)))
+			return -ECHILD;
+	} else {
+		dget(path.dentry);
+		err = follow_managed(&path, nd);
+		if (unlikely(err < 0))
+			return err;
+		inode = d_backing_inode(path.dentry);
+		seq = 0;
+	}
+	path_to_nameidata(&path, nd);
+	nd->inode = inode;
+	nd->seq = seq;
+	return 0;
+}
+
 /* Returns 0 and nd will be valid on success; Retuns error, otherwise. */
 static int path_lookupat(struct nameidata *nd, unsigned flags, struct path *path)
 {
@@ -2260,6 +2289,15 @@ static int path_lookupat(struct nameidata *nd, unsigned flags, struct path *path
 
 	if (IS_ERR(s))
 		return PTR_ERR(s);
+
+	if (unlikely(flags & LOOKUP_DOWN)) {
+		err = handle_lookup_down(nd);
+		if (unlikely(err < 0)) {
+			terminate_walk(nd);
+			return err;
+		}
+	}
+
 	while (!(err = link_path_walk(s, nd))
 		&& ((err = lookup_last(nd)) > 0)) {
 		s = trailing_symlink(nd);