fs: make sure to fail try_to_unlazy() and try_to_unlazy() for LOOKUP_CACHED

Otherwise the slowpath can be taken by the caller, defeating the flag.

This regressed after calls to legitimize_links() started being
conditionally elided and stems from the routine always failing
after seeing the flag, regardless if there were any links.

In order to address both the bug and the weird semantics make it illegal
to call legitimize_links() with LOOKUP_CACHED and handle the problem at
the two callsites.

Fixes: 7c17909 ("fs: add predicts based on nd->depth")
Reported-by: Chris Mason <[email protected]>
Signed-off-by: Mateusz Guzik <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Christian Brauner <[email protected]>

Author: mjguzik

fs/namei.c

@@ -830,11 +830,9 @@ static inline bool legitimize_path(struct nameidata *nd,
 static bool legitimize_links(struct nameidata *nd)
 {
 	int i;
-	if (unlikely(nd->flags & LOOKUP_CACHED)) {
-		drop_links(nd);
-		nd->depth = 0;
-		return false;
-	}
+
+	VFS_BUG_ON(nd->flags & LOOKUP_CACHED);
+
 	for (i = 0; i < nd->depth; i++) {
 		struct saved *last = nd->stack + i;
 		if (unlikely(!legitimize_path(nd, &last->link, last->seq))) {
@@ -883,6 +881,11 @@ static bool try_to_unlazy(struct nameidata *nd)
 
 	BUG_ON(!(nd->flags & LOOKUP_RCU));
 
+	if (unlikely(nd->flags & LOOKUP_CACHED)) {
+		drop_links(nd);
+		nd->depth = 0;
+		goto out1;
+	}
 	if (unlikely(nd->depth && !legitimize_links(nd)))
 		goto out1;
 	if (unlikely(!legitimize_path(nd, &nd->path, nd->seq)))
@@ -918,6 +921,11 @@ static bool try_to_unlazy_next(struct nameidata *nd, struct dentry *dentry)
 	int res;
 	BUG_ON(!(nd->flags & LOOKUP_RCU));
 
+	if (unlikely(nd->flags & LOOKUP_CACHED)) {
+		drop_links(nd);
+		nd->depth = 0;
+		goto out2;
+	}
 	if (unlikely(nd->depth && !legitimize_links(nd)))
 		goto out2;
 	res = __legitimize_mnt(nd->path.mnt, nd->m_seq);