link_path_walk(): be careful when failing with ENOTDIR

In RCU mode we might end up with dentry evicted just we check
that it's a directory.  In such case we should return ECHILD
rather than ENOTDIR, so that pathwalk would be retries in non-RCU
mode.

Breakage had been introduced in commit b18825a - prior to that
we were looking at nd->inode, which had been fetched before
verifying that ->d_seq was still valid.  That form of check
would only be satisfied if at some point the pathname prefix
would indeed have resolved to a non-directory.  The fix consists
of checking ->d_seq after we'd run into a non-directory dentry,
and failing with ECHILD in case of mismatch.

Note that all branches since 3.12 have that problem...

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

1 files changed, 6 insertions, 1 deletions

diff --git a/fs/namei.c b/fs/namei.c
index ae4e4c18b2ac..fbbcf0993312 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -1954,8 +1954,13 @@ OK:
                                 continue;
                         }
                 }
-                if (unlikely(!d_can_lookup(nd->path.dentry)))
+                if (unlikely(!d_can_lookup(nd->path.dentry))) {
+                        if (nd->flags & LOOKUP_RCU) {
+                                if (unlazy_walk(nd, NULL, 0))
+                                        return -ECHILD;
+                        }
                         return -ENOTDIR;
+                }
         }
 }