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>

diff --git a/fs/namei.c b/fs/namei.c
index ae4e4c1..fbbcf09 100644 (file)
--- 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;
+               }
        }
 }