ovl: unbless lower st_ino of unverified origin

On a malformed overlay, several redirected dirs can point to the same
dir on a lower layer. This presents a similar challenge as broken
hardlinks, because different objects in the overlay can return the same
st_ino/st_dev pair from stat(2).

For broken hardlinks, we do not provide constant st_ino on copy up to
avoid this inconsistency. When NFS export feature is enabled, apply
the same logic to files and directories with unverified lower origin.
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/overlayfs/inode.c

@@ -105,12 +105,20 @@ int ovl_getattr(const struct path *path, struct kstat *stat,
 			 * Lower hardlinks may be broken on copy up to different
 			 * upper files, so we cannot use the lower origin st_ino
 			 * for those different files, even for the same fs case.
+			 *
+			 * Similarly, several redirected dirs can point to the
+			 * same dir on a lower layer. With the "verify_lower"
+			 * feature, we do not use the lower origin st_ino, if
+			 * we haven't verified that this redirect is unique.
+			 *
 			 * With inodes index enabled, it is safe to use st_ino
-			 * of an indexed hardlinked origin. The index validates
-			 * that the upper hardlink is not broken.
+			 * of an indexed origin. The index validates that the
+			 * upper hardlink is not broken and that a redirected
+			 * dir is the only redirect to that origin.
 			 */
-			if (is_dir || lowerstat.nlink == 1 ||
-			    ovl_test_flag(OVL_INDEX, d_inode(dentry)))
+			if (ovl_test_flag(OVL_INDEX, d_inode(dentry)) ||
+			    (!ovl_verify_lower(dentry->d_sb) &&
+			     (is_dir || lowerstat.nlink == 1)))
 				stat->ino = lowerstat.ino;
 
 			if (samefs)