Patch "dcache: Handle escaped paths in prepend_path" has been added to the 4.2-stable tree

This is a note to let you know that I've just added the patch titled

dcache: Handle escaped paths in prepend_path

to the 4.2-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
dcache-handle-escaped-paths-in-prepend_path.patch
and it can be found in the queue-4.2 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: "Eric W. Biederman" <[email protected]>
Date: Sat, 15 Aug 2015 13:36:12 -0500
Subject: dcache: Handle escaped paths in prepend_path

From: "Eric W. Biederman" <[email protected]>

commit cde93be45a8a90d8c264c776fab63487b5038a65 upstream.

A rename can result in a dentry that by walking up d_parent will never reach it's mnt_root. For lack of a better term I call this an escaped path.

prepend_path is called by four different functions __d_path, d_absolute_path, d_path, and getcwd.

__d_path only wants to see paths are connected to the root it passes in. So __d_path needs prepend_path to return an error.

d_absolute_path similarly wants to see paths that are connected to some root. Escaped paths are not connected to any mnt_root so d_absolute_path needs prepend_path to return an error greater than 1. So escaped paths will be treated like paths on lazily unmounted mounts.

getcwd needs to prepend "(unreachable)" so getcwd also needs prepend_path to return an error.

d_path is the interesting hold out. d_path just wants to print something, and does not care about the weird cases. Which raises the question what should be printed?

Given that <escaped_path>/<anything> should result in -ENOENT I believe it is desirable for escaped paths to be printed as empty paths. As there are not really any meaninful path components when considered from the perspective of a mount tree.

So tweak prepend_path to return an empty path with an new error code of 3 when it encounters an escaped path.

---
 fs/dcache.c |    7 +++++++
 1 file changed, 7 insertions(+)

@@ -2926,6 +2926,13 @@ restart:
 
 		if (dentry == vfsmnt->mnt_root || IS_ROOT(dentry)) {
 			struct mount *parent = ACCESS_ONCE(mnt->mnt_parent);
+			/* Escaped? */
+			if (dentry != vfsmnt->mnt_root) {
+				bptr = *buffer;
+				blen = *buflen;
+				error = 3;
+				break;
+			}
 			/* Global root? */
 			if (mnt != parent) {
 				dentry = ACCESS_ONCE(mnt->mnt_mountpoint);

Patches currently in stable-queue which might be from [email protected] are

queue-4.2/dcache-handle-escaped-paths-in-prepend_path.patch queue-4.2/vfs-test-for-and-handle-paths-that-are-unreachable-from-their-mnt_root.patch