dcache: Handle escaped paths in prepend_path

commit cde93be4 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.
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
[bwh: For 2.6.32, implement the "(unreachable)" string in __d_path()]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Willy Tarreau <w@1wt.eu>

fs/dcache.c

@@ -1910,7 +1910,7 @@ char *__d_path(const struct path *path, struct path *root,
 	struct dentry *dentry = path->dentry;
 	struct vfsmount *vfsmnt = path->mnt;
 	char *end = buffer + buflen;
-	char *retval;
+	char *retval, *tail;
 
 	spin_lock(&vfsmount_lock);
 	prepend(&end, &buflen, "\0", 1);
@@ -1923,6 +1923,7 @@ char *__d_path(const struct path *path, struct path *root,
 	/* Get '/' right */
 	retval = end-1;
 	*retval = '/';
+	tail = end;
 
 	for (;;) {
 		struct dentry * parent;
@@ -1930,6 +1931,14 @@ char *__d_path(const struct path *path, struct path *root,
 		if (dentry == root->dentry && vfsmnt == root->mnt)
 			break;
 		if (dentry == vfsmnt->mnt_root || IS_ROOT(dentry)) {
+			/* Escaped? */
+			if (dentry != vfsmnt->mnt_root) {
+				buflen += (tail - end);
+				end = tail;
+				prepend(&end, &buflen, "(unreachable)/", 14);
+				retval = end;
+				goto out;
+			}
 			/* Global root? */
 			if (vfsmnt->mnt_parent == vfsmnt) {
 				goto global_root;