[PATCH 4.2 226/258] dcache: Handle escaped paths in prepend_path

From: Greg Kroah-Hartman
Date: Sat Oct 17 2015 - 23:39:37 EST


4.2-stable review patch. If anyone has any objections, please let me know.

------------------

From: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>

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.

Signed-off-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
Signed-off-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

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

--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -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);


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