Re: [PATCH] namei: revert old behaviour for filename_lookup with LOOKUP_PARENT flag

From: Vineeth Remanan Pillai
Date: Thu Oct 13 2016 - 16:43:50 EST

On 10/13/2016 01:26 PM, Al Viro wrote:
On Thu, Oct 13, 2016 at 01:09:04PM -0700, Christoph Hellwig wrote:
On Thu, Oct 13, 2016 at 07:58:51PM +0000, Vineeth Remanan Pillai wrote:
filename_lookup used to return success for non-existing file when called
with LOOKUP_PARENT flag. This behaviour was changed with
commit 8bcb77fabd7c ("namei: split off filename_lookupat()

The above patch split parent lookup functionality to a different function
filename_parentat and changed all calls to filename_lookup(LOOKUP_PARENT)
to the new function filename_parentat. But functions like kern_path which
passed the flags directly to filename_lookup regressed due to this.

This patch aims to fix the regressed behaviour by calling
filename_parentat from filename_lookup if the flags contain LOOKUP_PARENT.
What callers shows te problems? That's probaby were the fix need to got
in, and even if not that's still part of a good bug report.
Out-of-tree, at a guess... Incidentally, since filename_lookup() is not
exported, it's probably kern_path() that gets used wherever it is.
Depending on the details of what's being attempted, kern_path_locked()
might or might not be the right primitive to use, but I would probably
start with checking if that's what the code in question really wants.

// kernel_string is an kernel pointer to NUL-terminated array of char

struct path path;
struct dentry *dentry;

dentry = kern_path_locked(kernel_string, &path);
if (IS_ERR(dentry))
// failed while getting the parent, or not a regular last
// component (/, ., .., <something>/., <something>/..)
sod off // no cleanup needed

// now path contains the resolved parent and dentry points to the
// dentry of child, possibly negative; the last component of the
// name can be determined from dentry->d_name. Parent directory
// is locked, making sure that directory entry won't be changed
// until you are done.

if (d_is_really_negative(dentry)) {
// parent exists, but child doesn't
} else {
// child exists

// clean up: drop dentry, unlock parent, drop dentry/vfsmount of parent

Yes, the use case is out-of-tree and the code snippet above depicts the use .
Since kern_path_locked is also not exported, out-of-tree code used kern_path
for the existence check for directories.

One reference about this issue can be seen here.

We also have a customer who complained about this functionality change.

I understand that there has been no API promises been made to this API. But since this is an
exported function, the change in function could cause break in out-of-tree kernel code. I will
rephrase the commit message to say "change in functionality" instead of regression