Re: [PATCH v2] audit: Avoid excessive dput/dget in audit_context setup and reset paths

On 2/5/26 8:20 PM, Waiman Long wrote:
> On 2/5/26 6:53 PM, Al Viro wrote:
>> On Wed, Feb 04, 2026 at 11:45:17PM -0500, Waiman Long wrote:
>>
>>> @@ -70,6 +74,8 @@ void chroot_fs_refs(const struct path *old_root, 
>>> const
>>> struct>
>>>                                  count++;
>>>                                  path_get(new_root);
>>>                          }
>>> +                       count += fs->pwd_xrefs;
>>> +                       fs->pwd_xrefs = 0;
>>>                          write_sequnlock(&fs->seq);
>> Nope - you only need that for threads that have ->pwd equal to old_root.
>> Incidentally, I'd forgotten about that sucker - it kills the idea of
>> fdget-like tricks dead, more's the pity.  Third-party modification of
>> task->fs->pwd (under task->lock and task->fs->seq), possible even with
>> task->fs->users == 1.
>
> Yes, I am aware of that when I took a further look at the patch that I 
> sent out yesterday. I am testing the updated patch now and is trying 
> to figure out why I get a warning from mntput_no_expire_slowpath() 
> with a count of -1 when doing an umount. It is off by 1 somewhere. I 
> will post the patch once I resolve this bug. 

I now know why there are warnings. The problem is in the copy_mnt_ns() 
function in fs/namespace.c:

__latent_entropy
struct mnt_namespace *copy_mnt_ns(u64 flags, struct mnt_namespace *ns,
                 struct user_namespace *user_ns, struct fs_struct *new_fs)
{
   :
                 if (new_fs) {
                         if (&p->mnt == new_fs->root.mnt) {
                                 new_fs->root.mnt = mntget(&q->mnt);
                                 rootmnt = &p->mnt;
                         }
                         if (&p->mnt == new_fs->pwd.mnt) {
                                 new_fs->pwd.mnt = mntget(&q->mnt);
                                 pwdmnt = &p->mnt;
                         }
                 }

It is replacing the fs->pwd.mnt with a new one while pwd_refs is 1. I 
can make this work with the new fs_struct field. I do have one question 
though. Do we need to acquire write_seqlock(&new_fs->seq) if we are 
changing root or pwd here or if the new_fs are in such a state that it 
will never change when this copying operation is in progress?

Thanks in advance for your advice.

Cheers,
Longman