On Tue, May 06, 2025 at 04:43:56PM +0200, Mateusz Guzik wrote:
> On Tue, May 06, 2025 at 01:55:54PM +0200, Christian Brauner wrote:
> > Now that we have pidfs_{get,register}_pid() that needs to be paired with
> > pidfs_put_pid() it's possible that someone pairs them with put_pid().
> > Thus freeing struct pid while it's still used by pidfs. Notice when that
> > happens. I'll also add a scheme to detect invalid uses of
> > pidfs_get_pid() and pidfs_put_pid() later.
> >
> > Signed-off-by: Christian Brauner <brauner@xxxxxxxxxx>
> > ---
> > kernel/pid.c | 1 +
> > 1 file changed, 1 insertion(+)
> >
> > diff --git a/kernel/pid.c b/kernel/pid.c
> > index 26f1e136f017..8317bcbc7cf7 100644
> > --- a/kernel/pid.c
> > +++ b/kernel/pid.c
> > @@ -100,6 +100,7 @@ void put_pid(struct pid *pid)
> >
> > ns = pid->numbers[pid->level].ns;
> > if (refcount_dec_and_test(&pid->count)) {
> > + WARN_ON_ONCE(pid->stashed);
> > kmem_cache_free(ns->pid_cachep, pid);
> > put_pid_ns(ns);
> > }
> > --
> > 2.47.2
> >
>
> With the patch as proposed you are only catching the misuse if this is
> the last ref though.
>
> iow, the check should be hoisted above unrefing?
No, not really. If there's more than one reference then pid->stashed can
be legimitately != NULL.
