Re: [PATCH v1] pid: annotate data-races around pid_ns->pid_allocated

[Christian Brauner <brauner@kernel.org>]

On Wed, Apr 23, 2025 at 06:38:18PM +0200, Oleg Nesterov wrote:
> On 04/23, Jiayuan Chen wrote:
> >
> > April 23, 2025 at 21:51, "Oleg Nesterov" <oleg@redhat.com> wrote:
> >
> >
> >
> > >
> > > On 04/23, Jiayuan Chen wrote:
> > >
> > > >
> > > > Suppress syzbot reports by annotating these accesses using
> > > >
> > > >  READ_ONCE() / WRITE_ONCE().
> > > >
> > >
> > > ...
> > >
> > > >
> > > > --- a/kernel/pid.c
> > > >
> > > >  +++ b/kernel/pid.c
> > > >
> > > >  @@ -122,7 +122,8 @@ void free_pid(struct pid *pid)
> > > >
> > > >  for (i = 0; i <= pid->level; i++) {
> > > >
> > > >  struct upid *upid = pid->numbers + i;
> > > >
> > > >  struct pid_namespace *ns = upid->ns;
> > > >
> > > >  - switch (--ns->pid_allocated) {
> > > >
> > > >  + WRITE_ONCE(ns->pid_allocated, READ_ONCE(ns->pid_allocated) - 1);
> > > >
> > > >  + switch (READ_ONCE(ns->pid_allocated)) {
> > > >
> > >
> > > I keep forgetting how kcsan works, but we don't need
> > >
> > > READ_ONCE(ns->pid_allocated) under pidmap_lock?
> > >
> > > Same for other functions which read/modify ->pid_allocated with
> > >
> > > this lock held.
> > >
> > > Oleg.
> > >
> >
> > However, not all places that read/write pid_allocated are locked,
> > for example:
> > https://web.git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/tree/kernel/pid_namespace.c#n271
> > https://web.git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/tree/kernel/fork.c#n2602
> >
> > So, in fact, the pidmap_lock is not effective. And if we were to add locks
> > to all these places, it would be too heavy.
> 
> It seems you misunderstood me. I didn't argue with the lockless READ_ONCE()s
> outside of pidmap_lock.

Agreed. We should only add those annotations where they're really
needed (someone once taught me ;).