Re: [PATCH v4 4/8] uprobes: travers uprobe's consumer list locklessly under SRCU protection

[Oleg Nesterov <oleg@redhat.com>]

On 08/29, Andrii Nakryiko wrote:
>
> On Thu, Aug 29, 2024 at 4:10 PM Jiri Olsa <olsajiri@gmail.com> wrote:
> >
> > > @@ -2101,17 +2110,24 @@ static void handler_chain(struct uprobe *uprobe, struct pt_regs *regs)
> > >                       need_prep = true;
> > >
> > >               remove &= rc;
> > > +             has_consumers = true;
> > >       }
> > >       current->utask->auprobe = NULL;
> > >
> > >       if (need_prep && !remove)
> > >               prepare_uretprobe(uprobe, regs); /* put bp at return */
> > >
> > > -     if (remove && uprobe->consumers) {
> > > -             WARN_ON(!uprobe_is_active(uprobe));
> > > -             unapply_uprobe(uprobe, current->mm);
> > > +     if (remove && has_consumers) {
> > > +             down_read(&uprobe->register_rwsem);
> > > +
> > > +             /* re-check that removal is still required, this time under lock */
> > > +             if (!filter_chain(uprobe, current->mm)) {
> >
> > sorry for late question, but I do not follow this change..
> >
> > at this point we got 1 as handler's return value from all the uprobe's consumers,
> > why do we need to call filter_chain in here.. IIUC this will likely skip over
> > the removal?
> >
>
> Because we don't hold register_rwsem we are now racing with
> registration. So while we can get all consumers at the time we were
> iterating over the consumer list to request deletion, a parallel CPU
> can add another consumer that needs this uprobe+PID combination. So if
> we don't double-check, we are risking having a consumer that will not
> be triggered for the desired process.

Oh, yes, but this logic is wrong in that it assumes that uc->filter != NULL.
At least it adds the noticeable change in behaviour.

Suppose we have a singler consumer UC with ->filter == NULL. Now suppose
that UC->handler() returns UPROBE_HANDLER_REMOVE.

Before this patch handler_chain() calls unapply_uprobe(), and I think
we should keep this behaviour.

After this patch unapply_uprobe() won't be called: consumer_filter(UC)
returns true, UC->filter == NULL means "probe everything". But I think
that UPROBE_HANDLER_REMOVE must be respected in this case anyway.

Thanks Jiri, I missed that too :/

Oleg.