Re: [PATCH v3 07/11] perf: Add breakpoint information to siginfo on SIGTRAP

[Dmitry Vyukov]

On Wed, Mar 24, 2021 at 3:12 PM Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote:
> > On Wed, 24 Mar 2021 at 15:01, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> > >
> > > One last try, I'll leave it alone now, I promise :-)
> >
> > This looks like it does what you suggested, thanks! :-)
> >
> > I'll still need to think about it, because of the potential problem
> > with modify-signal-races and what the user's synchronization story
> > would look like then.
>
> I agree that this looks inherently racy. The attr can't be allocated
> on stack, user synchronization may be tricky and expensive. The API
> may provoke bugs and some users may not even realize the race problem.
>
> One potential alternative is use of an opaque u64 context (if we could
> shove it into the attr). A user can pass a pointer to the attr in
> there (makes it equivalent to this proposal), or bit-pack size/type
> (as we want), pass some sequence number or whatever.

Just to clarify what I was thinking about, but did not really state:
perf_event_attr_t includes u64 ctx, and we return it back to the user
in siginfo_t. Kernel does not treat it in any way. This is a pretty
common API pattern in general.


> > > --- a/include/linux/perf_event.h
> > > +++ b/include/linux/perf_event.h
> > > @@ -778,6 +778,9 @@ struct perf_event {
> > >         void *security;
> > >  #endif
> > >         struct list_head                sb_list;
> > > +
> > > +       unsigned long                   si_uattr;
> > > +       unsigned long                   si_data;
> > >  #endif /* CONFIG_PERF_EVENTS */
> > >  };
> > >
> > > --- a/kernel/events/core.c
> > > +++ b/kernel/events/core.c
> > > @@ -5652,13 +5652,17 @@ static long _perf_ioctl(struct perf_even
> > >                 return perf_event_query_prog_array(event, (void __user *)arg);
> > >
> > >         case PERF_EVENT_IOC_MODIFY_ATTRIBUTES: {
> > > +               struct perf_event_attr __user *uattr;
> > >                 struct perf_event_attr new_attr;
> > > -               int err = perf_copy_attr((struct perf_event_attr __user *)arg,
> > > -                                        &new_attr);
> > > +               int err;
> > >
> > > +               uattr = (struct perf_event_attr __user *)arg;
> > > +               err = perf_copy_attr(uattr, &new_attr);
> > >                 if (err)
> > >                         return err;
> > >
> > > +               event->si_uattr = (unsigned long)uattr;
> > > +
> > >                 return perf_event_modify_attr(event,  &new_attr);
> > >         }
> > >         default:
> > > @@ -6399,7 +6403,12 @@ static void perf_sigtrap(struct perf_eve
> > >         clear_siginfo(&info);
> > >         info.si_signo = SIGTRAP;
> > >         info.si_code = TRAP_PERF;
> > > -       info.si_errno = event->attr.type;
> > > +       info.si_addr = (void *)event->si_data;
> > > +
> > > +       info.si_perf = event->si_uattr;
> > > +       if (event->parent)
> > > +               info.si_perf = event->parent->si_uattr;
> > > +
> > >         force_sig_info(&info);
> > >  }
> > >
> > > @@ -6414,8 +6423,8 @@ static void perf_pending_event_disable(s
> > >                 WRITE_ONCE(event->pending_disable, -1);
> > >
> > >                 if (event->attr.sigtrap) {
> > > -                       atomic_set(&event->event_limit, 1); /* rearm event */
> > >                         perf_sigtrap(event);
> > > +                       atomic_set_release(&event->event_limit, 1); /* rearm event */
> > >                         return;
> > >                 }
> > >
> > > @@ -9121,6 +9130,7 @@ static int __perf_event_overflow(struct
> > >         if (events && atomic_dec_and_test(&event->event_limit)) {
> > >                 ret = 1;
> > >                 event->pending_kill = POLL_HUP;
> > > +               event->si_data = data->addr;
> > >
> > >                 perf_event_disable_inatomic(event);
> > >         }
> > > @@ -12011,6 +12021,8 @@ SYSCALL_DEFINE5(perf_event_open,
> > >                 goto err_task;
> > >         }
> > >
> > > +       event->si_uattr = (unsigned long)attr_uptr;
> > > +
> > >         if (is_sampling_event(event)) {
> > >                 if (event->pmu->capabilities & PERF_PMU_CAP_NO_INTERRUPT) {
> > >                         err = -EOPNOTSUPP;