Re: [PATCH RFC 4/4] perf/core: Add breakpoint information to siginfo on SIGTRAP

[Marco Elver <elver@google.com>]

On Tue, 23 Feb 2021 at 16:16, Dmitry Vyukov <dvyukov@google.com> wrote:
>
> On Tue, Feb 23, 2021 at 4:10 PM 'Marco Elver' via kasan-dev
> <kasan-dev@googlegroups.com> wrote:
> > > > Encode information from breakpoint attributes into siginfo_t, which
> > > > helps disambiguate which breakpoint fired.
> > > >
> > > > Note, providing the event fd may be unreliable, since the event may have
> > > > been modified (via PERF_EVENT_IOC_MODIFY_ATTRIBUTES) between the event
> > > > triggering and the signal being delivered to user space.
> > > >
> > > > Signed-off-by: Marco Elver <elver@google.com>
> > > > ---
> > > >  kernel/events/core.c | 11 +++++++++++
> > > >  1 file changed, 11 insertions(+)
> > > >
> > > > diff --git a/kernel/events/core.c b/kernel/events/core.c
> > > > index 8718763045fd..d7908322d796 100644
> > > > --- a/kernel/events/core.c
> > > > +++ b/kernel/events/core.c
> > > > @@ -6296,6 +6296,17 @@ static void perf_sigtrap(struct perf_event *event)
> > > >         info.si_signo = SIGTRAP;
> > > >         info.si_code = TRAP_PERF;
> > > >         info.si_errno = event->attr.type;
> > > > +
> > > > +       switch (event->attr.type) {
> > > > +       case PERF_TYPE_BREAKPOINT:
> > > > +               info.si_addr = (void *)(unsigned long)event->attr.bp_addr;
> > > > +               info.si_perf = (event->attr.bp_len << 16) | (u64)event->attr.bp_type;
> > > > +               break;
> > > > +       default:
> > > > +               /* No additional info set. */
> > >
> > > Should we prohibit using attr.sigtrap for !PERF_TYPE_BREAKPOINT if we
> > > don't know what info to pass yet?
> >
> > I don't think it's necessary. This way, by default we get support for
> > other perf events. If user space observes si_perf==0, then there's no
> > information available. That would require that any event type that
> > sets si_perf in future, must ensure that it sets si_perf!=0.
> >
> > I can add a comment to document the requirement here (and user space
> > facing documentation should get a copy of how the info is encoded,
> > too).
> >
> > Alternatively, we could set si_errno to 0 if no info is available, at
> > the cost of losing the type information for events not explicitly
> > listed here.

Note that PERF_TYPE_HARDWARE == 0, so setting si_errno to 0 does not
work. Which leaves us with:

1. Ensure si_perf==0 (or some other magic value) if no info is
available and !=0 otherwise.

2. Return error for events where we do not officially support
requesting sigtrap.

I'm currently leaning towards (1).

> > What do you prefer?
>
> Ah, I see.
> Let's wait for the opinions of other people. There are a number of
> options for how to approach this.