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From:   Marco Elver <elver@google.com>
Date:   Tue, 23 Feb 2021 16:10:42 +0100
Message-ID: <CANpmjNP1wQvG0SNPP2L9QO=natf0XU8HXj-r2_-U4QZxtr-dVA@mail.gmail.com>
Subject: Re: [PATCH RFC 4/4] perf/core: Add breakpoint information to siginfo
 on SIGTRAP
To:     Dmitry Vyukov <dvyukov@google.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Ingo Molnar <mingo@redhat.com>, Jiri Olsa <jolsa@redhat.com>,
        Mark Rutland <mark.rutland@arm.com>,
        Namhyung Kim <namhyung@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Alexander Potapenko <glider@google.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Arnd Bergmann <arnd@arndb.de>,
        Christian Brauner <christian@brauner.io>,
        Jann Horn <jannh@google.com>, Jens Axboe <axboe@kernel.dk>,
        Matt Morehouse <mascasa@google.com>,
        Peter Collingbourne <pcc@google.com>,
        Ian Rogers <irogers@google.com>,
        kasan-dev <kasan-dev@googlegroups.com>,
        linux-arch <linux-arch@vger.kernel.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        linux-m68k@lists.linux-m68k.org,
        "the arch/x86 maintainers" <x86@kernel.org>
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On Tue, 23 Feb 2021 at 16:01, Dmitry Vyukov <dvyukov@google.com> wrote:
>
> On Tue, Feb 23, 2021 at 3:34 PM Marco Elver <elver@google.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.

What do you prefer?

> > +               break;
> > +       }
> > +
> >         force_sig_info(&info);
> >  }
> >
> > --
> > 2.30.0.617.g56c4b15f3c-goog
> >