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

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Date:   Thu, 25 Mar 2021 16:17:40 +0100
From:   Marco Elver <elver@...gle.com>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     Dmitry Vyukov <dvyukov@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Ingo Molnar <mingo@...hat.com>, Jiri Olsa <jolsa@...hat.com>,
        Mark Rutland <mark.rutland@....com>,
        Namhyung Kim <namhyung@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Alexander Potapenko <glider@...gle.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Arnd Bergmann <arnd@...db.de>,
        Christian Brauner <christian@...uner.io>,
        Jann Horn <jannh@...gle.com>, Jens Axboe <axboe@...nel.dk>,
        Matt Morehouse <mascasa@...gle.com>,
        Peter Collingbourne <pcc@...gle.com>,
        Ian Rogers <irogers@...gle.com>,
        kasan-dev <kasan-dev@...glegroups.com>,
        linux-arch <linux-arch@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>
Subject: Re: [PATCH v3 07/11] perf: Add breakpoint information to siginfo on SIGTRAP

On Thu, 25 Mar 2021 at 15:18, Ingo Molnar <mingo@...nel.org> wrote:
>
> * Dmitry Vyukov <dvyukov@...gle.com> wrote:
>
> > On Wed, Mar 24, 2021 at 3:05 PM Marco Elver <elver@...gle.com> wrote:
> > >
> > > On Wed, 24 Mar 2021 at 15:01, Peter Zijlstra <peterz@...radead.org> 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.
>
> Yeah, so why cannot we allocate enough space from the signal handler
> user-space stack and put the attr there, and point to it from
> sig_info?
>
> The idea would be to create a stable, per-signal snapshot of whatever
> the perf_attr state is at the moment the event happens and the signal
> is generated - which is roughly what user-space wants, right?

I certainly couldn't say how feasible this is. Is there infrastructure
in place to do this? Or do we have to introduce support for stashing
things on the signal stack?

>From what we can tell, the most flexible option though appears to be
just some user settable opaque data in perf_event_attr, that is copied
to siginfo. It'd allow user space to store a pointer or a hash/key, or
just encode the relevant information it wants; but could also go
further, and add information beyond perf_event_attr, such as things
like a signal receiver filter (e.g. task ID or set of threads which
should process the signal etc.).

So if there's no strong objection to the additional field in
perf_event_attr, I think it'll give us the simplest and most flexible
option.

Thanks,
-- Marco

> Thanks,
>
>         Ingo