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

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Date:   Thu, 25 Mar 2021 15:18:20 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Dmitry Vyukov <dvyukov@...gle.com>
Cc:     Marco Elver <elver@...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


* 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?

Thanks,

	Ingo