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

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Date:   Thu, 25 Mar 2021 16:35:33 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Marco Elver <elver@...gle.com>
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


* Marco Elver <elver@...gle.com> wrote:

> > 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.

Sounds good to me - it's also probably measurably faster than copying 
the not-so-small-anymore perf_attr structure.

Thanks,

	Ingo