linux-kernel - Re: [PATCH v3 06/11] perf: Add support for SIGTRAP on perf events

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Date:   Mon, 29 Mar 2021 16:27:06 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Marco Elver <elver@...gle.com>, alexander.shishkin@...ux.intel.com,
        acme@...nel.org, mingo@...hat.com, jolsa@...hat.com,
        mark.rutland@....com, namhyung@...nel.org, tglx@...utronix.de,
        glider@...gle.com, viro@...iv.linux.org.uk, arnd@...db.de,
        christian@...uner.io, dvyukov@...gle.com, jannh@...gle.com,
        axboe@...nel.dk, mascasa@...gle.com, pcc@...gle.com,
        irogers@...gle.com, kasan-dev@...glegroups.com,
        linux-arch@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        linux-kselftest@...r.kernel.org, Jiri Olsa <jolsa@...nel.org>
Subject: Re: [PATCH v3 06/11] perf: Add support for SIGTRAP on perf events

On 03/29, Peter Zijlstra wrote:
>
> On Thu, Mar 25, 2021 at 09:14:39AM +0100, Marco Elver wrote:
> > @@ -6395,6 +6395,13 @@ static void perf_sigtrap(struct perf_event *event)
> >  {
> >  	struct kernel_siginfo info;
> >
> > +	/*
> > +	 * This irq_work can race with an exiting task; bail out if sighand has
> > +	 * already been released in release_task().
> > +	 */
> > +	if (!current->sighand)
> > +		return;

This is racy. If "current" has already passed exit_notify(), current->parent
can do release_task() and destroy current->sighand right after the check.

> Urgh.. I'm not entirely sure that check is correct, but I always forget
> the rules with signal. It could be we ought to be testing PF_EXISTING
> instead.

Agreed, PF_EXISTING check makes more sense in any case, the exiting task
can't receive the signal anyway.

Oleg.