linux-kernel - Re: [PATCH 2/2] signal: Better detection of synchronous signals

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Date:   Tue, 12 Feb 2019 18:21:04 +0100
From:   Oleg Nesterov <oleg@...hat.com>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Dmitry Vyukov <dvyukov@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        jolsa@...hat.com, Namhyung Kim <namhyung@...nel.org>,
        luca abeni <luca.abeni@...tannapisa.it>,
        syzkaller <syzkaller@...glegroups.com>
Subject: Re: [PATCH 2/2] signal: Better detection of synchronous signals

On 02/11, Eric W. Biederman wrote:
>
> Oleg Nesterov <oleg@...hat.com> writes:
>
> >> +	/*
> >> +	 * Check if there is another siginfo for the same signal.
> >> +	 */
> >> +	list_for_each_entry_continue(q, &pending->list, list) {
> >> +		if (q->info.si_signo == sync->info.si_signo)
> >> +			goto still_pending;
> >> +	}
> >
> > But this must not be possible? SYNCHRONOUS_MASK doesn't include real-time
> > signals, we can't have 2 siginfo's for the same signal < SIGRTMIN.
>
> Yes for that reason it should be safe to strip that logic out at the
> moment.  I overlooked that when writing the code.
>
> However.  I am not certain that is a limit we actually want to honor
> with synchronous signals.  As it results in a louzy quality of
> implementation.
>
> We start with an instruction in the program being debugged.  In
> principle before that instruction starts we know that no signals
> are pending because they were not delivered to that process.
>
> If we for some reason send signal A to the process and at the same time
> hit a fault that is reported as signal A.  It is currently a race which
> one wins.  I think we could legitimately say that the fault happened
> before signal A was enqueued, and deliver both. It is a bit murkier if
> signal A was blocked.
>
> If we let the enqueued signal A win (as we do today) we have SA_SIGNFO
> that is not useful for describing the fault the instruction generated.
> Which is a really lousy quality of implementation.

I doubt this would be really useful but this doesn't matter right now,

> Which is a long way of saying I think that hunk of code is useful as it
> allows us the possibility of fixing a lousy quality of implementation in
> our code today.

If we ever rework the legacy_queue() logic we can easily add this hunk back.

Until then it complicates the code for no reason imo, just to confuse the reader.

Oleg.