linux-kernel - Re: [PATCH 1/2] signal: Always notice exiting tasks

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Date:   Mon, 11 Feb 2019 15:13:40 +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 1/2] signal: Always notice exiting tasks

sorry again for delay...

On 02/07, Eric W. Biederman wrote:
>
> --- a/kernel/signal.c
> +++ b/kernel/signal.c
> @@ -2393,6 +2393,11 @@ bool get_signal(struct ksignal *ksig)
>  		goto relock;
>  	}
>  
> +	/* Has this task already been marked for death? */
> +	ksig->info.si_signo = signr = SIGKILL;
> +	if (signal_group_exit(signal))
> +		goto fatal;
> +
>  	for (;;) {
>  		struct k_sigaction *ka;
>  
> @@ -2488,6 +2493,7 @@ bool get_signal(struct ksignal *ksig)
>  			continue;
>  		}
>  
> +	fatal:
>  		spin_unlock_irq(&sighand->siglock);

Eric, but this is wrong. At least this is the serious user-visible change.

Afaics, with this patch the tracee will never stop in PTRACE_EVENT_EXIT in case
of group_exit/exec, because schedule() in TASK_TRACED state won't block due to
__fatal_signal_pending().

Yes, yes, as I said many times the semantics of PTRACE_EVENT_EXIT was never really
defined, it depends on /dev/random, but still I don't think we should break it even
more.

Oleg.