linux-kernel - Re: [PATCH v2 2/5] sched,ptrace: Fix ptrace_check_attach() vs PREEMPT

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Message-ID: <20220428205956.GG15485@redhat.com>
Date:   Thu, 28 Apr 2022 22:59:57 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     "Eric W. Biederman" <ebiederm@...ssion.com>, rjw@...ysocki.net,
        mingo@...nel.org, vincent.guittot@...aro.org,
        dietmar.eggemann@....com, rostedt@...dmis.org, mgorman@...e.de,
        bigeasy@...utronix.de, Will Deacon <will@...nel.org>,
        linux-kernel@...r.kernel.org, tj@...nel.org,
        linux-pm@...r.kernel.org
Subject: Re: [PATCH v2 2/5] sched,ptrace: Fix ptrace_check_attach() vs
 PREEMPT_RT

On 04/28, Peter Zijlstra wrote:
>
> Oleg pointed out that the tracee can already be killed such that
> fatal_signal_pending() is true. In that case signal_wake_up_state()
> cannot be relied upon to be responsible for the wakeup -- something
> we're going to want to rely on.

Peter, I am all confused...

If this patch is against the current tree, we don't need it.

If it is on top of JOBCTL_TRACED/DELAY_WAKEKILL changes (yours or Eric's),
then it can't help - SIGKILL can come right after the tracee drops siglock
and calls schedule().

Perhaps I missed something, but let me repeat the 3rd time: I'd suggest
to simply clear JOBCTL_TRACED along with LISTENING/DELAY_WAKEKILL before
return to close this race.

Oleg.

> --- a/kernel/signal.c
> +++ b/kernel/signal.c
> @@ -2226,6 +2226,10 @@ static int ptrace_stop(int exit_code, in
>  		spin_lock_irq(&current->sighand->siglock);
>  	}
>  
> +	/* Don't stop if the task is dying. */
> +	if (unlikely(__fatal_signal_pending(current)))
> +		return exit_code;
> +
>  	/*
>  	 * schedule() will not sleep if there is a pending signal that
>  	 * can awaken the task.
>