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

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Message-ID: <Ymr5Ga3gcqG4ZAMt@hirez.programming.kicks-ass.net>
Date:   Thu, 28 Apr 2022 22:29:13 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Oleg Nesterov <oleg@...hat.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 Tue, Apr 26, 2022 at 07:24:03PM -0500, Eric W. Biederman wrote:
> But doing:
> 
> 	/* Don't stop if the task is dying */
> 	if (unlikely(__fatal_signal_pending(current)))
> 		return exit_code;
> 
> Should work.

Something like so then...

---
Subject: signal,ptrace: Don't stop dying tasks
From: Peter Zijlstra <peterz@...radead.org>
Date: Thu Apr 28 22:17:56 CEST 2022

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.

As such, explicitly handle this case.

Suggested-by: "Eric W. Biederman" <ebiederm@...ssion.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
 kernel/signal.c |    4 ++++
 1 file changed, 4 insertions(+)

--- 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.