linux-kernel - Re: [PATCH] Discard notification signals when a tracer exits

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Date:	Tue, 25 Mar 2008 15:37:58 +0100
From:	Petr Tesarik <ptesarik@...e.cz>
To:	linux-kernel@...r.kernel.org
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Roland McGrath <roland@...hat.com>,
	Oleg Nesterov <oleg@...sign.ru>
Subject: Re: [PATCH] Discard notification signals when a tracer exits

On Tue, 2008-03-25 at 15:31 +0100, Petr Tesarik wrote:
> When a tracer exits without detaching from the traced process, the
> tracee may be at a tracer notification stop and will thus interpret
> the value in task->exit_code (SIGTRAP | 0x80) as the signal to be
> delivered.
> 
> This patch fixes the problem by clearing exit_code when detaching
> the traced process from a dying tracer.
> 
> Signed-off-by: Petr Tesarik <ptesarik@...e.cz>
> 

Oh, and here is a testing script for the first hunk. It fails on all
kernels I have tried. The second hunk can also be tested if you run
strace on the traced process instead of attaching to a running one, but
I didn't figure out how to get the PID of the traced process within a
script, so you'd have to trigger the bug manually.

> ---
>  exit.c |    8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> --- a/kernel/exit.c
> +++ b/kernel/exit.c
> @@ -642,8 +642,10 @@ reparent_thread(struct task_struct *p, s
>  			/*
>  			 * If it was at a trace stop, turn it into
>  			 * a normal stop since it's no longer being
> -			 * traced.
> +			 * traced.  Cancel the notification signal,
> +			 * or the tracee may get a SIGTRAP.
>  			 */
> +			p->exit_code = 0;
>  			ptrace_untrace(p);
>  		}
>  	}
> @@ -713,6 +715,10 @@ static void forget_original_parent(struc
>  			p->real_parent = reaper;
>  			reparent_thread(p, father, 0);
>  		} else {
> +			/* cancel the notification signal at a trace stop */
> +			if (p->state == TASK_TRACED)
> +				p->exit_code = 0;
> +
>  			/* reparent ptraced task to its real parent */
>  			__ptrace_unlink (p);
>  			if (p->exit_state == EXIT_ZOMBIE && p->exit_signal != -1 &&
> 
> --
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