linux-kernel - Re: [RFC PATCH] kernel/signal.c: avoid BUG

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Date:	Wed, 29 May 2013 10:19:32 -0700
From:	David Daney <ddaney.cavm@...il.com>
To:	James Hogan <james.hogan@...tec.com>
CC:	linux-kernel@...r.kernel.org, linux-mips@...ux-mips.org,
	Ralf Baechle <ralf@...ux-mips.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Kees Cook <keescook@...omium.org>
Subject: Re: [RFC PATCH] kernel/signal.c: avoid BUG_ON with SIG128 (MIPS)

On 05/29/2013 10:01 AM, James Hogan wrote:
> MIPS has 128 signals, the highest of which has the number 128. The

I wonder if we should change the ABI and reduce the number of signals to 
127 instead of this patch.

David Daney



> following command causes get_signal_to_deliver() to pass this signal
> number straight through to do_group_exit() as the exit code:
>
>    strace sleep 10 & sleep 1 && kill -128 `pidof sleep`
>
> However do_group_exit() checks for the core dump bit (0x80) in the exit
> code which matches in this particular case and the kernel panics:
>
>    BUG_ON(exit_code & 0x80); /* core dumps don't get here */
>
> This is worked around by changing get_signal_to_deliver() to pass
> min(info->si_signo, 127) instead of info->si_signo, so that this highest
> of signal numbers get rounded down to 127. This makes the exit code
> technically incorrect, but it's better than killing the whole kernel.
>
> Signed-off-by: James Hogan <james.hogan@...tec.com>
> Cc: Ralf Baechle <ralf@...ux-mips.org>
> Cc: Al Viro <viro@...iv.linux.org.uk>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Oleg Nesterov <oleg@...hat.com>
> Cc: Kees Cook <keescook@...omium.org>
> ---
>
> This is based on v3.10-rc3.
>
> It's a little hacky, but aside from reducing the number of signals to
> 127 to avoid this case (which isn't backwards compatible) I'm not sure
> what else can be done. Any comments?
>
>   kernel/signal.c | 6 +++++-
>   1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/signal.c b/kernel/signal.c
> index 113411b..69bc00f 100644
> --- a/kernel/signal.c
> +++ b/kernel/signal.c
> @@ -2366,8 +2366,12 @@ relock:
>
>   		/*
>   		 * Death signals, no core dump.
> +		 *
> +		 * MIPS has a signal number 128 which clashes with the core dump
> +		 * bit. If this was the signal we still want to report a valid
> +		 * exit code, so round it down to 127.
>   		 */
> -		do_group_exit(info->si_signo);
> +		do_group_exit(min(info->si_signo, 127));
>   		/* NOTREACHED */
>   	}
>   	spin_unlock_irq(&sighand->siglock);
>

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