linux-kernel - Re: [2.6.27.24] Kernel coredump to a pipe is failing

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Message-ID: <20090527183109.GA30574@redhat.com>
Date:	Wed, 27 May 2009 20:31:09 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	paul@...-scientist.net, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Roland McGrath <roland@...hat.com>
Subject: Re: [2.6.27.24] Kernel coredump to a pipe is failing

On 05/26, Andi Kleen wrote:
>
> When a signal happens during core dump the core dump to a pipe
> can fail, because the write returns short, but the ELF core dumpers
> cannot handle that.
>
> There's no reason to handle signals during core dumping, so just
> block them all.

Actually, I think there is a strong reason to handle signals during
core dumping. The coredump can take a lot of time/resources, not good
it looks like unkillable procees to users.

Please look at

	killable/interruptible coredumps
	http://marc.info/?l=linux-kernel&m=121665710711931

at least, I think SIGKILL should terminate core dumping.

> Open issue: ELF puts blocked signals into the core dump and
> that will be always fully blocked now.  Need to save it somewhere?
>
> --- linux-2.6.30-rc5-ak.orig/fs/exec.c	2009-05-14 11:46:24.000000000 +0200
> +++ linux-2.6.30-rc5-ak/fs/exec.c	2009-05-26 22:22:12.000000000 +0200
> @@ -1760,6 +1760,12 @@
>  		goto fail;
>  	}
>
> +	/* block all signals */
> +	spin_lock_irq(&current->sighand->siglock);
> +	sigfillset(&current->blocked);
> +	/* No recalc sigpending */
> +	spin_unlock_irq(&current->sighand->siglock);

Perhaps it makes sense to do

	--- a/kernel/signal.c
	+++ b/kernel/signal.c
	@@ -644,6 +644,7 @@ static int prepare_signal(int sig, struc
			/*
			 * The process is in the middle of dying, nothing to do.
			 */
	+		 return 0;
		} else if (sig_kernel_stop(sig)) {
			/*
			 * This is a stop signal.  Remove SIGCONT from all queues.

instead, this was discussed before. This way the exiting/coredumping
task ignores all signals, and we cab simplify complete_signal() a bit.


This all needs more discussion, but imho for now something like
Paul's patch http://marc.info/?l=linux-kernel&m=124340506200729
is the best workaround. Note that we have the same dump_write()
in binfmt_elf.c and binfmt_aout.c, perhaps it makes sense to
create coredump_file_write() helper in fs/exec.c.

Oleg.

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