linux-kernel - Re: [PATCH 1/2] coredump: ensure all coredumping tasks have

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Date:	Mon, 5 Oct 2015 18:25:38 +0200
From:	Michal Hocko <mhocko@...nel.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	David Rientjes <rientjes@...gle.com>,
	Kyle Walker <kwalker@...hat.com>,
	Stanislav Kozina <skozina@...hat.com>,
	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] coredump: ensure all coredumping tasks have

The subject is truncated. It is missing SIGNAL_GROUP_COREDUMP

On Tue 29-09-15 17:55:02, Oleg Nesterov wrote:
> task_will_free_mem() is wrong in many ways, and in particular the
> SIGNAL_GROUP_COREDUMP check is not reliable: a task can participate
> in the coredumping without SIGNAL_GROUP_COREDUMP bit set.
> 
> change zap_threads() paths to always set SIGNAL_GROUP_COREDUMP even
> if other CLONE_VM processes can't react to SIGKILL. Fortunately, at
> least oom-kill case if fine; it kills all tasks sharing the same mm,
> so it should also kill the process which actually dumps the core.

Yes I do not think it will make too much difference for the oom killer
but it is much better to handle all the processes sharing the mm the
same way.

> 
> The change in prepare_signal() is not strictly necessary, it just
> ensures that the patch does not bring another subtle behavioural
> change. But it reminds us that this SIGNAL_GROUP_EXIT/COREDUMP case
> needs more changes.
> 
> Signed-off-by: Oleg Nesterov <oleg@...hat.com>

Acked-by: Michal Hocko <mhocko@...e.com>

> ---
>  fs/coredump.c   | 12 ++++++------
>  kernel/signal.c |  2 +-
>  2 files changed, 7 insertions(+), 7 deletions(-)
> 
> diff --git a/fs/coredump.c b/fs/coredump.c
> index 53d7d46..4fed8d0 100644
> --- a/fs/coredump.c
> +++ b/fs/coredump.c
> @@ -282,11 +282,13 @@ out:
>  	return ispipe;
>  }
>  
> -static int zap_process(struct task_struct *start, int exit_code)
> +static int zap_process(struct task_struct *start, int exit_code, int flags)
>  {
>  	struct task_struct *t;
>  	int nr = 0;
>  
> +	/* ignore all signals except SIGKILL, see prepare_signal() */
> +	start->signal->flags = SIGNAL_GROUP_COREDUMP | flags;
>  	start->signal->group_exit_code = exit_code;
>  	start->signal->group_stop_count = 0;
>  
> @@ -313,10 +315,8 @@ static int zap_threads(struct task_struct *tsk, struct mm_struct *mm,
>  	spin_lock_irq(&tsk->sighand->siglock);
>  	if (!signal_group_exit(tsk->signal)) {
>  		mm->core_state = core_state;
> -		nr = zap_process(tsk, exit_code);
>  		tsk->signal->group_exit_task = tsk;
> -		/* ignore all signals except SIGKILL, see prepare_signal() */
> -		tsk->signal->flags = SIGNAL_GROUP_COREDUMP;
> +		nr = zap_process(tsk, exit_code, 0);
>  		clear_tsk_thread_flag(tsk, TIF_SIGPENDING);
>  	}
>  	spin_unlock_irq(&tsk->sighand->siglock);
> @@ -367,8 +367,8 @@ static int zap_threads(struct task_struct *tsk, struct mm_struct *mm,
>  			if (p->mm) {
>  				if (unlikely(p->mm == mm)) {
>  					lock_task_sighand(p, &flags);
> -					nr += zap_process(p, exit_code);
> -					p->signal->flags = SIGNAL_GROUP_EXIT;
> +					nr += zap_process(p, exit_code,
> +							  SIGNAL_GROUP_EXIT);
>  					unlock_task_sighand(p, &flags);
>  				}
>  				break;
> diff --git a/kernel/signal.c b/kernel/signal.c
> index f2cbd4e..c0b01fe 100644
> --- a/kernel/signal.c
> +++ b/kernel/signal.c
> @@ -788,7 +788,7 @@ static bool prepare_signal(int sig, struct task_struct *p, bool force)
>  	sigset_t flush;
>  
>  	if (signal->flags & (SIGNAL_GROUP_EXIT | SIGNAL_GROUP_COREDUMP)) {
> -		if (signal->flags & SIGNAL_GROUP_COREDUMP)
> +		if (!(signal->flags & SIGNAL_GROUP_EXIT))
>  			return sig == SIGKILL;
>  		/*
>  		 * The process is in the middle of dying, nothing to do.
> -- 
> 2.4.3

-- 
Michal Hocko
SUSE Labs
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