linux-kernel - Re: WARNING in do_jobctl

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Date:	Wed, 27 Jan 2016 17:04:57 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Dmitry Vyukov <dvyukov@...gle.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	"Amanieu d'Antras" <amanieu@...il.com>,
	Richard Weinberger <richard@....at>,
	Davidlohr Bueso <dave@...olabs.net>,
	Vladimir Davydov <vdavydov@...allels.com>,
	Palmer Dabbelt <palmer@...belt.com>,
	Andy Lutomirski <luto@...capital.net>,
	LKML <linux-kernel@...r.kernel.org>,
	syzkaller <syzkaller@...glegroups.com>,
	Kostya Serebryany <kcc@...gle.com>,
	Alexander Potapenko <glider@...gle.com>,
	Sasha Levin <sasha.levin@...cle.com>,
	Eric Dumazet <edumazet@...gle.com>
Subject: Re: WARNING in do_jobctl_trap

On 01/26, Oleg Nesterov wrote:
>
> On 01/26, Dmitry Vyukov wrote:
> >
> > Hello,
> >
> > The following program triggers WARN_ON_ONCE(!signr) in do_jobctl_trap:
>
> Can't reproduce, but most probably I understand the problem... I'll try to
> make the simplified test-case tomorrow, it should work if I am right.

This triggers the same problem immediately:


	#include <unistd.h>
	#include <sys/ptrace.h>
	#include <sys/wait.h>

	void test(void)
	{
		for (;;) {
			if (fork()) {
				wait(NULL);
				continue;
			}

			ptrace(PTRACE_SEIZE, getppid(), 0, 0);
			ptrace(PTRACE_INTERRUPT, getppid(), 0, 0);
			_exit(0);
		}
	}

	int main(void)
	{
		int np;

		for (np = 0; np < 8; ++np)
			if (!fork())
				test();

		while (wait(NULL) > 0)
			;
		return 0;
	}

and just in case, your test-case works too if I run

	perl -e 'fork; fork; fork; 1 while 1' &

in the background, it has too many delays to work on the idle system.

> Does it work for you reliably?  If yes, any chance you can try the patch
> below?
>
> Oleg.
>
> --- x/kernel/ptrace.c
> +++ x/kernel/ptrace.c
> @@ -73,11 +73,11 @@ void __ptrace_unlink(struct task_struct *child)
>  {
>  	BUG_ON(!child->ptrace);
>
> -	child->ptrace = 0;
>  	child->parent = child->real_parent;
>  	list_del_init(&child->ptrace_entry);
>
>  	spin_lock(&child->sighand->siglock);
> +	child->ptrace = 0;
>
>  	/*
>  	 * Clear all pending traps and TRAPPING.  TRAPPING should be

The change above helps, but I need to think more...

Oleg.