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Subject: Re: [PATCH 1/2] wait/ptrace: always assume __WALL if the child is
 traced
To: syzkaller@googlegroups.com, Oleg Nesterov <oleg@redhat.com>
References: <20151020171740.GA29290@redhat.com>
 <20151020171754.GA29304@redhat.com>
 <20151020153155.e03f4219da4014efe6f810b0@linux-foundation.org>
Cc: Dmitry Vyukov <dvyukov@google.com>,
        Alexander Potapenko <glider@google.com>,
        Denys Vlasenko <dvlasenk@redhat.com>,
        Eric Dumazet <edumazet@google.com>,
        Jan Kratochvil <jan.kratochvil@redhat.com>,
        Julien Tinnes <jln@google.com>, Kees Cook <keescook@google.com>,
        Kostya Serebryany <kcc@google.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>,
        Pedro Alves <palves@redhat.com>, Robert Swiecki <swiecki@google.com>,
        Roland McGrath <roland@hack.frob.com>, linux-kernel@vger.kernel.org
From: Vasily Averin <vvs@virtuozzo.com>
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Date: Wed, 21 Oct 2015 06:27:39 +0300
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On 21.10.2015 01:31, Andrew Morton wrote:
> On Tue, 20 Oct 2015 19:17:54 +0200 Oleg Nesterov <oleg@redhat.com> wrote:
> 
>> The following program (simplified version of generated by syzkaller)
>>
>> 	#include <pthread.h>
>> 	#include <unistd.h>
>> 	#include <sys/ptrace.h>
>> 	#include <stdio.h>
>> 	#include <signal.h>
>>
>> 	void *thread_func(void *arg)
>> 	{
>> 		ptrace(PTRACE_TRACEME, 0,0,0);
>> 		return 0;
>> 	}
>>
>> 	int main(void)
>> 	{
>> 		pthread_t thread;
>>
>> 		if (fork())
>> 			return 0;
>>
>> 		while (getppid() != 1)
>> 			;
>>
>> 		pthread_create(&thread, NULL, thread_func, NULL);
>> 		pthread_join(thread, NULL);
>> 		return 0;
>> 	}
>>
>> creates the unreapable zombie if /sbin/init doesn't use __WALL.
>>
>> This is not a kernel bug, at least in a sense that everything works as
>> expected: debugger should reap a traced sub-thread before it can reap
>> the leader, but without __WALL/__WCLONE do_wait() ignores sub-threads.
>>
>> Unfortunately, it seems that /sbin/init in most (all?) distributions
>> doesn't use it and we have to change the kernel to avoid the problem.
> 
> Well, to fix this a distro needs to roll out a new kernel.  Or a new
> init(8).  Is there any reason to believe that distributing/deploying a
> new kernel is significantly easier for everyone?  Because fixing init
> sounds like a much preferable solution to this problem.

Patched kernel allows to run obsoleted distro inside containers.