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Date: Tue, 20 Oct 2015 15:31:55 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Oleg Nesterov <oleg@redhat.com>
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>, syzkaller@googlegroups.com,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/2] wait/ptrace: always assume __WALL if the child is
 traced
Message-Id: <20151020153155.e03f4219da4014efe6f810b0@linux-foundation.org>
In-Reply-To: <20151020171754.GA29304@redhat.com>
References: <20151020171740.GA29290@redhat.com>
	<20151020171754.GA29304@redhat.com>
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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.