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Date: Wed, 21 Oct 2015 19:41:50 +0200
From: Oleg Nesterov <oleg@redhat.com>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Dmitry Vyukov <dvyukov@google.com>,
        Alexander Potapenko <glider@google.com>,
        Denys Vlasenko <dvlasenk@redhat.com>,
        Eric Dumazet <edumazet@google.com>,
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        Linus Torvalds <torvalds@linux-foundation.org>,
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        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: <20151021174150.GA24546@redhat.com>
References: <20151020171740.GA29290@redhat.com> <20151020171754.GA29304@redhat.com> <20151020153155.e03f4219da4014efe6f810b0@linux-foundation.org>
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On 10/20, 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.

I will be happy if we decide that this is userpace problem and we should
not fix the kernel. I simply do not know.

However, please look at 2/2 which imho makes sense regardless and looks
"obviously safe". Without this patch waitid() can not use __WALL, so if
/sbin/init uses waitid() then the userspace fix won't be one-liner. And
at least Fedora22 and Ubuntu use waitid().

So personally I'd prefer 2/2 + fix-init, not sure if this can work...

Oleg.