linux-kernel - Re: [PATCH 1/2] wait/ptrace: always assume _

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Message-Id: <20151020153155.e03f4219da4014efe6f810b0@linux-foundation.org>
Date:	Tue, 20 Oct 2015 15:31:55 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Dmitry Vyukov <dvyukov@...gle.com>,
	Alexander Potapenko <glider@...gle.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Jan Kratochvil <jan.kratochvil@...hat.com>,
	Julien Tinnes <jln@...gle.com>,
	Kees Cook <keescook@...gle.com>,
	Kostya Serebryany <kcc@...gle.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>,
	Pedro Alves <palves@...hat.com>,
	Robert Swiecki <swiecki@...gle.com>,
	Roland McGrath <roland@...k.frob.com>,
	syzkaller@...glegroups.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] wait/ptrace: always assume __WALL if the child is
 traced

On Tue, 20 Oct 2015 19:17:54 +0200 Oleg Nesterov <oleg@...hat.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.
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