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Date: Thu, 22 Oct 2015 15:51:35 +0200
From: Denys Vlasenko <dvlasenk@redhat.com>
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To: Oleg Nesterov <oleg@redhat.com>, Andrew Morton <akpm@linux-foundation.org>,
        Dmitry Vyukov <dvyukov@google.com>
CC: Alexander Potapenko <glider@google.com>,
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        Linus Torvalds <torvalds@linux-foundation.org>,
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        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
References: <20151020171754.GA29304@redhat.com>
In-Reply-To: <20151020171754.GA29304@redhat.com>
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On 10/20/2015 07:17 PM, Oleg Nesterov wrote:
> 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.
> 
> This patch just adds the "ptrace" check into eligible_child(). To some
> degree this matches the "tsk->ptrace" in exit_notify(), ->exit_signal
> is mostly ignored when the tracee reports to debugger.
> 
> This obviously means the user-visible change: __WCLONE and __WALL no
> longer have any meaning for debugger. And I can only hope that this
> won't break something.
> 
> We could make a more conservative change. Say, we can take __WCLONE
> into account, or !thread_group_leader(). But it would be nice to not
> complicate these historical/confusing checks.

For the record, I like this way of fixing the problem too.

It removes the need to ever use __WALL in userspace:
IIRC the only case where it was needed are debuggers.

Which in turn makes *all* underscored wait flags (__WCLONE, __WALL,
__WNOTHREAD) unnecessary for any sane application use.