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

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Message-ID: <5628E9E7.4020500@redhat.com>
Date:	Thu, 22 Oct 2015 15:51:35 +0200
From:	Denys Vlasenko <dvlasenk@...hat.com>
To:	Oleg Nesterov <oleg@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dmitry Vyukov <dvyukov@...gle.com>
CC:	Alexander Potapenko <glider@...gle.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 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.

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