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

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Message-ID: <20151028190235.GA31914@redhat.com>
Date:	Wed, 28 Oct 2015 20:02:35 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Pedro Alves <palves@...hat.com>
Cc:	Denys Vlasenko <vda.linux@...glemail.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dmitry Vyukov <dvyukov@...gle.com>,
	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>,
	Robert Swiecki <swiecki@...gle.com>,
	Roland McGrath <roland@...k.frob.com>,
	syzkaller@...glegroups.com,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"gdb@...rceware.org" <gdb@...rceware.org>
Subject: Re: [PATCH 1/2] wait/ptrace: always assume __WALL if the child is
	traced

On 10/28, Pedro Alves wrote:
>
> On 10/28/2015 04:11 PM, Oleg Nesterov wrote:
> > On 10/26, Pedro Alves wrote:
> >>
> >> (Also, in the original test case, if the child gets/raises a signal or execs
> >> before exiting, the bash/init/whatever process won't be issuing PTRACE_CONT,
> >> and the child will thus end up stuck (though should be SIGKILLable,
> >
> > Oh, but if it is killable everything is fine. How does this differ from the
> > case when, say, you jusr reparent to init and do kill(getpid(), SIGSTOP) ?
>
> The difference is that if the child called PTRACE_TRACEME, then it goes
> to ptrace-stop instead and no amount of SIGCONT unstucks it -- the only way
> out is force killing.  I agree it's not a major issue as there's a way out
> (and thus made it a parens), but I wouldn't call it nice either.

IOW, the difference is that it is TASK_TRACED, not TASK_STOPPED. I agree,
this is not nice. But this is not nice simply because PTRACE_TRACEME is
not nice.

> >> All this because PTRACE_TRACEME is broken by design
> >
> > Heh. I agree. But we can't fix it now.
>
> Perhaps the man page could document it as deprecated, suggesting
> PTRACE_ATTACH/PTRACE_SEIZE instead?

I don't know. but I won't mind if you mark PTRACE_ATTACH as deprecated
too ;) PTRACE_SEIZE can be used instead and it doesn't abuse SIGSTOP.

Oleg.

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