From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S966170AbbJ1Pnj (ORCPT ); Wed, 28 Oct 2015 11:43:39 -0400 Received: from mx1.redhat.com ([209.132.183.28]:48702 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752668AbbJ1PnY (ORCPT ); Wed, 28 Oct 2015 11:43:24 -0400 Message-ID: <5630ED16.50900@redhat.com> Date: Wed, 28 Oct 2015 15:43:18 +0000 From: Pedro Alves User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.7.0 MIME-Version: 1.0 To: Oleg Nesterov CC: Denys Vlasenko , Denys Vlasenko , Andrew Morton , Dmitry Vyukov , Alexander Potapenko , Eric Dumazet , Jan Kratochvil , Julien Tinnes , Kees Cook , Kostya Serebryany , Linus Torvalds , "Michael Kerrisk (man-pages)" , Robert Swiecki , Roland McGrath , syzkaller@googlegroups.com, Linux Kernel Mailing List , "gdb@sourceware.org" Subject: Re: [PATCH 1/2] wait/ptrace: always assume __WALL if the child is traced References: <20151020171740.GA29290@redhat.com> <20151020171754.GA29304@redhat.com> <20151020153155.e03f4219da4014efe6f810b0@linux-foundation.org> <5627EE9E.8040600@redhat.com> <5627F607.4050506@redhat.com> <20151021214703.GA1810@redhat.com> <20151025155440.GB2043@redhat.com> <562E17D8.4000108@redhat.com> <20151028161152.GA24042@redhat.com> In-Reply-To: <20151028161152.GA24042@redhat.com> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 10/28/2015 04:11 PM, Oleg Nesterov wrote: > On 10/26, Pedro Alves wrote: >> >> On 10/25/2015 03:54 PM, Oleg Nesterov wrote: >>> >>> In any case, the real question is whether we should change the kernel to >>> fix the problem, or ask the distros to fix their init's. In the former >>> case 1/2 looks simpler/safer to me than the change in ptrace_traceme(), >>> and you seem to agree that 1/2 is not that bad. >> >> A risk here seems to be that waitpid will start returning unexpected >> (thread) PIDs to parent processes, > > I don't see how this change can make the things worse, > >> and it's not unreasonable to assume >> that e.g., a program asserts that waitpid either returns error or a >> known (process) PID. > > Well. /sbin/init can never assume this, obviously. Right. I was actually thinking of !init processes -- basically code that spawns helper processes, keeps a data structure indexed by pid, then discards the structure when the child exits. Something like: pid = waitpid(-1, &status, 0); if (pid > 0) { struct child_process *child = find_process(pid); assert (child != NULL); } As in, before your change, the child could get stuck forever, but after your change, the parent could die/assert instead. But ... > >> That's not an init-only issue, > > Yes. Because we have CLONE_PARENT. So "waitpid either returns error or a > known (process) PID" is only true if you trust your children. ... OK, that's indeed a good point. >> (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. >> 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? Thanks, Pedro Alves