From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932341AbbJUUr4 (ORCPT ); Wed, 21 Oct 2015 16:47:56 -0400 Received: from mx1.redhat.com ([209.132.183.28]:49200 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750846AbbJUUrz (ORCPT ); Wed, 21 Oct 2015 16:47:55 -0400 Date: Wed, 21 Oct 2015 22:44:19 +0200 From: Oleg Nesterov To: Andrew Morton Cc: Dmitry Vyukov , Alexander Potapenko , Denys Vlasenko , Eric Dumazet , Jan Kratochvil , Julien Tinnes , Kees Cook , Kostya Serebryany , Linus Torvalds , "Michael Kerrisk (man-pages)" , Pedro Alves , Robert Swiecki , Roland McGrath , syzkaller@googlegroups.com, linux-kernel@vger.kernel.org Subject: Re: [PATCH 1/2] wait/ptrace: always assume __WALL if the child is traced Message-ID: <20151021204419.GA31402@redhat.com> References: <20151020171740.GA29290@redhat.com> <20151020171754.GA29304@redhat.com> <20151020153155.e03f4219da4014efe6f810b0@linux-foundation.org> <20151021174150.GA24546@redhat.com> <20151021124737.53825611940d7a353fee2bca@linux-foundation.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20151021124737.53825611940d7a353fee2bca@linux-foundation.org> User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 10/21, Andrew Morton wrote: > > On Wed, 21 Oct 2015 19:41:50 +0200 Oleg Nesterov wrote: > > > On 10/20, Andrew Morton wrote: > > > > > > On Tue, 20 Oct 2015 19:17:54 +0200 Oleg Nesterov wrote: > > > > > > > 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. > > > > I will be happy if we decide that this is userpace problem and we should > > not fix the kernel. I simply do not know. > > The kernel patch sounds pretty sketchy - something we should avoid > doing if at all possible. Yes, I agree. > > However, please look at 2/2 which imho makes sense regardless and looks > > "obviously safe". Without this patch waitid() can not use __WALL, so if > > /sbin/init uses waitid() then the userspace fix won't be one-liner. And > > at least Fedora22 and Ubuntu use waitid(). > > 2/2 does look sensible (needs a better changelog if it's to be a > standalone thing), Yes. Without 1/2 the changlelog should menetion that at least __WALL makes sense because /sbin/init has a good reason to use waitid(WALL). Plus it should cc -stable. > but if we're expecting distros to fix this with an > updated init(8) only, then they can't assume that the kernel's waitid() > has been altered. Well, 2/2 looks safe for every kernel version... starting from git history at least. > So init will need the not-one-liner version of the > fix. Then I think this fix will stay forever ;) > > So personally I'd prefer 2/2 + fix-init, not sure if this can work... > > I'm just guessing here. Are you (or someone) able to find out which > approach the distros will prefer, and why? No, I have no idea, sorry. > And what has to be done to init(8) to fix this bug when running current > kernels? Say, http://git.busybox.net/busybox/tree/init/init.c at first glance it just needs - wpid = waitpid(-1, NULL, maybe_WNOHANG); + wpid = waitpid(-1, NULL, maybe_WNOHANG | __WALL); I have a testing machine running Fedora22, according to strace /bin/systemd does waitid(P_ALL, 0, {}, WNOHANG|WEXITED|WNOWAIT, NULL); ... waitid(P_PID, 25558, {INFO}, WEXITED, NULL) so it probably wants siginfo and thus it can't use waitpid. Without 2/2 systemd can probably just do something like while (waitpid(-1, NULL, __WCLONE | WNOHANG) != ESRCH) { log("Dmitry Vyukov detected"); } every time it does waitid() to reap the traced subthreads. Unless of course systemd itself uses ptrace or forks a child with (clone_flags & CSIGNAL) != SIGCHLD. Unlikely, but who knows. In any case I think the fix should be simple. 2/2 can help, most probably systemd can too just add __WALL to wait options. Oleg.