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Date: Wed, 21 Oct 2015 22:44:19 +0200 From: Oleg Nesterov <oleg@...hat.com> To: Andrew Morton <akpm@...ux-foundation.org> Cc: Dmitry Vyukov <dvyukov@...gle.com>, Alexander Potapenko <glider@...gle.com>, Denys Vlasenko <dvlasenk@...hat.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 10/21, Andrew Morton wrote: > > On Wed, 21 Oct 2015 19:41:50 +0200 Oleg Nesterov <oleg@...hat.com> wrote: > > > On 10/20, Andrew Morton wrote: > > > > > > On Tue, 20 Oct 2015 19:17:54 +0200 Oleg Nesterov <oleg@...hat.com> 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. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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