From: Oleg Nesterov <oleg@redhat.com>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Dmitry Vyukov <dvyukov@google.com>,
Alexander Potapenko <glider@google.com>,
Denys Vlasenko <dvlasenk@redhat.com>,
Eric Dumazet <edumazet@google.com>,
Jan Kratochvil <jan.kratochvil@redhat.com>,
Julien Tinnes <jln@google.com>, Kees Cook <keescook@google.com>,
Kostya Serebryany <kcc@google.com>,
Linus Torvalds <torvalds@linux-foundation.org>,
"Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>,
Pedro Alves <palves@redhat.com>,
Robert Swiecki <swiecki@google.com>,
Roland McGrath <roland@hack.frob.com>,
syzkaller@googlegroups.com, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/2] wait/ptrace: always assume __WALL if the child is traced
Date: Wed, 21 Oct 2015 22:44:19 +0200 [thread overview]
Message-ID: <20151021204419.GA31402@redhat.com> (raw)
In-Reply-To: <20151021124737.53825611940d7a353fee2bca@linux-foundation.org>
On 10/21, Andrew Morton wrote:
>
> On Wed, 21 Oct 2015 19:41:50 +0200 Oleg Nesterov <oleg@redhat.com> wrote:
>
> > On 10/20, Andrew Morton wrote:
> > >
> > > On Tue, 20 Oct 2015 19:17:54 +0200 Oleg Nesterov <oleg@redhat.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.
next prev parent reply other threads:[~2015-10-21 20:47 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-10-20 17:17 [PATCH 0/2] wait/ptrace: always assume __WALL if the child is traced Oleg Nesterov
2015-10-20 17:17 ` [PATCH 1/2] " Oleg Nesterov
2015-10-20 22:31 ` Andrew Morton
2015-10-21 3:27 ` Vasily Averin
2015-10-21 17:41 ` Oleg Nesterov
2015-10-21 19:47 ` Andrew Morton
2015-10-21 20:44 ` Oleg Nesterov [this message]
2015-10-21 19:59 ` Denys Vlasenko
2015-10-21 20:31 ` Denys Vlasenko
2015-10-21 21:47 ` Oleg Nesterov
2015-10-21 23:27 ` Denys Vlasenko
2015-10-25 15:54 ` Oleg Nesterov
2015-10-26 12:08 ` Pedro Alves
2015-10-28 16:11 ` Oleg Nesterov
2015-10-28 15:43 ` Pedro Alves
2015-10-28 19:02 ` Oleg Nesterov
2015-10-22 13:51 ` Denys Vlasenko
2015-10-20 17:17 ` [PATCH 2/2] wait: allow sys_waitid() to use __WNOTHREAD/__WCLONE/__WALL Oleg Nesterov
2015-10-20 17:36 ` [PATCH 0/2] wait/ptrace: always assume __WALL if the child is traced Oleg Nesterov
2015-10-22 14:40 ` Pedro Alves
2015-10-25 15:42 ` Oleg Nesterov
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