Re: [PATCH 1/2] wait/ptrace: always assume __WALL if the child is traced

[Oleg Nesterov <oleg@redhat.com>]

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.