Re: WARNING in task_participate_group_stop

[Jamie Iles]

On Mon, Nov 06, 2017 at 12:56:09PM +0100, Dmitry Vyukov wrote:
> On Mon, Nov 6, 2017 at 12:25 PM, Jamie Iles <jamie.iles@xxxxxxxxxx> wrote:
> > Hi Dmitry,
> >
> > On Mon, Nov 06, 2017 at 12:02:19PM +0100, Dmitry Vyukov wrote:
> >> On Thu, Nov 2, 2017 at 6:01 PM, Oleg Nesterov <oleg@xxxxxxxxxx> wrote:
> >> > On 11/01, Dmitry Vyukov wrote:
> >> >>
> >> >> On Tue, Oct 31, 2017 at 7:34 PM, Oleg Nesterov <oleg@xxxxxxxxxx> wrote:
> >> >> > Hmm. I do not see reproducer in this email...
> >> >>
> >> >> Ah, sorry. You can see full thread with attachments here:
> >> >> https://groups.google.com/forum/#!topic/syzkaller-bugs/EUmYZU4m5gU
> >> >
> >> > Heh. I can't say I enjoyed reading the reproducer ;)
> >> >
> >> >> >> > WARNING: CPU: 0 PID: 1 at kernel/signal.c:340
> >> >> >> > task_participate_group_stop+0x1ce/0x230 kernel/signal.c:340
> >> >> >> > Kernel panic - not syncing: panic_on_warn set ...
> >> >> >> >
> >> >> >> > CPU: 0 PID: 1 Comm: init Not tainted 4.13.0-mm1+ #5
> >> >> >
> >> >> > So this is init process with SIGNAL_UNKILLABLE flag set. And I hope it has
> >> >> > the pending SIGKILL, otherwise there is something else.
> >> >
> >> > From repro.c
> >> >
> >> >         line 111    r[8] = syscall(__NR_ptrace, 0x10ul, r[7]);
> >> >
> >> > this is PTRACE_ATTACH
> >> >
> >> >         line 115        syscall(__NR_ptrace, 0x4200ul, r[7], 0x40000012ul, 0x100012ul);
> >> >
> >> > this is PTRACE_SETOPTIONS and "data" includes PTRACE_O_EXITKILL.
> >> >
> >> > r[7] is initialized at
> >> >
> >> >         line 110      r[7] = *(uint32_t*)0x20f9cffc;
> >> >
> >> > so if it is eq to 1 then it can attach to init and in this case the problem
> >> > can be explained by the wrong SIGNAL_UNKILLABLE/SIGKILL logic.
> >> >
> >> > But how *(uint32_t*)0x20f9cffc can be 1 ?
> >> >
> >> >         line 108    r[6] = syscall(__NR_fcntl, r[1], 0x10ul, 0x20f9cff8ul);
> >> >
> >> > this is F_GETOWN_EX, addr = 0x20f9cff8 == 0x20f9cffc + 4, so if fcntl()
> >> > actually succeeds then r[7] == f_owner_ex->pid.
> >> >
> >> > It _can_ be 1, but the reproducer doesn't work for me. If you can reproduce,
> >> > could you try the patch below?
> >>
> >> Hi,
> >>
> >> I would like to understand why you were not able to reproduce it. I
> >> won't be sitting here all the time, and we are tracking hundreds of
> >> bugs across different linux kernels and other OSes, so it's
> >> problematic to do any extensive work on all of them. That's why we try
> >> to provide reproducers.
> >>
> >> I've just tried the repro on the latest upstream
> >> (39dae59d66acd86d1de24294bd2f343fd5e7a625) and it triggered the
> >> WARNING within a second.
> >> Did you use the config provided? Did you use qemu or real hardware?
> >> Can you try in qemu (with -smp>1)?
> >
> > I'm unable to reproduce the warning in qemu with SMP (on a 32 CPU VM).
> > Instead I get the following instant traceback which is different to what
> > you report when run as root:
> 
> 
> Uh, it seems to be racy. I am getting either the WARNING or "attempt
> to kill init" in ~1/5 proportion.
> Please try this simplified program, it triggers the WARNING all the time for me:

I can't reproduce the warning with that reproducer, but I do see that it 
runs and exits once and init is left in state T then the second run ends 
up killing init and the VM crashes.

If I run once and then 'kill -CONT 1' then init goes back to sleeping 
and won't accept a SIGSTOP so it looks like SIGNAL_UNKILLABLE isn't gone 
though.

Jamie