Re: ptrace() hangs on attempt to seize/attach stopped & frozen task

From: Oleg Nesterov
Date: Mon Nov 09 2015 - 12:59:06 EST


On 11/09, Andrey Ryabinin wrote:
> Hi,
> So, the ptrace() hangs if we try to attach to stopped task from freezing cgroup.
> It seems this was introduced by 5d8f72b55c2756("freezer: change ptrace_stop/do_signal_stop to use freezable_schedule()").

quite possible...

> See below for the exact scenario and small script to reproduce this.
> Tracee: Tracer:
> static bool do_signal_stop(int signr)
> __set_current_state(TASK_STOPPED);
> freezable_schedule();
> freezer_do_not_count();
> schedule(); /* waiting for wake up */
> ptrace_attach()
> if (task_is_stopped(task) &&
> task_set_jobctl_pending(task, JOBCTL_TRAP_STOP | JOBCTL_TRAPPING))
> signal_wake_up_state(task, __TASK_STOPPED);
> /* woken up by ptrace_attach() */
> freezer_count();
> __refrigerator()
> /* And here we will hang, because tracee is now frozen in __refrigerator() */
> wait_on_bit(&task->jobctl, JOBCTL_TRAPPING_BIT,

Thanks. All I can say I never liked this wait_on_bit() ;)

I need to think, but *at first glance* we can move this wait-for-stopped-
traced-transition into do_wait() path, and this way clear_jobctl_trapping()
can use __wake_up_parent(). Perhaps we just need to modify task_stopped_code()
to take JOBCTL_TRAPPING into account...

Sure, debugger will block in sys_wait() after PTRACE_ATTACH/SEIZE. But this
does not really differ from the case when the tracee was already frozen;
SIGSTOP sent by ATTACH or PTRACE_INTERRUPT, so debugger will equally block
in do_wait() until the tracee is unfrozen.


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