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

From: Tejun Heo
Date: Mon Nov 09 2015 - 13:02:16 EST

Hello, Oleg.

> 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.

We simply need to reimplement cgroup freezer so that its userland
visible state is well defined (most likely jobctl stop). Right now,
it's allowing userland to trigger "stuck somewhere in the kernel"
condition, so interactions with frozen tasks are naturally broken.


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