[PATCH v4 12/12] sched,signal,ptrace: Rework TASK_TRACED, TASK_STOPPED state

[Sven Schnelle]

Peter Zijlstra <peterz at infradead.org> writes:

> On Wed, Jul 06, 2022 at 09:58:55AM +0200, Sven Schnelle wrote:
>
>> >> [   86.218551] kill_chi-343805    6d.... 79990141us : ptrace_stop: JOBCTL_TRACED already set, state=0 <------ valid combination of flags?
>> >
>> > Yeah, that's not supposed to be so. JOBCTL_TRACED is supposed to follow
>> > __TASK_TRACED for now. Set when __TASK_TRACED, cleared when
>> > TASK_RUNNING.
>> >
>> > Specifically {ptrace_,}signal_wake_up() in signal.h clear JOBCTL_TRACED
>> > when they would wake a __TASK_TRACED task.
>> 
>> try_to_wake_up() clears TASK_TRACED in this case because a signal
>> (SIGKILL) has to be delivered. As a test I put the following change
>> on top, and it "fixes" the problem:
>> 
>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
>> index da0bf6fe9ecd..f2e0f5e70e77 100644
>> --- a/kernel/sched/core.c
>> +++ b/kernel/sched/core.c
>> @@ -4141,6 +4149,9 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
>>          * TASK_WAKING such that we can unlock p->pi_lock before doing the
>>          * enqueue, such as ttwu_queue_wakelist().
>>          */
>> +       if (p->__state & TASK_TRACED)
>> +               trace_printk("clearing TASK_TRACED 2\n");
>> +       p->jobctl &= ~JOBCTL_TRACED;
>>         WRITE_ONCE(p->__state, TASK_WAKING);
>> 
>>         /*
>> 
>> There are several places where the state is changed from TASK_TRACED to
>> something else without clearing JOBCTL_TRACED.
>
> I'm having difficulty spotting them; I find:
>
> TASK_WAKEKILL: signal_wake_up()
> __TASK_TRACED: ptrace_signal_wake_up(), ptrace_unfreeze_traced(), ptrace_resume()
>
> And all those sites dutifully clear JOBCTL_TRACED.
>
> I'd be most interested in the calstack for the 'clearing TASK_TRACED 2'
> events to see where we miss a spot.

The calltrace is:
[    9.863613] Call Trace:
[    9.863616]  [<00000000d3105f0e>] try_to_wake_up+0xae/0x620
[    9.863620] ([<00000000d3106164>] try_to_wake_up+0x304/0x620)
[    9.863623]  [<00000000d30d1e46>] ptrace_unfreeze_traced+0x9e/0xa8
[    9.863629]  [<00000000d30d2ef0>] __s390x_sys_ptrace+0xc0/0x160
[    9.863633]  [<00000000d3c5d8f4>] __do_syscall+0x1d4/0x200
[    9.863678]  [<00000000d3c6c332>] system_call+0x82/0xb0
[    9.863685] Last Breaking-Event-Address:
[    9.863686]  [<00000000d3106176>] try_to_wake_up+0x316/0x620
[    9.863688] ---[ end trace 0000000000000000 ]---

ptrace_unfreeze_traced() is:

static void ptrace_unfreeze_traced(struct task_struct *task)
{
        unsigned long flags;

        /*
         * The child may be awake and may have cleared
         * JOBCTL_PTRACE_FROZEN (see ptrace_resume).  The child will
         * not set JOBCTL_PTRACE_FROZEN or enter __TASK_TRACED anew.
         */
        if (lock_task_sighand(task, &flags)) {
                task->jobctl &= ~JOBCTL_PTRACE_FROZEN;
                if (__fatal_signal_pending(task)) {
                        task->jobctl &= ~TASK_TRACED;

Looking at this, shouldn't the line above read task->jobctl &= ~JOBCTL_TRACED?

                        wake_up_state(task, __TASK_TRACED);
                }
                unlock_task_sighand(task, &flags);
        }
}