Oleg Nesterov <o...@redhat.com> writes:
2> On 04/26, Eric W. Biederman wrote:
>>
>> static void ptrace_unfreeze_traced(struct task_struct *task)
>> {
>> - if (READ_ONCE(task->__state) != __TASK_TRACED)
>> + if (!(READ_ONCE(task->jobctl) & JOBCTL_DELAY_WAKEKILL))
>> return;
>>
>> WARN_ON(!task->ptrace || task->parent != current);
>> @@ -213,11 +213,10 @@ static void ptrace_unfreeze_traced(struct task_struct
>> *task)
>> * Recheck state under the lock to close this race.
>> */
>> spin_lock_irq(&task->sighand->siglock);
>
> Now that we do not check __state = __TASK_TRACED, we need lock_task_sighand().
> The tracee can be already woken up by ptrace_resume(), but it is possible that
> it didn't clear DELAY_WAKEKILL yet.
Yes. The subtle differences in when __TASK_TRACED and
JOBCTL_DELAY_WAKEKILL are cleared are causing me some minor issues.
This "WARN_ON(!task->ptrace || task->parent != current);" also now
needs to be inside siglock, because the __TASK_TRACED is insufficient.
> Now, before we take ->siglock, the tracee can exit and another thread can do
> wait() and reap this task.
>
> Also, I think the comment above should be updated. I agree, it makes sense to
> re-check JOBCTL_DELAY_WAKEKILL under siglock just for clarity, but we no
> longer
> need to do this to close the race; jobctl &= ~JOBCTL_DELAY_WAKEKILL and
> wake_up_state() are safe even if JOBCTL_DELAY_WAKEKILL was already
> cleared.
I think you are right about it being safe, but I am having a hard time
convincing myself that is true. I want to be very careful sending
__TASK_TRACED wake_ups as ptrace_stop fundamentally can't handle
spurious wake_ups.
So I think adding task_is_traced to the test to verify the task
is still frozen.
static void ptrace_unfreeze_traced(struct task_struct *task)
{
unsigned long flags;
/*
* Verify the task is still frozen before unfreezing it,
* ptrace_resume could have unfrozen us.
*/
if (lock_task_sighand(task, &flags)) {
if ((task->jobctl & JOBCTL_DELAY_WAKEKILL) &&
task_is_traced(task)) {
task->jobctl &= ~JOBCTL_DELAY_WAKEKILL;
if (__fatal_signal_pending(task))
wake_up_state(task, __TASK_TRACED);
}
unlock_task_sighand(task, &flags);
}
}
>> @@ -2307,6 +2307,7 @@ static int ptrace_stop(int exit_code, int why, int
>> clear_code,
>>
>> /* LISTENING can be set only during STOP traps, clear it */
>> current->jobctl &= ~JOBCTL_LISTENING;
>> + current->jobctl &= ~JOBCTL_DELAY_WAKEKILL;
>
> minor, but
>
> current->jobctl &= ~(JOBCTL_LISTENING | JOBCTL_DELAY_WAKEKILL);
>
> looks better.
Yes.
Eric
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