Re: [PATCH v4 10/12] ptrace: Don't change __state

Oleg Nesterov
2022-05-10T14:23:18Z

On 05/05, Eric W. Biederman wrote:
>
> static void ptrace_unfreeze_traced(struct task_struct *task)
> {
> - if (READ_ONCE(task->__state) != __TASK_TRACED)
> - return;
> -
> - WARN_ON(!task->ptrace || task->parent != current);
> + unsigned long flags;
>
> /*
> - * PTRACE_LISTEN can allow ptrace_trap_notify to wake us up remotely.
> - * Recheck state under the lock to close this race.
> + * 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.
> */
> - spin_lock_irq(&task->sighand->siglock);
> - if (READ_ONCE(task->__state) = __TASK_TRACED) {
> + if (lock_task_sighand(task, &flags)) {

But I still think that a lockless

if (!(task->jobctl & JOBCTL_PTRACE_FROZEN))
return;

check at the start of ptrace_unfreeze_traced() makes sense to avoid
lock_task_sighand() if possible.

And ptrace_resume() can probably clear JOBCTL_PTRACE_FROZEN along with
JOBCTL_TRACED to make this optimization work better. The same for
ptrace_signal_wake_up().

Oleg.