Re: [PATCH v2 07/12] ptrace: Don't change __state

Oleg Nesterov
2022-05-03T13:41:50Z

On 05/02, Eric W. Biederman wrote:
>
> Oleg Nesterov <oleg@redhat.com> writes:
>
> >> #define TASK_KILLABLE (TASK_WAKEKILL | TASK_UNINTERRUPTIBLE)
> >> #define TASK_STOPPED (TASK_WAKEKILL | __TASK_STOPPED)
> >> -#define TASK_TRACED (TASK_WAKEKILL | __TASK_TRACED)
> >> +#define TASK_TRACED __TASK_TRACED
> > ...
> >> static inline void signal_wake_up(struct task_struct *t, bool resume)
> >> {
> >> - signal_wake_up_state(t, resume ? TASK_WAKEKILL : 0);
> >> + unsigned int state = 0;
> >> + if (resume) {
> >> + state = TASK_WAKEKILL;
> >> + if (!(t->jobctl & JOBCTL_PTRACE_FROZEN))
> >> + state |= __TASK_TRACED;
> >> + }
> >> + signal_wake_up_state(t, state);
> >
> > Can't understand why is this better than the previous version which removed
> > TASK_WAKEKILL if resume... Looks a bit strange to me. But again, I didn't
> > look at the next patches yet.
>
> The goal is to replace the existing mechanism with an equivalent one,
> so that we don't have to be clever and deal with it being slightly
> different in one case.
>
> The difference is how does signal_pending_state affect how schedule will
> sleep in ptrace_stop.

But why is it bad if the tracee doesn't sleep in schedule ? If it races
with SIGKILL. I still can't understand this.

Yes, wait_task_inactive() can fail, so you need to remove WARN_ON_ONCE()
in 11/12.

Why is removing TASK_WAKEKILL from TASK_TRACED and complicating
*signal_wake_up() better?

And even if we need to ensure the tracee will always block after
ptrace_freeze_traced(), we can change signal_pending_state() to
return false if JOBCTL_PTRACE_FROZEN. Much simpler, imo. But still
looks unnecessary to me.

> Peter's plans to fix PREEMPT_RT or the freezer wait_task_inactive needs
> to cope with the final being changed by something else. (TASK_FROZEN in
> the freezer case). I can only see that happening by removing the
> dependency on the final state in wait_task_inactive. Which we can't do
> if we depend on wait_task_inactive failing if the process is in the
> wrong state.

OK, I guess this is what I do not understand. Could you spell please?

And speaking of RT, wait_task_inactive() still can fail because
cgroup_enter_frozen() takes css_set_lock? And it is called under
preempt_disable() ? I don't understand the plan :/

> At a practical level I think it also has an impact on patch:
> "10/12 ptrace: Only return signr from ptrace_stop if it was provided".

I didn't look at JOBCTL_PTRACE_SIGNR yet. But this looks minor to me,
I mean, I am not sure it worth the trouble.

Oleg.