Re: [PATCH v3 00/13] Virtually mapped stacks with guard pages (x86, core)

Oleg Nesterov
2016-06-23T14:31:31Z

On 06/22, Linus Torvalds wrote:
>
> Oleg, what do you think? Would it be reasonable to free the stack and
> thread_info synchronously at exit time, clear the pointer (to catch
> any odd use), and only RCU-delay the task_struct itself?

I didn't see the patches yet, quite possibly I misunderstood... But no,
I don't this we can do this (if we are not going to move ti->flags to
task_struct at least).

> (Obviously, we can't release it in do_exit() itself like we do some of
> the other state - it would need to be released after we've scheduled
> away to another process' stack, but we already have that TASK_DEAD
> handling in finish_task_switch for this exact reason).

Yes, but the problem is that a zombie thread can do its last schedule
before it is reaped.

Just for example, syscall_regfunc() does

read_lock(&tasklist_lock);
for_each_process_thread(p, t) {
set_tsk_thread_flag(t, TIF_SYSCALL_TRACEPOINT);
}
read_unlock(&tasklist_lock);

and this can easily hit a TASK_DEAD thread with ->stack == NULL.

And we can't free/nullify it when the parent/debuger reaps a zombie,
say, mark_oom_victim() expects that get_task_struct() protects
thread_info as well.

Oleg.