Messages in this thread | | | Date | Fri, 26 Nov 2010 16:40:09 +0100 | From | Oleg Nesterov <> | Subject | Re: [PATCH 05/14] signal: fix premature completion of group stop when interfered by ptrace |
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On 11/26, Tejun Heo wrote: > > task->signal->group_stop_count is used to tracke the progress of group > stop. It's initialized to the number of tasks which need to stop for > group stop to finish and each stopping or trapping task decrements. > However, each task doesn't keep track of whether it decremented the > counter or not and if woken up before the group stop is complete and > stops again, it can decrement the counter multiple times.
Everything is fine without ptrace, I hope.
(ignoring the deprecated CLONE_STOPPED)
> Please consider the following example code.
I didn't even read the test-case ;)
Yes, known problems. ptrace is very wrong when it comes to group_stop_count/SIGNAL_STOP_STOPPED/etc. Almost everything is wrong.
Cough, this is fixed in utrace ;) It doesn't use ptrace_stop/ ptrace_resume/etc at all.
> This patch adds a new field task->group_stop which is protected by > siglock and uses GROUP_STOP_CONSUME flag to track which task is still > to consume group_stop_count to fix this bug.
Yes, currently the tracee never knows how it should react to ->group_stop_count.
> @@ -1645,7 +1658,7 @@ static void ptrace_stop(int exit_code, int clear_code, siginfo_t *info) > * we must participate in the bookkeeping. > */ > if (current->signal->group_stop_count > 0) > - --current->signal->group_stop_count; > + consume_group_stop();
This doesn't look exactly right. If we participate (decrement the counter), we should stop even if we race with ptrace_detach().
And what if consume_group_stop() returns true? We should set SIGNAL_STOP_STOPPED and notify ->parent.
Otherwise looks correct at first glance...
Of course, there are more problems. To me, even ptrace_resume()->wake_up_process() is very wrt jctl.
Cosmetic nit,
> +static bool consume_group_stop(void) > +{ > + if (!(current->group_stop & GROUP_STOP_CONSUME)) > + return false; > + > + current->group_stop &= ~GROUP_STOP_CONSUME; > + > + if (!WARN_ON_ONCE(current->signal->group_stop_count == 0)) > + current->signal->group_stop_count--;
Every caller should check ->group_stop_count != 0. do_signal_stop() does this too in fact. May be it would be cleaner to move this check into consume_group_stop() and remove WARN_ON_ONCE().
This way it is more clear why prepare_signal(SIGCONT) do not reset task->group_stop, it has no effect unless ->group_stop_count is set by do_signal_stop() which updates ->group_stop for every thread.
Probably consume_group_stop() should also set SIGNAL_STOP_STOPPED if it returns true.
But, I didn't read the next patches yet...
Oleg.
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