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SubjectRe: PATCH? fix SIGNAL_STOP_DEQUEUED vs SIGCONT race
On 09/01, Roland McGrath wrote:
>
> > However, this changes the behaviour when the task is ptraced. If the debugger
> > doesn't clear ->exit_code, SIGSTOP always succeeds after ptrace_stop(), even
> > if SIGCONT was sent in between. I can't decide whether this change is good
> > or bad, hopefully Roland can clarify.
>
> Hmm. I think this is bad.
>
> First, considering only the single-threaded case, there are debugger vs
> SIGCONT races. Someone does kill(pid,SIGSTOP);kill(pid,SIGCONT); while pid
> is debugged. The mandate for end user behavior here is that pid cannot
> wind up sitting in job control stop in the end. Say the debugger is
> e.g. strace, simply printing every signal and passing it through.
> So say it goes:
> T K D
> merrily running ... blocked in wait4
> kill(K, SIGSTOP)
> dequeue SIGSTOP
> -> ptrace_stop
> wait4 -> K,{SIGSTOP}
> kill(K, SIGCONT)
> PTRACE_CONT,K,SIGSTOP
> do_signal_stop(SIGSTOP)
> wait4 -> K,{SIGSTOP}

Thanks Roland.

Yes, that is what I was worrying about.

> It's still probably a worthwhile cleanup to have the logic only in
> get_signal_to_deliver, and to fix the problem you cited. It will take only
> a little extra code to handle the ptrace case too, i.e.
> if (sig_kernel_stop(signr) &&
> current->sighand->action[signr-1] == SIG_DFL &&
> !(current->signal->flags & SIGNAL_GROUP_EXIT))
> current->signal->flags |= SIGNAL_STOP_DEQUEUED;
> ptrace_stop(signr, signr, info);
> if (sig_kernel_stop(signr) && current->exit_code == signr &&
> !(current->signal->flags & SIGNAL_STOP_DEQUEUED) &&
> current->sighand->action[signr-1] == SIG_DFL)
> current->exit_code = 0;

Yes, I also thought about something like this, but tried to avoid because
it adds some complications. OTOH, this is not the fast path.

I'll try to think a bit more about this, and update the patch according
to your comments. Looks like we don't need to check SIGNAL_GROUP_EXIT
here, we are doing this later anyway.

Thanks!

Oleg.

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