Messages in this thread | | | Date | Fri, 23 Sep 2016 15:40:59 +0200 | From | Michal Hocko <> | Subject | Re: strace lockup when tracing exec in go |
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On Fri 23-09-16 15:21:02, Oleg Nesterov wrote: > On 09/23, Michal Hocko wrote: > > > > On Fri 23-09-16 12:21:41, Oleg Nesterov wrote: > > > On 09/22, Michal Hocko wrote: > > > > > > > > --- a/kernel/signal.c > > > > +++ b/kernel/signal.c > > > > @@ -91,6 +91,10 @@ static int sig_ignored(struct task_struct *t, int sig, bool force) > > > > if (!sig_task_ignored(t, sig, force)) > > > > return 0; > > > > > > > > + /* Do not ignore signals sent from child to the parent */ > > > > + if (current->ptrace && current->parent == t) > > > > + return 0; > > > > > > This doesn't look right in general, and this can't really help. > > > > > > This assumes that the tracer will call do_wait() after mm_access() > > > fails, but this is not necessarily true. > > > > > > Note also ptrace_attach(), -ERESTARTNOINTR means that the tracer won't > > > even return to user-space if SIGCHLD is ignored, the tracer will silently > > > restart the syscall. > > > > Well, it apparently does help the strace case. > > Only because strace doesn't even try to handle -EINTR; it assumes this is not > possible, gives up, and calls wait() after that. So this change actually > breaks strace.
Hmm, OK. In this case process_vm_readv wouldn't give a usuful data which still sounds better to me than a hang.
> And once again, this can't really help. SIGCHLD can come before strace calls > process_vm_readv(), and in this case it will enter the syscall without > signal_pending() == T. IOW, this hack can only help if the tracer already > sleeps in process_vm_readv().
True.
> Plus, again, "strace -f" can equally hang if mt-exec races with PTRACE_ATTACH. > > > So I am not arguing this > > is the best fix but can it be harmful? > > This change is simply wrong no matter what.
I've just tried to extend the existing
/* * Tracers may want to know about even ignored signals. */ return !t->ptrace;
but I probably just do not understand what that actually means. I thought that the tracer is _really_ interested in hearing about the signal.
> We could change do_notify_parent() > to call signal_wake_up() if tsk->ptrace, but see above, this won't help.
So does this mean WONTFIX? Can we at least document this behavior? It surely is unexpected.
-- Michal Hocko SUSE Labs
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