strace lockup when tracing exec in go

[Michal Hocko]

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