linux-kernel - Re: [RFC][PATCH 11/11] signal: Ignore all but multi-process signals that come in during fork.

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Date:   Thu, 12 Jul 2018 15:42:20 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, Wen Yang <wen.yang99@....com.cn>,
        majiang <ma.jiang@....com.cn>
Subject: Re: [RFC][PATCH 11/11] signal: Ignore all but multi-process signals
 that come in during fork.

On 07/11, Eric W. Biederman wrote:
>
> Oleg Nesterov <oleg@...hat.com> writes:
>
> >> -	recalc_sigpending();
> >> -	if (signal_pending(current)) {
> >> +	if (read_seqcount_retry(&current->signal->multi_process_seq, seq) ||
> >> +	    fatal_signal_pending(current)) {
> >>  		retval = -ERESTARTNOINTR;
> >>  		goto bad_fork_cancel_cgroup;
> >
> > So once again, I think this is not right, see the discussion on
> > bugzilla.
>
> I am trying to dig through and understand your concerns.  I am having
> difficulty understanding your concerns.
>
> Do the previous patches look good to you?

Yes, yes, personally I like 1-10 after a quick glance. I'll try to read this
series carefully later, but I don't think I will find something really wrong.

> If I understand you correctly.  Your concern is that since we added the:
>
> 	recalc_sigpending();
>         if (signal_pending(current))
>         	return -ERESTARTNOINTR;
>
> Other (non-signal) code such as the freezer has come to depend upon that
> test.  Changing the test in the proposed way will allow the new child to
> escape the freezer, as it is not guaranteed the new child will be
> frozen.

Yes.

>
> It seems reasonable to look at other things that set TIF_SIGPENDING and
> see if any of them are broken by the fork changes.

Again, please look at do_signal_stop(). If it was the source of signal_pending(),
copy_process() should fail. Or we should update the new thread to participate in
group-stop, but then we need to set TIF_SIGPENDING, copy the relevant part of
current->jobctl, and increment ->group_stop_count at least.

> A practical (and fixable) problem with your patch was that you modified
> task->blocked which was then copied to the child.  So all children now
> would start with all signals being blocked.

What are you talking about, this pseudo-code has a lot more bugs ;)

OK, at least I certainly agree that this approach needs more changes in copy_process().

> > This also makes another difference in multi-threaded case, a signal with a handler
> > sent to a forking process will be re-targeted to another thread which can handle it;
> > with your patch this signal will be "blocked" until fork() finishes or until another
> > thread gets TIF_SIGPENDING. Not that I think this is that important,
> > but still.
>
> I would not object to wants_signal deciding that a task in the middle of
> copy_process does not want signals.

This is not enough, we need to signal all in-fork threads...

Oleg.