linux-kernel - Re: [PATCH 01/17] signal: Make SIGKILL during coredumps an explicit special case

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Date: Fri, 21 Jun 2024 12:40:32 +0200
From: Oleg Nesterov <oleg@...hat.com>
To: "Eric W. Biederman" <ebiederm@...ssion.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, Tejun Heo <tj@...nel.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 01/17] signal: Make SIGKILL during coredumps an explicit
 special case

Another case when I can hardly understand your reply...

This patch adds a minor user visible change, that was my point.

If you say that the new behaviour is better / more consistent -
I won't really argue, "I expect no one cares" below is probably
true. In my opinion group_exit_code = SIGKILL makes more sense
in this special case, but again, I won't insist.

But then this change should be mentioned and explained in the
changelog, agree?

As for "zap_threads that tests if SIGNAL_GROUP_EXIT is already set",
this is another thing but probably I misundertood you. It is not that
zap_threads/zap_process do not set ->group_exit_code in this case,
in this case do_coredump() will be aborted.

And to remind, zap_threads() used to set SIGNAL_GROUP_COREDUMP, not
SIGNAL_GROUP_EXIT. Because to me the coredumping process is not exiting
yet, it tries to handle the coredumping signal. That is why I prefer
group_exit_code = SIGKILL if it is killed during the dump. But this is
slightly offtopic today.

Oleg.

On 06/21, Eric W. Biederman wrote:
>
> Oleg Nesterov <oleg@...hat.com> writes:
>
> > On 06/19, Eric W. Biederman wrote:
> >>
> >> Oleg Nesterov <oleg@...hat.com> writes:
> >>
> >> > Hi Eric,
> >> >
> >> > I'll _try_ to read this (nontrivial) changes this week. To be honest,
> >> > right now I don't really understand your goals after the quick glance...
> >> >
> >> > So far I have only looked at this simple 1/17 and it doesn't look right
> >> > to me.
> >>
> >> It might be worth applying them all on a branch and just looking at the
> >> end result.
> >
> > Perhaps. Say, the next 2/17 patch. I'd say it is very difficult to understand
> > the purpose unless you read the next patches. OK, at least the change log
> > mentions "in preparation".
> >
> >> > 	- complete_signal() won't be called, so signal->group_exit_code
> >> > 	  won't be updated.
> >> >
> >> > 	  coredump_finish() won't change it too so the process will exit
> >> > 	  with group_exit_code == signr /* coredumping signal */.
> >> >
> >> > 	  Yes, the fix is obvious and trivial...
> >>
> >> The signal handling from the coredump is arguably correct.  The process
> >> has already exited, and gotten an exit code.
> >
> > And zap_process() sets roup_exit_code = signr. But,
> >
> >> But I really don't care about the exit_code either way.  I just want to
> >> make ``killing'' a dead process while it core dumps independent of
> >> complete_signal.
> >>
> >> That ``killing'' of a dead process is a completely special case.
> >
> > Sorry I fail to understand...
> >
> > If the coredumping process is killed by SIGKILL, it should exit with
> > group_exit_code = SIGKILL, right? At least this is what we have now.
>
> In general when a fatal signal is sent:
> - It is short circuit delivered.
> - If SIGNAL_GROUP_EXIT is not set
>    SIGNAL_GROUP_EXIT is set
>    signal->group_exit_code is set
>
> Under those rules group_exit_code should not be updated.  Frankly no
> signals should even be processed (except to be queued) after a fatal
> signal comes in.
>
> There is an issue that short circuit delivery does not work on coredump
> signals (because of the way the coredump code works).  So it winds up
> being zap_threads that tests if SIGNAL_GROUP_EXIT is already set and
> zap_process that sets SIGNAL_GROUP_EXIT.  Essentially the logic remains
> the same, and importantly no later signal is able to set
> group_exit_code.  Or really have any effect because the signal sent was
> fatal.
>
> Except except except when the kernel in the context of the userspace
> process is writing a coredump for that userspace process.  Then we allow
> the kernel to be sent SIGKILL to stop it's coredumping activities
> because sometimes it can block indefinitely otherwise.
>
> Which is why I call handling that SIGKILL after a coredump has
> begun and SIGNAL_GROUP_EXIT is already set a completely special case.
>
> We might have to change group_exit_code to SIGKILL in that special case,
> if someone in userspace cares.  However I expect no one cares.
>
> Further if adding support for SIGKILL during a coredump were being added
> from scratch.  The semantics I would choose would be for that SIGKILL
> and indeed all of the coredumping activity would be invisible to
> userspace except for the delay to make it happen.  Otherwise a coredump
> which every occasionally gets it's return code changed could introduce
> heisenbugs.
>
> But none of this is documented in the change description and at a bare
> minimum this change of behavior should be before such code is merged.
>
> Eric
>