Re: wait(2) and SIGCHLD

[Johnny Billquist <bqt%softjar.se@localhost>]

On 2020-08-16 21:17, Mouse wrote:
> > > They don't vanish, they get reparented to init(8) which then wakes
> > > up and reaps them.
> > That probably would work, approximately,
>
> Well, it does work, to at least a first approximation.
>
> > but isn't what's supposed to happen when a child's parent is ignoring
> > SIGCHLD - the child should skip zombie state, and simply be cleaned
> > up.
>
> And how is "reparent to init" not an acceptable means of implementing
> that?

Acceptable or not, it would seem to not match our own documentation.

From the sigaction() man-page:

     SA_NOCLDWAIT   If set, the system will not create a zombie when 
the child
                    exits, but the child process will be automatically 
waited
                    for.  The same effect can be achieved by setting the
                    signal handler for SIGCHLD to SIG_IGN.

> > The difference would be detectable if init were sent a SIGSTOP
> > (assuming that isn't one which would cause a system panic)
>
> I don't think it would panic, but I think that, if it really does stop
> init, it's a bug that it does so.  I thought I'd seen some code that
> rendered init immune to SIGKILL and possibly SIGSTOP too (maybe by
> forcing them into init's blocked-signals set? I forget).  But I can't
> seem to find it now.

SIGSTOP is one of two signals that a process supposedly should not be 
able to intercept. Of course, init is special enough that normal rules 
might not apply...

> > so it would stop reaping children (temporarily) - processes of the
> > type in question should not be showing up as zombies.
>
> Right, they shouldn't be.  But init shouldn't be stopped, either.
>
> Similarly, I think it should be impossible to ptrace init, and I have a
> fuzzy memory that it was on at least one system I tried it on.
>
> I'll be poking around a bit more.

How special do one really want init to be?

  Johnny

--
Johnny Billquist                  || "I'm on a bus
                                  ||  on a psychedelic trip
email: bqt%softjar.se@localhost             ||  Reading murder books
pdp is alive!                     ||  tryin' to stay hip" - B. Idol