Nope. Hiding all zombie processes from ps is going to be both confusing and
misleading.

Philip Guenther

On Monday, August 11, 2025, Claudio Jeker <cjeker@diehard.n-r-g.com> wrote:

> On Mon, Aug 11, 2025 at 10:45:05AM +0000, Gerhard Roth wrote:
> > About a year ago, the call to uvm_exit() was moved outside of the
> > KERNEL_LOCK() in the reaper() by mpi@. Now we observed a kernel
> > protection fault that results from this change.
> >
> > In fill_kproc() we read the vmspace pointer (vm) right at the very
> > beginning of the function:
> >
> >         struct vmspace *vm = pr->ps_vmspace;
> >
> > Sometime later, we try to access it:
> >
> >       /* fixups that can only be done in the kernel */
> >       if ((pr->ps_flags & PS_ZOMBIE) == 0) {
> >               if ((pr->ps_flags & PS_EMBRYO) == 0 && vm != NULL)
> >                       ki->p_vm_rssize = vm_resident_count(vm);
> >
> >
> > In the meantime the process might have exited and the reaper() can free
> > the vmspace by calling uvm_exit(). After that, the 'vm' pointer in
> > fill_kproc() points to stale memory. Accessing it will yield a kernel
> > protection fault.
> >
> > BTW: only after freeing the vmspace of the process, the PS_ZOMBIE flag
> > is set by the reaper().
> >
> > I propose to put the reaper()'s call to uvm_exit() back under the
> > kernel lock to avoid the fault.
>
> In my opinion the fill_kproc() code is wrong and it should not look at
> pr->ps_vmspace if the PS_EXITING flag is set for the process.
>
> exit1() sets PS_EXITING flag early on and after that point the vm can be
> purged so the vm_resident_count() is probably wrong anyway.
>
>
> > Gerhard
> >
> >
> > Index: sys/kern/kern_exit.c
> > ===================================================================
> > RCS file: /cvs/src/sys/kern/kern_exit.c,v
> > diff -u -p -u -p -r1.252 kern_exit.c
> > --- sys/kern/kern_exit.c      10 Aug 2025 15:17:57 -0000      1.252
> > +++ sys/kern/kern_exit.c      11 Aug 2025 10:30:57 -0000
> > @@ -498,10 +498,15 @@ reaper(void *arg)
> >               } else {
> >                       struct process *pr = p->p_p;
> >
> > -                     /* Release the rest of the process's vmspace */
> > +                     /*
> > +                      * Release the rest of the process's vmspace
> > +                      * Use the kernel lock to avoid a race with
> fill_kproc()
> > +                      * accessing the vmspace while the process isn't
> yet a
> > +                      * zombie.
> > +                      */
> > +                     KERNEL_LOCK();
> >                       uvm_exit(pr);
> >
> > -                     KERNEL_LOCK();
> >                       if ((pr->ps_flags & PS_NOZOMBIE) == 0) {
> >                               /* Process is now a true zombie. */
> >                               atomic_setbits_int(&pr->ps_flags,
> PS_ZOMBIE);
> >
>
>
>
> --
> :wq Claudio
>
>