On Fri, Sep 4, 2020 at 1:06 PM Mateusz Guzik <mjgu...@gmail.com> wrote:
> On 9/4/20, Vitaliy Makkoveev <m...@openbsd.org> wrote:
> > On Fri, Sep 04, 2020 at 05:24:42PM +0200, Mateusz Guzik wrote:
> >> getppid blindly follows the parent pointer and reads the pid.
> >>
> >> The problem is that ptrace reparents the traced process, so in
> >> particular if you gdb -p $something, the target proc will start seeing
> >> gdb instead of its actual parent.
> >>
> >> There is a lot to say about the entire reparenting business or storing
> >> the original pid in ps_oppid (instead of some form of a reference to
> >> the process).
> >>
> >> However, I think the most feasible fix for now is the same thing
> >> FreeBSD did: *always* store the actual parent pid in ps_oppid. This
> >> means all repareting will keep updating it (most notably when
> >> abandoning children on exit), while ptrace will skip that part.
> >>
> >> Side effect of such a change be that getppid will stop requiring the
> >> kernel lock.
> >>
> >
> > Thanks for report. But we are in beta stage now so such modification is
> > impossible until next iteration.
> >
> > Since original parent identifier is stored as `ps_oppid' while process
> > is traced we just return it to userland for this case. This is the way I
> > propose to fix this bug for now.
> >
> > Comments? OKs?
> >
> > Index: sys/kern/kern_prot.c
> > ===================================================================
> > RCS file: /cvs/src/sys/kern/kern_prot.c,v
> > retrieving revision 1.76
> > diff -u -p -r1.76 kern_prot.c
> > --- sys/kern/kern_prot.c 9 Jul 2019 12:23:25 -0000 1.76
> > +++ sys/kern/kern_prot.c 4 Sep 2020 21:12:15 -0000
> > @@ -84,7 +84,11 @@ int
> > sys_getppid(struct proc *p, void *v, register_t *retval)
> > {
> >
> > - *retval = p->p_p->ps_pptr->ps_pid;
> > + if (p->p_p->ps_flags & PS_TRACED)
> > + *retval = p->p_p->ps_oppid;
> > + else
> > + *retval = p->p_p->ps_pptr->ps_pid;
> > +
> > return (0);
> > }
>
> This is definitely a bare minimum fix, but it does the job.
>
ptrace() has behaved like this for the life of OpenBSD and an indefinite
number of years previous in the BSD releases. What has happened that a
definitely incomplete fix is needed Right Now?
Philip Guenther
