On 9/5/20, Philip Guenther <guent...@gmail.com> wrote:
> 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?
>
I don't see how this reads as a demand this is fixed Right Now.
I don't see how the fix is incomplete either. It can be done better
with more effort, but AFAICS the above results in correct behavior.
--
Mateusz Guzik <mjguzik gmail.com>
