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);

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