Only reparent a traced process to its old parent if the tracing process is not the old parent.

[jhb]

Summary

Only reparent a traced process to its old parent if the tracing process is
not the old parent. Otherwise, proc_reap() will leave the zombie in place
resulting in the process' status being returned twice to its parent.

Add test cases for PT_TRACE_ME and PT_ATTACH which are fixed by
this change.

Test Plan

• I wrote an ATF test case for this that is included in the change. It currently fails as the last waitpid does not fail, but returns the child process a second time (verified via ktrace).

[kib]

Isn't the same situation occur when PT_ATTACH is performed on the direct child ?

Myself, I would fix the issue by the check in proc_reap(), comparing the p_oppid to the parent pid. Note that PT_DETACH already does the same check.

[jhb]

Mmm, how about I fix in both places? That is, fix PT_ATTACH to only set oppid hen it reparents, but also add the check in proc_reap() (or we could change proc_reap() to assert that if oppid is set it doesn't match the existing parent).

[kib]

In D2594#48133, @jhb wrote:

Mmm, how about I fix in both places? That is, fix PT_ATTACH to only set oppid hen it reparents, but also add the check in proc_reap() (or we could change proc_reap() to assert that if oppid is set it doesn't match the existing parent).

I sounds good either way. If you change PT_ATTACH, then assert in the proc_reap() seems to be most logical.

[jhb]

Hmm, I actually think I will just fix proc_reap(). It is a smaller change. Otherwise I'd need to fix PT_DETACH to ignore a oppid of 0. Note that procfs only sets oppid if it reparents, but it always tries to reparent in detach. I will fix procfs to always set oppid in my next update, though I probably won't bother testing procfs, just ptrace.

[jhb]

• Add the same double-wait test for PT_ATTACH().
• Revert previous and check for parent == tracer in exit.

[jhb]

Tested.

[kib]

sys/kern/kern_exit.c

856 ↗

(On Diff #5503)

I believe that p_oppid must be cleared if not zero, regardless of its value.

[jhb]

• Always clear p_oppid.