What inherent TOCTOU vulnerability exists with process groups? Process groups are the way to shoot down a collection of processes atomically in Unix. getpgrp re

What inherent TOCTOU vulnerability exists with process groups? Process groups are the way to shoot down a collection of processes atomically in Unix. getpgrp returns the current processes' process group, and killpg(pgrp) sends a signal atomically; so long as the process doesn't switch process groups between those two calls (and using normal APIs only the process can do this itself), there's no race. (IOW, there's no race in `killpg(getpgrp(), signo)`) There are other ways, too, like using SIGHUP delivered when a controlling TTY is closed (not strictly POSIX, but reliable semantics across all the major extant Unix-like systems). cgroups also allows killing a group of processes race-free, but Linux only got this feature (cgroup.kill) in 2021.[1] Before then even systemd would just walk the cgroup PID list and send a signal to each individual process, which was inherently racy. If this was done from PID 1--init--this could maybe be done correctly by ensuring zombie's weren't reaped--preventing PID recycling--while walking the PID list. OTOH, Linux supports a prctl, PR_SET_CHILD_SUBREAPER, to change a reaper, so perhaps even systemd could never guaranteed to not accidentally shoot down an innocent process. [1] Merged https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/kernel/cgroup/cgroup.c?id=661ee6280931548f7b3b887ad26a157474ae5ac4 https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux... I'm not sure what the subsequent release history looks like, but I wouldn't be surprised if some popular LTS distro versions lack this capability.