On Mon, Nov 19, 2018 at 1:21 PM, Christian Brauner [off-list ref] wrote:

That can be done without a loop by comparing the level counter for the
two pid namespaces.

quoted

And you can rewrite pidns_get_parent to use it. So you would instead be
doing:

    if (pidns_is_descendant(proc_pid_ns, task_active_pid_ns(current)))
        return -EPERM;

(Or you can just copy the 5-line loop into procfd_signal -- though I
imagine we'll need this for all of the procfd_* APIs.)

Why is any of this even necessary? Why does the child namespace we're
considering even have a file descriptor to its ancestor's procfs? If
it has one of these FDs, it can already *read* all sorts of
information it really shouldn't be able to acquire, so the additional
ability to send a signal (subject to the usual permission checks)
feels like sticking a finger in a dike that's already well-perforated.
IMHO, we shouldn't bother with this check. The patch would be simpler
without it.