bpf: Add a way to mark functions as requiring privilege

This is horribly incomplete:

 - I only marked one function as requiring privilege, and there are
   surely more.

 - Checking is_priv is probably not the right thing to do.  This should
   probably do something more clever.  At the very lease, it needs to
   integrate with the upcoming lockdown LSM infrastructure.

 - The seen_privileged_funcs mechanism is probably not a good solution.
   Instead we should check something while we still have enough context
   to give a good error message.  But we *don't* want to check for
   capabilities up front before even seeing a function call, since we
   don't want to inadvertently generate audit events for privileges that
   are never used.

So it's the idea that counts :)

Signed-off-by: Andy Lutomirski <luto@kernel.org>
4 files changed