Merge branch 'bpf-fix-null-arg-semantics'

Gianluca Borello says:

====================
This set includes some fixes in semantics and usability issues that emerged
recently, and would be good to have them in net before the next release.

In particular, ARG_CONST_SIZE_OR_ZERO semantics was recently changed in
commit 9fd29c08e520 ("bpf: improve verifier ARG_CONST_SIZE_OR_ZERO
semantics") with the goal of letting the compiler generate simpler code
that the verifier can more easily accept.

To handle this change in semantics, a few checks in some helpers were
added, like in commit 9c019e2bc4b2 ("bpf: change helper bpf_probe_read arg2
type to ARG_CONST_SIZE_OR_ZERO"), and those checks are less than ideal
because once they make it into a released kernel bpf programs can start
relying on them, preventing the possibility of being removed later on.

This patch tries to fix the issue by introducing a new argument type
ARG_PTR_TO_MEM_OR_NULL that can be used for helpers that can receive a
<NULL, 0> tuple. By doing so, we can fix the semantics of the other helpers
that don't need <NULL, 0> and can just handle <!NULL, 0>, allowing the code
to get rid of those checks.
====================

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>