Merge branch 'enable-bpf_skc-cast-for-networking-progs'

Martin KaFai Lau says:

====================
This set allows networking prog type to directly read fields from
the in-kernel socket type, e.g. "struct tcp_sock".

Patch 2 has the details on the use case.

v3:
- Pass arg_btf_id instead of fn into check_reg_type() in Patch 1 (Lorenz)
- Move arg_btf_id from func_proto to struct bpf_reg_types in Patch 2 (Lorenz)
- Remove test_sock_fields from .gitignore in Patch 8 (Andrii)
- Add tests to have better coverage on the modified helpers (Alexei)
  Patch 13 is added.
- Use "void *sk" as the helper argument in UAPI bpf.h

v3:
- ARG_PTR_TO_SOCK_COMMON_OR_NULL was attempted in v2.  The _OR_NULL was
  needed because the PTR_TO_BTF_ID could be NULL but note that a could be NULL
  PTR_TO_BTF_ID is not a scalar NULL to the verifier.  "_OR_NULL" implicitly
  gives an expectation that the helper can take a scalar NULL which does
  not make sense in most (except one) helpers.  Passing scalar NULL
  should be rejected at the verification time.

  Thus, this patch uses ARG_PTR_TO_BTF_ID_SOCK_COMMON to specify that the
  helper can take both the btf-id ptr or the legacy PTR_TO_SOCK_COMMON but
  not scalar NULL.  It requires the func_proto to explicitly specify the
  arg_btf_id such that there is a very clear expectation that the helper
  can handle a NULL PTR_TO_BTF_ID.

v2:
- Add ARG_PTR_TO_SOCK_COMMON_OR_NULL (Lorenz)
====================

Signed-off-by: Alexei Starovoitov <ast@kernel.org>