Merge branch 'libbpf: stricter BPF program section name handling'

Merge branch 'libbpf: stricter BPF program section name handling'

Andrii Nakryiko says:

====================

Implement opt-in stricter BPF program section name (SEC()) handling logic. For
a lot of supported ELF section names, enforce exact section name match with no
arbitrary characters added at the end. See patch #9 for more details.

To allow this, patches #2 through #4 clean up and preventively fix selftests,
normalizing existing SEC() usage across multiple selftests. While at it, those
patches also reduce the amount of remaining bpf_object__find_program_by_title()
uses, which should be completely removed soon, given it's an API with
ambiguous semantics and will be deprecated and eventually removed in libbpf 1.0.

Patch #1 also introduces SEC("tc") as an alias for SEC("classifier"). "tc" is
a better and less misleading name, so patch #3 replaces all classifier* uses
with nice and short SEC("tc").

Last patch is also fixing "sk_lookup/" definition to not require and not allow
extra "/blah" parts after it, which serve no meaning.

All the other patches are gradual internal libbpf changes to:
  - allow this optional strict logic for ELF section name handling;
  - allow new use case (for now for "struct_ops", but that could be extended
    to, say, freplace definitions), in which it can be used stand-alone to
    specify just type (SEC("struct_ops")), or also accept extra parameters
    which can be utilized by libbpf to either get more data or double-check
    valid use (e.g., SEC("struct_ops/dctcp_init") to specify desired
    struct_ops operation that is supposed to be implemented);
  - get libbpf's internal logic ready to allow other libraries and
    applications to specify their custom handlers for ELF section name for BPF
    programs. All the pieces are in place, the only thing preventing making
    this as public libbpf API is reliance on internal type for specifying BPF
    program load attributes. The work is planned to revamp related low-level
    libbpf APIs, at which point it will be possible to just re-use such new
    types for coordination between libbpf and custom handlers.

These changes are a part of libbpf 1.0 effort ([0]). They are also intended to
be applied on top of the previous preparatory series [1], so currently CI will
be failing to apply them to bpf-next until that patch set is landed. Once it
is landed, kernel-patches daemon will automatically retest this patch set.

  [0] https://github.com/libbpf/libbpf/wiki/Libbpf:-the-road-to-v1.0#stricter-and-more-uniform-bpf-program-section-name-sec-handling
  [1] https://patchwork.kernel.org/project/netdevbpf/list/?series=547675&state=*

v3->v4:
  - replace SEC("classifier*") with SEC("tc") (Daniel);
v2->v3:
  - applied acks, addressed most feedback, added comments to new flags (Dave);
v1->v2:
  - rebase onto latest bpf-next and resolve merge conflicts w/ Dave's changes.
====================

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