Merge branch 'libbpf-side-__arg_ctx-fallback-support'

Andrii Nakryiko says:

====================
Libbpf-side __arg_ctx fallback support

Support __arg_ctx global function argument tag semantics even on older kernels
that don't natively support it through btf_decl_tag("arg:ctx").

Patches #2-#6 are preparatory work to allow to postpone BTF loading into the
kernel until after all the BPF program relocations (including global func
appending to main programs) are done. Patch #4 is perhaps the most important
and establishes pre-created stable placeholder FDs, so that relocations can
embed valid map FDs into ldimm64 instructions.

Once BTF is done after relocation, what's left is to adjust BTF information to
have each main program's copy of each used global subprog to point to its own
adjusted FUNC -> FUNC_PROTO type chain (if they use __arg_ctx) in such a way
as to satisfy type expectations of BPF verifier regarding the PTR_TO_CTX
argument definition. See patch #8 for details.

Patch #8 adds few more __arg_ctx use cases (edge cases like multiple arguments
having __arg_ctx, etc) to test_global_func_ctx_args.c, to make it simple to
validate that this logic indeed works on old kernels. It does. But just to be
100% sure patch #9 adds a test validating that libbpf uploads func_info with
properly modified BTF data.

v2->v3:
  - drop renaming patch (Alexei, Eduard);
  - use memfd_create() instead of /dev/null for placeholder FD (Eduard);
  - add one more test for validating BTF rewrite logic (Eduard);
  - fixed wrong -errno usage, reshuffled some BTF rewrite bits (Eduard);
v1->v2:
  - do internal functions renaming in patch #1 (Alexei);
  - extract cloning of FUNC -> FUNC_PROTO information into separate function
    (Alexei);
====================

Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20240104013847.3875810-1-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>