The positive path for signed BPF loaders is covered today by the
signed lskels (fentry_test, fexit_test, atomics).

But the runtime metadata check the generated loader performs (libbpf
gen_loader's emit_signature_match), the map content hash it relies
on, the load-time signature, and the immutability invariants of its
metadata map are not yet covered.

Thus, add a new, extensive test suite which drives libbpf's gen_loader
(bpf_object__gen_loader, gen_hash=true), the same machinery which
bpftool uses for signed light skeletons, and exercise corner cases
so that we can assert this in BPF CI:

  # LDLIBS=-static PKG_CONFIG='pkg-config --static' ./vmtest.sh -- ./test_progs -t signed_loader
  [...]
  [    1.840842] clocksource: Switched to clocksource tsc
  #405/1   signed_loader/metadata_check_shape:OK
  #405/2   signed_loader/metadata_match:OK
  #405/3   signed_loader/metadata_sha_mismatch:OK
  #405/4   signed_loader/metadata_not_exclusive:OK
  #405/5   signed_loader/metadata_hash_not_computed:OK
  #405/6   signed_loader/signature_enforced:OK
  #405/7   signed_loader/signature_too_large:OK
  #405/8   signed_loader/signature_bad_keyring:OK
  #405/9   signed_loader/metadata_ctx_max_entries_ignored:OK
  #405/10  signed_loader/metadata_ctx_initial_value_ignored:OK
  #405/11  signed_loader/signature_authenticates_insns:OK
  #405/12  signed_loader/hash_requires_frozen:OK
  #405/13  signed_loader/no_update_after_freeze:OK
  #405/14  signed_loader/freeze_writable_mmap:OK
  #405/15  signed_loader/no_writable_mmap_frozen:OK
  #405/16  signed_loader/map_hash_matches_libbpf:OK
  #405/17  signed_loader/map_hash_multi_element:OK
  #405/18  signed_loader/map_hash_bad_size:OK
  #405/19  signed_loader/map_hash_unsupported_type:OK
  #405     signed_loader:OK
  Summary: 1/19 PASSED, 0 SKIPPED, 0 FAILED

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/r/20260603211658.471212-2-daniel@iogearbox.net
Signed-off-by: Alexei Starovoitov <ast@kernel.org>