Re: [PATCH bpf-next] selftests/bpf: Add arena direct-value one-past-end reject test

[Woojin Ji <random6.xyz@gmail.com>]

Thanks, I dug into the inline assembly route more.

A hand-encoded BPF_PSEUDO_MAP_VALUE ldimm64 in verifier_arena.c can produce
the right instruction bytes only with __imm_addr(arena), but that emits
R_BPF_64_NODYLD32. The selftests libbpf/bpftool link step rejects that
relocation type.

I also tried register, immediate, and memory constraints with raw bytes. Those
either fail in the BPF backend address matching or NOP padding path, or lower
to a normal R_BPF_64_64 load followed by an ALU add. The latter does not
exercise arena_map_direct_value_addr() with off == arena_size; it tests a
later ALU-add path instead.

I also checked the legacy test_verifier route. It can express the raw insn
directly, but it requires adding arena-specific map creation and mmap fixup
plumbing to the legacy runner. That seems more intrusive and less aligned with
new selftests than the small prog_tests raw-instruction test.

So my current preference is still to keep the userspace raw-instruction test.
If that sounds reasonable, I can send v3 with this rationale added to the
commit message/changelog.