Merge branch 'bpf-allow-arena-pointers-in-by-value-returns'

Yonghong Song says:

====================
bpf: Allow arena pointers in by-value returns

A function returning a struct by value may only return one whose members
are all scalars. That is stricter than it needs to be: an arena pointer
is safe to hand over as raw register bits, and both a global function
and a kfunc can already return one on its own.

This patch set allows returning arena pointer(s) (as member(s) of a
struct) for global functions and kfuncs. Any other pointer member stays
rejected, as it would be laundered into a scalar and escape provenance
and reference tracking.

Patch 1 fixes a diagnostics bug. Patches 2-4 are refactoring with no
functional change. Patch 5 checked (nested) arrays until non-array type
to be checked by struct or other types. Patch 6 improves the diagnostics
for an unsupported return type, and patches 7-8 allow arena pointer members.
Patches 9-11 are selftests. Patch 12 updates the kfunc's doc.

Changelog:
  v3 -> v4:
    - v3: https://lore.kernel.org/bpf/20260827061114.2514603-1-yonghong.song@linux.dev/
    - Generate type string(s) for those verifier rejected kfunc's.
    - For global function, disable returning arena pointer if main prog.
    - Add additional tests for rejected kfunc type string(s).
  v2 -> v3:
    - v2: https://lore.kernel.org/bpf/20260825205412.1320099-1-yonghong.song@linux.dev/
    - Fix two more verifier diagnostics.
    - Fix btf_member_path_str() by detecting anonymous union/struct.
    - Fix a few arena tests with non-zero arena pointers.
    - Update kfuncs doc for supporting return value with arena pointers.
  v1 -> v2:
    - v1: https://lore.kernel.org/bpf/20260824144943.991316-1-yonghong.song@linux.dev/
    - Add nested struct field names for diagnostics, and report the
      nesting depth for a type nested past the walk limit.
    - Allow to return arena pointers for global functions and kfuncs.
    - Necessary selftests for newly supported arena pointers.
====================

Link: https://patch.msgid.link/20260829061514.1690730-1-yonghong.song@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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