[PATCH bpf-next 2/3] bpf: Don't reference arena kfuncs when arena isn't built

[Sasha Levin <sashal@kernel.org>]

kernel/bpf/Makefile builds arena.o only for CONFIG_MMU && CONFIG_64BIT,
but special_kfunc_list references bpf_arena_alloc_pages() and
bpf_arena_free_pages() unconditionally, so on 32-bit and !MMU kernels
resolve_btfids can't resolve them:

  WARN: resolve_btfids: unresolved symbol bpf_arena_free_pages
  WARN: resolve_btfids: unresolved symbol bpf_arena_alloc_pages

Since commit 201a454df948 ("resolve_btfids: Fix the --fatal_warnings
exit status") these warnings fail the build with CONFIG_WERROR=y, which
breaks i386 builds with CONFIG_DEBUG_INFO_BTF=y:

  Failed to generate BTF for vmlinux
  Try to disable CONFIG_DEBUG_INFO_BTF

Only reference them under the condition arena.o is built with, and use
BTF_ID_UNUSED otherwise to keep the list in sync with enum
special_kfunc_type. An unresolved ID is zero, like BTF_ID_UNUSED, so the
contents of the list don't change.

Fixes: b8467290edab ("bpf: arena: make arena kfuncs any context safe")
Assisted-by: LLM
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 kernel/bpf/verifier.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 22f095b9dca2d..12229713a4528 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -13639,8 +13639,13 @@ BTF_ID(func, bpf_task_work_schedule_signal)
 BTF_ID(func, bpf_task_work_schedule_resume)
 BTF_ID(func, bpf_call_rcu)
 BTF_ID(func, bpf_call_rcu_tasks_trace)
+#if defined(CONFIG_MMU) && defined(CONFIG_64BIT)
 BTF_ID(func, bpf_arena_alloc_pages)
 BTF_ID(func, bpf_arena_free_pages)
+#else
+BTF_ID_UNUSED
+BTF_ID_UNUSED
+#endif
 #ifdef CONFIG_BPF_EVENTS
 BTF_ID(func, bpf_session_is_return)
 #else
-- 
2.53.0