[bpf-ci-bot] libarena free() symbol clash breaks s390x selftest build

[kernel-patches-review-bot]

Summary

The recently-merged libarena library (commit 86426a28c52d) declares void free(void __arena *ptr) which collides with an enum constant free = 1 in the s390x-generated vmlinux.h. This breaks the s390x selftest build deterministically for every PR.

Failure Details

• Test / Component: libarena BPF selftest build (all .bpf.c files including libarena/common.h)
• Frequency: Every CI run since libarena was merged (~Apr 27, 2026). Observed in 8+ independent PRs.
• Failure mode: Build error — clang rejects redefinition of free as different kind of symbol
• Affected architectures: s390x only (x86_64 and aarch64 vmlinux.h do not contain the conflicting enum)
• CI runs observed:
  • https://github.com/kernel-patches/bpf/actions/runs/24993512634 (mm: remove page_mapped())
  • https://github.com/kernel-patches/bpf/actions/runs/24972312447 (bpf: decap flags)
  • https://github.com/kernel-patches/bpf/actions/runs/24972282298 (bpf: resizable hash map)
  • https://github.com/kernel-patches/bpf/actions/runs/24972250516 ([test] for-next_test)

Root Cause Analysis

libarena/include/libarena/common.h:53 declares:

void free(void __arena *ptr);

This header includes <vmlinux.h> which on s390x contains an enum constant:

free = 1,   /* vmlinux.h:9209 */

The enum constant comes from BTF extraction of a kernel type specific to the s390x kernel configuration. When clang processes the libarena BPF sources, it first sees free as an enum enumerator (integer constant) from vmlinux.h, then encounters the function declaration void free(...), producing:

error: redefinition of 'free' as different kind of symbol

The Makefile already had -Wno-incompatible-library-redeclaration (line 55) to suppress warnings about redefining the C library free(), but this flag only handles type-mismatched redeclarations of the same kind of symbol. An enum-constant-to-function conflict is a "different kind" error that cannot be suppressed by any warning flag.

The issue was introduced by commit 86426a28c52d ("selftests/bpf: Add buddy allocator for libarena") which added the free() declaration and definition. The libarena v9 patches were merged without s390x CI validation catching this because the s390x build failure was already masked by the same error.

Proposed Fix

Rename the function from free() to arena_free() and provide a #define free(ptr) arena_free(ptr) macro for API compatibility. This mirrors the existing pattern for malloc() / malloc_internal(). See attached patch: 0001-selftests-bpf-Rename-libarena-free-to-arena_free-to-.patch.

Changes:

1. common.h: Replace void free(...) declaration with void arena_free(...) + #define free(ptr) arena_free(ptr)
2. common.bpf.c: Rename function definition from free() to arena_free()
3. Makefile: Remove the now-unnecessary -Wno-incompatible-library-redeclaration flag

Impact

Without this fix, every s390x CI build fails at the selftest compilation stage. All s390x test jobs are blocked — no s390x test results are produced for any PR. This means s390x-specific BPF regressions go undetected.

References

• libarena patch series: https://lore.kernel.org/bpf/20260426190338.4615-1-emil@etsalapatis.com/
• Introducing commit: 86426a28c52d ("selftests/bpf: Add buddy allocator for libarena")
• Author: Emil Tsalapatis emil@etsalapatis.com

[kernel-patches-review-bot]

0001-selftests-bpf-Rename-libarena-free-to-arena_free-to-.patch

From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
From: BPF CI Bot <bot+bpf-ci@kernel.org>
Date: Sun, 27 Apr 2026 00:00:00 +0000
Subject: [PATCH] selftests/bpf: Rename libarena free() to arena_free() to fix
 s390x build

The libarena library declares `void free(void __arena *ptr)` in
common.h and defines it in common.bpf.c. On s390x, the generated
vmlinux.h contains an enum constant named `free` (at line ~9209:
`free = 1,`), likely from a s390x-specific kernel type exposed
through BTF. When clang compiles the libarena BPF programs for
s390x, it sees the enum constant first via `#include <vmlinux.h>`
and then hits the function declaration, producing:

  error: redefinition of 'free' as different kind of symbol

This breaks the s390x selftest build deterministically for every PR.

The existing `-Wno-incompatible-library-redeclaration` flag in the
Makefile suppresses warnings about redefining C library functions
with different signatures, but it cannot suppress this error because
the conflict is between an enum constant and a function — a different
kind of symbol entirely.

Fix this by renaming the actual function to `arena_free()` and
providing a `#define free(ptr) arena_free(ptr)` compatibility macro,
matching the existing pattern used for `malloc()` / `malloc_internal()`.
This avoids any possible namespace collision with vmlinux.h symbols
while preserving the `free()` API for libarena consumers via the
macro. Also remove the now-unnecessary `-Wno-incompatible-library-
redeclaration` compiler flag.

Fixes: 86426a28c52d ("selftests/bpf: Add buddy allocator for libarena")
Signed-off-by: BPF CI Bot <bot+bpf-ci@kernel.org>
Generated-by: BPF CI Bot (claude-opus-4-6-20250514) <bot+bpf-ci@kernel.org>
---
 tools/testing/selftests/bpf/libarena/Makefile              | 2 --
 tools/testing/selftests/bpf/libarena/include/libarena/common.h | 3 ++-
 tools/testing/selftests/bpf/libarena/src/common.bpf.c         | 2 +-
 3 files changed, 3 insertions(+), 4 deletions(-)

diff --git a/tools/testing/selftests/bpf/libarena/Makefile b/tools/testing/selftests/bpf/libarena/Makefile
index 3c695f9c0054..5e2ab514805e 100644
--- a/tools/testing/selftests/bpf/libarena/Makefile
+++ b/tools/testing/selftests/bpf/libarena/Makefile
@@ -51,8 +51,6 @@ ASAN_FLAGS += -mllvm -asan-destructor-kind=none
 override BPF_CFLAGS += -DENABLE_ATOMICS_TESTS
 override BPF_CFLAGS += -O2 -g
 override BPF_CFLAGS += -Wno-incompatible-pointer-types-discards-qualifiers
-# Required to define our own arena-based free()
-override BPF_CFLAGS += -Wno-incompatible-library-redeclaration
 # Required for suppressing harmless vmlinux.h-related warnings.
 override BPF_CFLAGS += -Wno-missing-declarations
 override BPF_CFLAGS += $(INCLUDES)
diff --git a/tools/testing/selftests/bpf/libarena/include/libarena/common.h b/tools/testing/selftests/bpf/libarena/include/libarena/common.h
index e54cb7b869bd..8f6151a44450 100644
--- a/tools/testing/selftests/bpf/libarena/include/libarena/common.h
+++ b/tools/testing/selftests/bpf/libarena/include/libarena/common.h
@@ -50,7 +50,8 @@ int arena_fls(__u64 word);

 u64 malloc_internal(size_t size);
 #define malloc(size) ((void __arena *)malloc_internal((size)))
-void free(void __arena *ptr);
+void arena_free(void __arena *ptr);
+#define free(ptr) arena_free(ptr)

 /*
  * The verifier associates arenas with programs by checking LD.IMM
diff --git a/tools/testing/selftests/bpf/libarena/src/common.bpf.c b/tools/testing/selftests/bpf/libarena/src/common.bpf.c
index e5da1e37e83e..205fe93ac9c3 100644
--- a/tools/testing/selftests/bpf/libarena/src/common.bpf.c
+++ b/tools/testing/selftests/bpf/libarena/src/common.bpf.c
@@ -43,7 +43,7 @@ __weak u64 malloc_internal(size_t size)
 	return buddy_alloc_internal(&buddy, size);
 }

-__weak void free(void __arg_arena __arena *ptr)
+__weak void arena_free(void __arg_arena __arena *ptr)
 {
 	buddy_free_internal(&buddy, (u64)ptr);
 }
--
2.53.0