arena_base() casts the map definition to the kernel's struct bpf_arena and reads user_vm_start out of it. In the object file that definition is only the 32-byte BTF placeholder the loader rewrites, so the access is past the end of the object and GCC says so: libarena/include/bpf_arena_common.h:52:69: error: array subscript 'struct bpf_arena[0]' is partly outside array bounds of 'struct [1]' [-Werror=array-bounds=] 52 | #define arena_base(map) ((void __arena *)((struct bpf_arena *)(map))->user_vm_start) progs/arena_race.c:58:30: note: in expansion of macro 'arena_base' GCC is right; clang has the same warning but does not apply it to a member access through a cast. With BPF_CFLAGS carrying -Werror this is a build failure, today, for every prog that calls arena_base() and is compiled by a GCC with arena support: verifier_arena.c, verifier_arena_large.c, verifier_arena_globals1.c, verifier_arena_globals2.c and arena_race.c. Make the pointer opaque instead, which is what progs/stream.c already does for its own open-coded copy of the same access. The bounds analysis then has nothing to reason about, and every caller is covered without a per-file opt-out that has to be maintained -- arena_race.c was added without one and broke the bpf_gcc build. The barrier is behind #ifdef __clang__ so clang keeps the plain macro. It does not need it, and the barrier is not free: it blocks reuse of the map address, so each call site reloads it. Across the five progs and 20 call sites that is +35 instructions under GCC (2528 -> 2563), about 1.4% on those objects and nothing elsewhere. Clang output is unchanged, bit for bit. Signed-off-by: Vineet Gupta --- .../bpf/libarena/include/bpf_arena_common.h | 16 ++++++++++++++++ 1 file changed, 16 insertions(+) diff --git a/tools/testing/selftests/bpf/libarena/include/bpf_arena_common.h b/tools/testing/selftests/bpf/libarena/include/bpf_arena_common.h index 82aafe879fae..4f2c6050215f 100644 --- a/tools/testing/selftests/bpf/libarena/include/bpf_arena_common.h +++ b/tools/testing/selftests/bpf/libarena/include/bpf_arena_common.h @@ -49,7 +49,23 @@ void __arena* bpf_arena_alloc_pages(void *map, void __arena *addr, __u32 page_cn int bpf_arena_reserve_pages(void *map, void __arena *addr, __u32 page_cnt) __ksym __weak; void bpf_arena_free_pages(void *map, void __arena *ptr, __u32 page_cnt) __ksym __weak; +/* + * GCC's -Warray-bounds fires on the member access: the map definition the + * object file carries is smaller than struct bpf_arena. Make the pointer + * opaque to it, as the open-coded form in progs/stream.c already does. + * clang does not warn and does not need the barrier, which costs a reload + * of the map address at every call site. + */ +#ifdef __clang__ #define arena_base(map) ((void __arena *)((struct bpf_arena *)(map))->user_vm_start) +#else +#define arena_base(map) \ +({ \ + struct bpf_arena *__ma = (void *)(map); \ + barrier_var(__ma); \ + (void __arena *)__ma->user_vm_start; \ +}) +#endif #else /* when compiled as user space code */ -- 2.53.0-Meta