[PATCH] bpf: Replace 0-length arrays with flexible arrays

[Kees Cook <keescook@chromium.org>]

Zero-length arrays are deprecated[1]. Replace struct bpf_array's
union of 0-length arrays with flexible arrays. (How are the
sizes of these arrays verified?) Detected with GCC 13, using
-fstrict-flex-arrays=3:

arch/x86/net/bpf_jit_comp.c: In function 'bpf_tail_call_direct_fixup':
arch/x86/net/bpf_jit_comp.c:606:37: warning: array subscript <unknown> is outside array bounds of 'void *[0]' [-Warray-bounds=]
  606 |                 target = array->ptrs[poke->tail_call.key];
      |                          ~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~
In file included from include/linux/filter.h:9,
                 from arch/x86/net/bpf_jit_comp.c:9:
include/linux/bpf.h:1527:23: note: while referencing 'ptrs'
 1527 |                 void *ptrs[0] __aligned(8);
      |                       ^~~~

[1] https://www.kernel.org/doc/html/latest/process/deprecated.html#zero-length-and-one-element-arrays

Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: John Fastabend <john.fastabend@gmail.com>
Cc: Andrii Nakryiko <andrii@kernel.org>
Cc: Martin KaFai Lau <martin.lau@linux.dev>
Cc: Song Liu <song@kernel.org>
Cc: Yonghong Song <yhs@fb.com>
Cc: KP Singh <kpsingh@kernel.org>
Cc: Stanislav Fomichev <sdf@google.com>
Cc: Hao Luo <haoluo@google.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Cc: bpf@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
---
 include/linux/bpf.h | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 3de24cfb7a3d..2131000f711e 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -1523,9 +1523,9 @@ struct bpf_array {
 	u32 index_mask;
 	struct bpf_array_aux *aux;
 	union {
-		char value[0] __aligned(8);
-		void *ptrs[0] __aligned(8);
-		void __percpu *pptrs[0] __aligned(8);
+		DECLARE_FLEX_ARRAY(char, value) __aligned(8);
+		DECLARE_FLEX_ARRAY(void *, ptrs) __aligned(8);
+		DECLARE_FLEX_ARRAY(void __percpu *, pptrs) __aligned(8);
 	};
 };
 
-- 
2.34.1