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

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Date:   Thu, 5 Jan 2023 11:56:26 -0800
From:   Stanislav Fomichev <sdf@...gle.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        John Fastabend <john.fastabend@...il.com>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Song Liu <song@...nel.org>, Yonghong Song <yhs@...com>,
        KP Singh <kpsingh@...nel.org>, Hao Luo <haoluo@...gle.com>,
        Jiri Olsa <jolsa@...nel.org>,
        "Gustavo A. R. Silva" <gustavoars@...nel.org>, bpf@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH] bpf: Replace 0-length arrays with flexible arrays

On Thu, Jan 5, 2023 at 11:26 AM Kees Cook <keescook@...omium.org> wrote:
>
> 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

Haven't looked closely, but these arrays should be produced somewhere
around kernel/bpf/arraymap.c (fd_array).

> -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@...nel.org>
> Cc: Daniel Borkmann <daniel@...earbox.net>
> Cc: John Fastabend <john.fastabend@...il.com>
> Cc: Andrii Nakryiko <andrii@...nel.org>
> Cc: Martin KaFai Lau <martin.lau@...ux.dev>
> Cc: Song Liu <song@...nel.org>
> Cc: Yonghong Song <yhs@...com>
> Cc: KP Singh <kpsingh@...nel.org>
> Cc: Stanislav Fomichev <sdf@...gle.com>
> Cc: Hao Luo <haoluo@...gle.com>
> Cc: Jiri Olsa <jolsa@...nel.org>
> Cc: "Gustavo A. R. Silva" <gustavoars@...nel.org>
> Cc: bpf@...r.kernel.org
> Signed-off-by: Kees Cook <keescook@...omium.org>

Acked-by: Stanislav Fomichev <sdf@...gle.com>

> ---
>  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
>