linux-kernel - Re: [PATCH] bpf: Annotate struct bpf_cand_cache with __counted

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Message-ID: <CAADnVQKEgG5bXvLMLYupAZO6xahWHU7mc06KFfseNoYUvoJbRQ@mail.gmail.com>
Date: Tue, 13 Aug 2024 09:28:56 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Thorsten Blum <thorsten.blum@...lux.com>
Cc: Martin KaFai Lau <martin.lau@...ux.dev>, Alexei Starovoitov <ast@...nel.org>, 
	Daniel Borkmann <daniel@...earbox.net>, Andrii Nakryiko <andrii@...nel.org>, Eddy Z <eddyz87@...il.com>, 
	Song Liu <song@...nel.org>, Yonghong Song <yonghong.song@...ux.dev>, 
	John Fastabend <john.fastabend@...il.com>, KP Singh <kpsingh@...nel.org>, 
	Stanislav Fomichev <sdf@...ichev.me>, Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>, 
	Kees Cook <kees@...nel.org>, "Gustavo A. R. Silva" <gustavoars@...nel.org>, bpf <bpf@...r.kernel.org>, 
	LKML <linux-kernel@...r.kernel.org>, linux-hardening@...r.kernel.org
Subject: Re: [PATCH] bpf: Annotate struct bpf_cand_cache with __counted_by()

On Tue, Aug 13, 2024 at 8:19 AM Thorsten Blum <thorsten.blum@...lux.com> wrote:
>
> Add the __counted_by compiler attribute to the flexible array member
> cands to improve access bounds-checking via CONFIG_UBSAN_BOUNDS and
> CONFIG_FORTIFY_SOURCE.
>
> Increment cnt before adding a new struct to the cands array.

why? What happens otherwise?

>
> Signed-off-by: Thorsten Blum <thorsten.blum@...lux.com>
> ---
>  kernel/bpf/btf.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> index 520f49f422fe..42bc70a56fcd 100644
> --- a/kernel/bpf/btf.c
> +++ b/kernel/bpf/btf.c
> @@ -7240,7 +7240,7 @@ struct bpf_cand_cache {
>         struct {
>                 const struct btf *btf;
>                 u32 id;
> -       } cands[];
> +       } cands[] __counted_by(cnt);
>  };
>
>  static DEFINE_MUTEX(cand_cache_mutex);
> @@ -8784,9 +8784,9 @@ bpf_core_add_cands(struct bpf_cand_cache *cands, const struct btf *targ_btf,
>                 memcpy(new_cands, cands, sizeof_cands(cands->cnt));
>                 bpf_free_cands(cands);
>                 cands = new_cands;
> -               cands->cands[cands->cnt].btf = targ_btf;
> -               cands->cands[cands->cnt].id = i;
>                 cands->cnt++;
> +               cands->cands[cands->cnt - 1].btf = targ_btf;
> +               cands->cands[cands->cnt - 1].id = i;
>         }
>         return cands;
>  }
> --
> 2.46.0
>