netdev - Re: [PATCH bpf-next] libbpf: Remove redundant check in btf_ext_

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Date:   Tue, 15 Mar 2022 11:36:56 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Yuntao Wang <ytcoode@...il.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH bpf-next] libbpf: Remove redundant check in btf_ext__new()

On Sat, Mar 12, 2022 at 9:14 AM Yuntao Wang <ytcoode@...il.com> wrote:
>
> Since 'core_relo_len' is the last field of 'struct btf_ext_header', if
> 'xxx->hdr_len' is not less than 'offsetofend(xxx, core_relo_len)', then
> 'xxx->hdr_len' must also be not less than 'offsetofend(xxx, line_info_len)'.
>
> We can check 'xxx->hdr_len < offsetofend(xxx, core_relo_len)' first, if it
> passes, the 'xxx->hdr_len < offsetofend(xxx, line_info_len)' check will be
> redundant, it can be removed.
>
> Signed-off-by: Yuntao Wang <ytcoode@...il.com>
> ---
>  tools/lib/bpf/btf.c | 7 +------
>  1 file changed, 1 insertion(+), 6 deletions(-)
>
> diff --git a/tools/lib/bpf/btf.c b/tools/lib/bpf/btf.c
> index 1383e26c5d1f..d55b44124c3e 100644
> --- a/tools/lib/bpf/btf.c
> +++ b/tools/lib/bpf/btf.c
> @@ -2813,7 +2813,7 @@ struct btf_ext *btf_ext__new(const __u8 *data, __u32 size)
>         if (err)
>                 goto done;
>
> -       if (btf_ext->hdr->hdr_len < offsetofend(struct btf_ext_header, line_info_len)) {
> +       if (btf_ext->hdr->hdr_len < offsetofend(struct btf_ext_header, core_relo_len)) {
>                 err = -EINVAL;
>                 goto done;
>         }
> @@ -2826,11 +2826,6 @@ struct btf_ext *btf_ext__new(const __u8 *data, __u32 size)
>         if (err)
>                 goto done;
>
> -       if (btf_ext->hdr->hdr_len < offsetofend(struct btf_ext_header, core_relo_len)) {
> -               err = -EINVAL;
> -               goto done;
> -       }

it seems like it's actually a bug. If header is smaller then core
relos parsing should be skipped, I think. Maybe let's fix that
instead?

basically the logic should be:

1. if size of header is exactly == offsetof(core_relo_off) then skip core relos
2. otherwise check that it has enough size to cover core_relo_off and
core_relo_len, and error out if not
3. otherwise proceed to parsing core relos

> -
>         err = btf_ext_setup_core_relos(btf_ext);
>         if (err)
>                 goto done;
> --
> 2.35.1
>