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Date:   Fri, 9 Sep 2022 10:19:39 -0700
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Subject: Re: [PATCH bpf] bpf: btf: fix truncated last_member_type_id in btf_struct_resolve
From:   sdf@google.com
To:     Lorenz Bauer <oss@lmb.io>
Cc:     Martin KaFai Lau <martin.lau@linux.dev>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii@kernel.org>,
        Song Liu <song@kernel.org>, Yonghong Song <yhs@fb.com>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@kernel.org>, Hao Luo <haoluo@google.com>,
        Jiri Olsa <jolsa@kernel.org>, bpf@vger.kernel.org,
        linux-kernel@vger.kernel.org
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On 09/09, Lorenz Bauer wrote:
> When trying to finish resolving a struct member, btf_struct_resolve
> saves the member type id in a u16 temporary variable. This truncates
> the 32 bit type id value if it exceeds UINT16_MAX.

> As a result, structs that have members with type ids > UINT16_MAX and
> which need resolution will fail with a message like this:

>      [67414] STRUCT ff_device size=120 vlen=12
>          effect_owners type_id=67434 bits_offset=960 Member exceeds  
> struct_size

> Fix this by changing the type of last_member_type_id to u32.

Makes sense to me; I'm surprised the compiler isn't raising a warning on  
line:
last_member_type_id = last_member->type;

Reviewed-by: Stanislav Fomichev <sdf@google.com>

> Fixes: eb3f595dab40 ("bpf: btf: Validate type reference")
> Signed-off-by: Lorenz Bauer <oss@lmb.io>
> ---
>   kernel/bpf/btf.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)

> diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> index 7e64447659f3..36fd4b509294 100644
> --- a/kernel/bpf/btf.c
> +++ b/kernel/bpf/btf.c
> @@ -3128,7 +3128,7 @@ static int btf_struct_resolve(struct  
> btf_verifier_env *env,
>   	if (v->next_member) {
>   		const struct btf_type *last_member_type;
>   		const struct btf_member *last_member;
> -		u16 last_member_type_id;
> +		u32 last_member_type_id;

>   		last_member = btf_type_member(v->t) + v->next_member - 1;
>   		last_member_type_id = last_member->type;
> --
> 2.34.1