Re: [RFC PATCH bpf-next] bpftool: avoid sign extension in unsigned enum JSON dumps

[Andrii Nakryiko]

On Fri, Oct 30, 2026 at 2:09 AM Hungyu Lin <dennylin0707@xxxxxxxxx> wrote:
>
> The BTF enum value is stored in a signed 32-bit field even when the
> enum is unsigned. Passing it directly to jsonw_uint_field(), which
> takes a uint64_t, sign-extends values with bit 31 set. For example,
> 0xffffffffU is printed as 18446744073709551615 instead of 4294967295
> by bpftool -j btf dump.
>
> Cast the value to __u32 before widening it, matching the conversion
> already used for plain-text output.
>
> Fixes: c93cc69004df ("bpftool: add ability to dump BTF types")
> Signed-off-by: Hungyu Lin <dennylin0707@xxxxxxxxx>
> ---
>  tools/bpf/bpftool/btf.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>

please drop RFC tag and report with Quentin's ack

> diff --git a/tools/bpf/bpftool/btf.c b/tools/bpf/bpftool/btf.c
> index abd661db91e6..385521e6b028 100644
> --- a/tools/bpf/bpftool/btf.c
> +++ b/tools/bpf/bpftool/btf.c
> @@ -482,7 +482,7 @@ static int dump_btf_type(const struct btf *btf, __u32 id,
>                                 if (btf_kflag(t))
>                                         jsonw_int_field(w, "val", v->val);
>                                 else
> -                                       jsonw_uint_field(w, "val", v->val);
> +                                       jsonw_uint_field(w, "val", (__u32)v->val);
>                                 jsonw_end_object(w);
>                         } else {
>                                 if (btf_kflag(t))
>
> base-commit: 7101b10cf281f629ac58c99697ca6f6d082b99a7
> --
> 2.43.0
>