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

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

[Hungyu Lin]

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@gmail.com>
---
 tools/bpf/bpftool/btf.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

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

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

[Quentin Monnet]

2026-10-30 09:09 UTC+0000 ~ Hungyu Lin <dennylin0707@gmail.com>
> 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@gmail.com>

Looks correct, thank you.

Acked-by: Quentin Monnet <qmo@kernel.org>

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@gmail.com> 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@gmail.com>
> ---
>  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
>

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

[Hungyu Lin]

> please drop RFC tag and report with Quentin's ack

Reposted as v2 with the RFC tag removed and Quentin's ack added.

For future bpftool fixes during the merge window, would you prefer
regular bpf-next patches or RFCs?

Thanks,
Hungyu

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

[Andrii Nakryiko]

On Fri, Oct 30, 2026 at 1:00 PM Hungyu Lin <dennylin0707@gmail.com> wrote:
>
> > please drop RFC tag and report with Quentin's ack
>
> Reposted as v2 with the RFC tag removed and Quentin's ack added.
>
> For future bpftool fixes during the merge window, would you prefer
> regular bpf-next patches or RFCs?
>

regular, bpf-next is generally open during merge window

> Thanks,
> Hungyu