[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>