Re: [PATCH bpf-next] libbpf: Match unsigned enum values when dumping data

["Alexei Starovoitov" <alexei.starovoitov@gmail.com>]

On Sat, Oct 31, 2026 at 04:32 PM Hungyu Lin <dennylin0707@gmail.com> wrote:
> @@ -2373,7 +2373,7 @@ static int btf_dump_enum_data(struct btf_dump *d,
> -			if (value != e->val)
> +			if (value != (is_signed ? (__s64)e->val : (__u32)e->val))

This is the third patch in two days about the sign of BTF enum values.
btf_enum->val is __s32 for unsigned enums too.
libbpf widens it to 64 bits in three places and ignores kflag in
all three:

  function                      val = 0x80000000, kflag = 0
  ----------------------------  ----------------------------------
  btf_dump_enum_data()          no match, prints 2147483648
  get_map_field_long()          map_extra = 0xffffffff80000000
  bpf_core_calc_enumval_relo()  *val = 0xffffffff80000000

__ulong(map_extra, 0x80000000) gets to the second one.
The arena cannot be mmap-ed at that address.

When the same mistake is made in N places the interface is the problem.
Lets add a helper next to btf_enum64_value(). bpftool can use it too.

The diff leaves bpf_core_calc_enumval_relo() alone.
Pls check what clang puts into ld_imm64 for such enumerator.

Pls send libbpf, bpftool and the selftest as one series.

Something like this

diff --git a/tools/lib/bpf/btf.h b/tools/lib/bpf/btf.h
index 57f1263..d6a770c 100644
--- a/tools/lib/bpf/btf.h
+++ b/tools/lib/bpf/btf.h
@@ -636,6 +636,12 @@ static inline struct btf_enum *btf_enum(const struct btf_type *t)
 	return (struct btf_enum *)(t + 1);
 }

+/* btf_enum->val is __s32 for unsigned enums too. kflag of the enum has the sign. */
+static inline __s64 btf_enum_value(const struct btf_type *t, const struct btf_enum *e)
+{
+	return btf_kflag(t) ? (__s64)e->val : (__u32)e->val;
+}
+
 struct btf_enum64;

 static inline struct btf_enum64 *btf_enum64(const struct btf_type *t)
diff --git a/tools/lib/bpf/btf_dump.c b/tools/lib/bpf/btf_dump.c
index ea9a34d..37c9c4a 100644
--- a/tools/lib/bpf/btf_dump.c
+++ b/tools/lib/bpf/btf_dump.c
@@ -2373,7 +2373,7 @@ static int btf_dump_enum_data(struct btf_dump *d,
 		const struct btf_enum *e;

 		for (i = 0, e = btf_enum(t); i < btf_vlen(t); i++, e++) {
-			if (value != e->val)
+			if (value != btf_enum_value(t, e))
 				continue;
 			btf_dump_type_values(d, "%s", btf_name_of(d, e->name_off));
 			return 0;
diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 02e75b1..cecd6b5 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -2627,7 +2627,7 @@ static bool get_map_field_long(const char *map_name, const struct btf *btf,
 	if (btf_is_enum(t)) {
 		const struct btf_enum *e = btf_enum(t);

-		*res = e->val;
+		*res = btf_enum_value(t, e);
 	} else {
 		const struct btf_enum64 *e = btf_enum64(t);

diff --git a/tools/testing/selftests/bpf/prog_tests/btf_dump.c b/tools/testing/selftests/bpf/prog_tests/btf_dump.c
index dbef014..f3d165a 100644
--- a/tools/testing/selftests/bpf/prog_tests/btf_dump.c
+++ b/tools/testing/selftests/bpf/prog_tests/btf_dump.c
@@ -919,6 +919,9 @@ static void test_btf_dump_typedef_data(struct btf *btf, struct btf_dump *d,
 static void test_btf_dump_enum_data(struct btf *btf, struct btf_dump *d,
 				    char *str)
 {
+	__u32 high = 0x80000000;
+	__s32 neg = -1;
+
 	/* enum where enum value does (and does not) exist */
 	TEST_BTF_DUMP_DATA_C(btf, d, "enum", str, enum bpf_cmd, BTF_F_COMPACT,
 			     BPF_MAP_CREATE);
@@ -950,6 +953,22 @@ static void test_btf_dump_enum_data(struct btf *btf, struct btf_dump *d,

 	TEST_BTF_DUMP_DATA_OVER(btf, d, "enum", str, enum bpf_cmd,
 				sizeof(enum bpf_cmd) - 1, "", BPF_MAP_CREATE);
+
+	/* enumerators with bit 31 set in unsigned and in signed enum */
+	ASSERT_GT(btf__add_enum(btf, "test_enum_u32", 4), 0, "add unsigned enum");
+	ASSERT_OK(btf__add_enum_value(btf, "TEST_ENUM_HIGH", 0x80000000), "add TEST_ENUM_HIGH");
+	ASSERT_GT(btf__add_enum(btf, "test_enum_s32", 4), 0, "add signed enum");
+	ASSERT_OK(btf__add_enum_value(btf, "TEST_ENUM_NEG", -1), "add TEST_ENUM_NEG");
+
+	/* btf_dump__dump_type_data() doesn't resize 'd' for the new types */
+	d = btf_dump__new(btf, btf_dump_snprintf, str, NULL);
+	if (!ASSERT_OK_PTR(d, "btf_dump__new"))
+		return;
+	ASSERT_OK(btf_dump_data(btf, d, "test_enum_u32", NULL, 0, &high, 4, str,
+				"(enum test_enum_u32)TEST_ENUM_HIGH"), "dump unsigned enum");
+	ASSERT_OK(btf_dump_data(btf, d, "test_enum_s32", NULL, 0, &neg, 4, str,
+				"(enum test_enum_s32)TEST_ENUM_NEG"), "dump signed enum");
+	btf_dump__free(d);
 }

 static void test_btf_dump_struct_data(struct btf *btf, struct btf_dump *d,

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