[PATCH bpf-next v4 1/3] libbpf: Normalize enum values according to signedness

[Hungyu Lin <dennylin0707@gmail.com>]

BTF_KIND_ENUM stores enumerators in a signed 32-bit field even for
unsigned enums. btf_dump_enum_data() compares unsigned data against that
signed field, so values with bit 31 set fail to match their names.
get_map_field_long() also sign-extends the field: for example,
__ulong(map_extra, 0x80000000) becomes 0xffffffff80000000 instead of the
requested arena mapping address.

Add btf_enum_value() to extend an enumerator according to kflag and use
it for data-name matching and enum-backed map fields. Return __u64,
consistent with btf_enum64_value(), while sign-extending signed values.

Fixes: d90ec262b35b ("libbpf: Add enum64 support for btf_dump")
Fixes: d147357e2e59 ("libbpf: Allow specifying 64-bit integers in map BTF.")
Suggested-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/DMJG1R2YIJRK.16CZLV9S5U7Z7@gmail.com/
Signed-off-by: Hungyu Lin <dennylin0707@gmail.com>
---
Changes in v4:
- Return __u64 from the shared helper and compare normalized values.
- Drop the legacy comparison and rely on kflag, as requested by Andrii.

 tools/lib/bpf/btf.h      | 6 ++++++
 tools/lib/bpf/btf_dump.c | 2 +-
 tools/lib/bpf/libbpf.c   | 2 +-
 3 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/tools/lib/bpf/btf.h b/tools/lib/bpf/btf.h
index 57f12630c5d1..d74c92056f0f 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 signed even for unsigned enums; kflag records the sign. */
+static inline __u64 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 fcd97aa90589..602664a6adef 100644
--- a/tools/lib/bpf/btf_dump.c
+++ b/tools/lib/bpf/btf_dump.c
@@ -2378,7 +2378,7 @@ 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 ((__u64)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 0e2a93a42908..44134421bd7d 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -2650,7 +2650,7 @@ 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);
 
-- 
2.43.0