[PATCH bpf-next v3 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 in both places.

For data dumps, keep the original comparison as well. Older BTF producers
did not record enum signedness; an eight-byte BTF_KIND_ENUM can contain
signed data such as -1 with kflag clear. Using only the helper would lose
an existing name match. This compatibility comparison does not truncate
the data value or change the numeric fallback.

For an eight-byte BTF_KIND_ENUM with kflag clear, a negative stored
enumerator can therefore match both its sign- and zero-extended values.
The record cannot distinguish legacy signed metadata from unsigned
metadata with the same bits.

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>
---
 tools/lib/bpf/btf.h      | 6 ++++++
 tools/lib/bpf/btf_dump.c | 3 ++-
 tools/lib/bpf/libbpf.c   | 2 +-
 3 files changed, 9 insertions(+), 2 deletions(-)

diff --git a/tools/lib/bpf/btf.h b/tools/lib/bpf/btf.h
index 57f12630c5d1..eeb4984d6c57 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 __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 ea9a34db4c00..b636dc4185a0 100644
--- a/tools/lib/bpf/btf_dump.c
+++ b/tools/lib/bpf/btf_dump.c
@@ -2373,7 +2373,8 @@ 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)
+			/* Keep names from legacy signed enums without kflag. */
+			if (value != e->val && 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 02e75b183ed0..cecd6b5f27de 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);
 
-- 
2.43.0