[PATCH bpf-next v2 1/2] libbpf: Fix sign extension of unaligned enums in btf_dump_get_enum_value()

[Hungyu Lin <dennylin0707@gmail.com>]

btf_dump_get_enum_value() reads unaligned enum values through
btf_dump_get_bitfield_value() with a zero bit size. That makes
right_shift_bits 64 in

	*value = (num << left_shift_bits) >> right_shift_bits;

which is undefined behavior for a __u64 on both endians. The path also
bypasses the signed loads used for aligned data, so a negative value of
a signed enum can lose its enumerator name when the data is unaligned.

Fix this by copying unaligned enum data into an aligned buffer, as
btf_dump_int_data() does, and reading it with the same loads as aligned
data.

Fixes: 920d16af9b42 ("libbpf: BTF dumper support for typed data")
Signed-off-by: Hungyu Lin <dennylin0707@gmail.com>
---
 tools/lib/bpf/btf_dump.c | 13 ++++---------
 1 file changed, 4 insertions(+), 9 deletions(-)

diff --git a/tools/lib/bpf/btf_dump.c b/tools/lib/bpf/btf_dump.c
index bbe86928b3ec..a80762a70cd0 100644
--- a/tools/lib/bpf/btf_dump.c
+++ b/tools/lib/bpf/btf_dump.c
@@ -2329,16 +2329,11 @@ static int btf_dump_get_enum_value(struct btf_dump *d,
 				   __s64 *value)
 {
 	bool is_signed = btf_kflag(t);
+	char buf[8] __attribute__((aligned(8)));
 
-	if (!ptr_is_aligned(d->btf, id, data)) {
-		__u64 val;
-		int err;
-
-		err = btf_dump_get_bitfield_value(d, t, data, 0, 0, &val);
-		if (err)
-			return err;
-		*value = (__s64)val;
-		return 0;
+	if (t->size <= sizeof(buf) && !ptr_is_aligned(d->btf, id, data)) {
+		memcpy(buf, data, t->size);
+		data = buf;
 	}
 
 	switch (t->size) {
-- 
2.43.0