libbpf: Fix parsing of multi-split BTF

[ Upstream commit 4f596ac ]

When creating multi-split BTF we correctly set the start string offset
to be the size of the base string section plus the base BTF start
string offset; the latter is needed for multi-split BTF since the
offset is non-zero there.

Unfortunately the BTF parsing case needed that logic and it was
missed.

Fixes: 4e29128 ("libbpf/btf: Fix string handling to support multi-split BTF")
Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20251104203309.318429-2-alan.maguire@oracle.com
Signed-off-by: Sasha Levin <sashal@kernel.org>

Author: alan-maguire

tools/lib/bpf/btf.c

@@ -1046,7 +1046,7 @@ static struct btf *btf_new(const void *data, __u32 size, struct btf *base_btf)
 	if (base_btf) {
 		btf->base_btf = base_btf;
 		btf->start_id = btf__type_cnt(base_btf);
-		btf->start_str_off = base_btf->hdr->str_len;
+		btf->start_str_off = base_btf->hdr->str_len + base_btf->start_str_off;
 	}
 
 	btf->raw_data = malloc(size);
@@ -5504,7 +5504,7 @@ void btf_set_base_btf(struct btf *btf, const struct btf *base_btf)
 {
 	btf->base_btf = (struct btf *)base_btf;
 	btf->start_id = btf__type_cnt(base_btf);
-	btf->start_str_off = base_btf->hdr->str_len;
+	btf->start_str_off = base_btf->hdr->str_len + base_btf->start_str_off;
 }
 
 int btf__relocate(struct btf *btf, const struct btf *base_btf)