libbpf: Fix realloc usage in bpf_core_find_cands

Fix bug requesting invalid size of reallocated array when constructing CO-RE
relocation candidate list. This can cause problems if there are many potential
candidates and a very fine-grained memory allocator bucket sizes are used.

Fixes: ddc7c30 ("libbpf: implement BPF CO-RE offset relocation algorithm")
Reported-by: William Smith <williampsmith@fb.com>
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20200124201847.212528-1-andriin@fb.com

tools/lib/bpf/libbpf.c

@@ -3869,7 +3869,9 @@ static struct ids_vec *bpf_core_find_cands(const struct btf *local_btf,
 		if (strncmp(local_name, targ_name, local_essent_len) == 0) {
 			pr_debug("[%d] %s: found candidate [%d] %s\n",
 				 local_type_id, local_name, i, targ_name);
-			new_ids = realloc(cand_ids->data, cand_ids->len + 1);
+			new_ids = reallocarray(cand_ids->data,
+					       cand_ids->len + 1,
+					       sizeof(*cand_ids->data));
 			if (!new_ids) {
 				err = -ENOMEM;
 				goto err_out;