libbpf: Improve sanity checking during BTF fix up

If BTF is corrupted DATASEC's variable type ID might be incorrect.
Prevent this easy to detect situation with extra NULL check.
Reported by oss-fuzz project.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20211103173213.1376990-3-andrii@kernel.org

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 71f5a00..f836a19 100644 (file)
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -2752,13 +2752,12 @@ static int btf_fixup_datasec(struct bpf_object *obj, struct btf *btf,
 
        for (i = 0, vsi = btf_var_secinfos(t); i < vars; i++, vsi++) {
                t_var = btf__type_by_id(btf, vsi->type);
-               var = btf_var(t_var);
-
-               if (!btf_is_var(t_var)) {
+               if (!t_var || !btf_is_var(t_var)) {
                        pr_debug("Non-VAR type seen in section %s\n", name);
                        return -EINVAL;
                }
 
+               var = btf_var(t_var);
                if (var->linkage == BTF_VAR_STATIC)
                        continue;