bpf: Change return value of verifier function add_subprog()

Currently, verifier function add_subprog() returns 0 for success
and negative value for failure. Change the return value
to be the subprog number for success. This functionality will be
used in the next patch to save a call to find_subprog().
Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210226204924.3884848-1-yhs@fb.com

kernel/bpf/verifier.c

@@ -1530,7 +1530,7 @@ static int add_subprog(struct bpf_verifier_env *env, int off)
 	}
 	ret = find_subprog(env, off);
 	if (ret >= 0)
-		return 0;
+		return ret;
 	if (env->subprog_cnt >= BPF_MAX_SUBPROGS) {
 		verbose(env, "too many subprograms\n");
 		return -E2BIG;
@@ -1538,7 +1538,7 @@ static int add_subprog(struct bpf_verifier_env *env, int off)
 	env->subprog_info[env->subprog_cnt++].start = off;
 	sort(env->subprog_info, env->subprog_cnt,
 	     sizeof(env->subprog_info[0]), cmp_subprogs, NULL);
-	return 0;
+	return env->subprog_cnt - 1;
 }
 
 static int check_subprogs(struct bpf_verifier_env *env)