bpf: Allow void global functions in the verifier

Global subprogs are currently not allowed to return void. Adjust
verifier logic to allow global functions with a void return type.

Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Emil Tsalapatis <emil@etsalapatis.com>
Link: https://lore.kernel.org/r/20260228184759.108145-5-emil@etsalapatis.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/btf.c

@@ -7836,15 +7836,16 @@ int btf_prepare_func_args(struct bpf_verifier_env *env, int subprog)
 			tname, nargs, MAX_BPF_FUNC_REG_ARGS);
 		return -EINVAL;
 	}
-	/* check that function returns int, exception cb also requires this */
+	/* check that function is void or returns int, exception cb also requires this */
 	t = btf_type_by_id(btf, t->type);
 	while (btf_type_is_modifier(t))
 		t = btf_type_by_id(btf, t->type);
-	if (!btf_type_is_int(t) && !btf_is_any_enum(t)) {
+	if (!btf_type_is_void(t) && !btf_type_is_int(t) && !btf_is_any_enum(t)) {
 		if (!is_global)
 			return -EINVAL;
 		bpf_log(log,
-			"Global function %s() doesn't return scalar. Only those are supported.\n",
+			"Global function %s() return value not void or scalar. "
+			"Only those are supported.\n",
 			tname);
 		return -EINVAL;
 	}

kernel/bpf/verifier.c

@@ -444,6 +444,29 @@ static bool subprog_is_global(const struct bpf_verifier_env *env, int subprog)
 	return aux && aux[subprog].linkage == BTF_FUNC_GLOBAL;
 }
 
+static bool subprog_returns_void(struct bpf_verifier_env *env, int subprog)
+{
+	const struct btf_type *type, *func, *func_proto;
+	const struct btf *btf = env->prog->aux->btf;
+	u32 btf_id;
+
+	btf_id = env->prog->aux->func_info[subprog].type_id;
+
+	func = btf_type_by_id(btf, btf_id);
+	if (verifier_bug_if(!func, env, "btf_id %u not found", btf_id))
+		return false;
+
+	func_proto = btf_type_by_id(btf, func->type);
+	if (!func_proto)
+		return false;
+
+	type = btf_type_skip_modifiers(btf, func_proto->type, NULL);
+	if (!type)
+		return false;
+
+	return btf_type_is_void(type);
+}
+
 static const char *subprog_name(const struct bpf_verifier_env *env, int subprog)
 {
 	struct bpf_func_info *info;
@@ -10889,9 +10912,11 @@ static int check_func_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
 		subprog_aux(env, subprog)->called = true;
 		clear_caller_saved_regs(env, caller->regs);
 
-		/* All global functions return a 64-bit SCALAR_VALUE */
-		mark_reg_unknown(env, caller->regs, BPF_REG_0);
-		caller->regs[BPF_REG_0].subreg_def = DEF_NOT_SUBREG;
+		/* All non-void global functions return a 64-bit SCALAR_VALUE. */
+		if (!subprog_returns_void(env, subprog)) {
+			mark_reg_unknown(env, caller->regs, BPF_REG_0);
+			caller->regs[BPF_REG_0].subreg_def = DEF_NOT_SUBREG;
+		}
 
 		/* continue with next insn after call */
 		return 0;
@@ -17956,7 +17981,7 @@ static bool return_retval_range(struct bpf_verifier_env *env, struct bpf_retval_
 static bool program_returns_void(struct bpf_verifier_env *env)
 {
 	const struct bpf_prog *prog = env->prog;
-	enum bpf_prog_type prog_type = resolve_prog_type(prog);
+	enum bpf_prog_type prog_type = prog->type;
 
 	switch (prog_type) {
 	case BPF_PROG_TYPE_LSM:
@@ -17969,6 +17994,16 @@ static bool program_returns_void(struct bpf_verifier_env *env)
 		if (!prog->aux->attach_func_proto->type)
 			return true;
 		break;
+	case BPF_PROG_TYPE_EXT:
+		/*
+		 * If the actual program is an extension, let it
+		 * return void - attaching will succeed only if the
+		 * program being replaced also returns void, and since
+		 * it has passed verification its actual type doesn't matter.
+		 */
+		if (subprog_returns_void(env, 0))
+			return true;
+		break;
 	default:
 		break;
 	}
@@ -18063,8 +18098,12 @@ static int check_return_code(struct bpf_verifier_env *env, int regno, const char
 static int check_global_subprog_return_code(struct bpf_verifier_env *env)
 {
 	struct bpf_reg_state *reg = reg_state(env, BPF_REG_0);
+	struct bpf_func_state *cur_frame = cur_func(env);
 	int err;
 
+	if (subprog_returns_void(env, cur_frame->subprogno))
+		return 0;
+
 	err = check_reg_arg(env, BPF_REG_0, SRC_OP);
 	if (err)
 		return err;
@@ -24564,10 +24603,18 @@ static int do_check_common(struct bpf_verifier_env *env, int subprog)
 
 		if (subprog_is_exc_cb(env, subprog)) {
 			state->frame[0]->in_exception_callback_fn = true;
-			/* We have already ensured that the callback returns an integer, just
-			 * like all global subprogs. We need to determine it only has a single
-			 * scalar argument.
+
+			/*
+			 * Global functions are scalar or void, make sure
+			 * we return a scalar.
 			 */
+			if (subprog_returns_void(env, subprog)) {
+				verbose(env, "exception cb cannot return void\n");
+				ret = -EINVAL;
+				goto out;
+			}
+
+			/* Also ensure the callback only has a single scalar argument. */
 			if (sub->arg_cnt != 1 || sub->args[0].arg_type != ARG_ANYTHING) {
 				verbose(env, "exception cb only supports single integer argument\n");
 				ret = -EINVAL;

tools/testing/selftests/bpf/progs/exceptions_fail.c

@@ -51,7 +51,7 @@ __noinline int exception_cb_ok_arg_small(int a)
 
 SEC("?tc")
 __exception_cb(exception_cb_bad_ret_type)
-__failure __msg("Global function exception_cb_bad_ret_type() doesn't return scalar.")
+__failure __msg("Global function exception_cb_bad_ret_type() return value not void or scalar.")
 int reject_exception_cb_type_1(struct __sk_buff *ctx)
 {
 	bpf_throw(0);

tools/testing/selftests/bpf/progs/test_global_func7.c

@@ -12,7 +12,7 @@ void foo(struct __sk_buff *skb)
 }
 
 SEC("tc")
-__failure __msg("foo() doesn't return scalar")
+__success
 int global_func7(struct __sk_buff *skb)
 {
 	foo(skb);