selftests/bpf: Add tests for KF_RCU_PROTECTED

Add a couple of test cases to ensure RCU protection is kicked in
automatically, and the return type is as expected.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20250917032755.4068726-3-memxor@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Author: kkdwvd

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

@@ -123,3 +123,49 @@ int iter_next_ptr_mem_not_trusted(const void *ctx)
 	bpf_iter_num_destroy(&num_it);
 	return 0;
 }
+
+SEC("?fentry.s/" SYS_PREFIX "sys_getpgid")
+__failure __msg("kernel func bpf_kfunc_ret_rcu_test requires RCU critical section protection")
+int iter_ret_rcu_test_protected(const void *ctx)
+{
+	struct task_struct *p;
+
+	p = bpf_kfunc_ret_rcu_test();
+	return p->pid;
+}
+
+SEC("?fentry.s/" SYS_PREFIX "sys_getpgid")
+__failure __msg("R1 type=rcu_ptr_or_null_ expected=")
+int iter_ret_rcu_test_type(const void *ctx)
+{
+	struct task_struct *p;
+
+	bpf_rcu_read_lock();
+	p = bpf_kfunc_ret_rcu_test();
+	bpf_this_cpu_ptr(p);
+	bpf_rcu_read_unlock();
+	return 0;
+}
+
+SEC("?fentry.s/" SYS_PREFIX "sys_getpgid")
+__failure __msg("kernel func bpf_kfunc_ret_rcu_test_nostruct requires RCU critical section protection")
+int iter_ret_rcu_test_protected_nostruct(const void *ctx)
+{
+	void *p;
+
+	p = bpf_kfunc_ret_rcu_test_nostruct(4);
+	return *(int *)p;
+}
+
+SEC("?fentry.s/" SYS_PREFIX "sys_getpgid")
+__failure __msg("R1 type=rdonly_rcu_mem_or_null expected=")
+int iter_ret_rcu_test_type_nostruct(const void *ctx)
+{
+	void *p;
+
+	bpf_rcu_read_lock();
+	p = bpf_kfunc_ret_rcu_test_nostruct(4);
+	bpf_this_cpu_ptr(p);
+	bpf_rcu_read_unlock();
+	return 0;
+}

tools/testing/selftests/bpf/test_kmods/bpf_testmod.c

@@ -218,6 +218,16 @@ __bpf_kfunc void bpf_kfunc_rcu_task_test(struct task_struct *ptr)
 {
 }
 
+__bpf_kfunc struct task_struct *bpf_kfunc_ret_rcu_test(void)
+{
+	return NULL;
+}
+
+__bpf_kfunc int *bpf_kfunc_ret_rcu_test_nostruct(int rdonly_buf_size)
+{
+	return NULL;
+}
+
 __bpf_kfunc struct bpf_testmod_ctx *
 bpf_testmod_ctx_create(int *err)
 {
@@ -623,6 +633,8 @@ BTF_ID_FLAGS(func, bpf_kfunc_trusted_vma_test, KF_TRUSTED_ARGS)
 BTF_ID_FLAGS(func, bpf_kfunc_trusted_task_test, KF_TRUSTED_ARGS)
 BTF_ID_FLAGS(func, bpf_kfunc_trusted_num_test, KF_TRUSTED_ARGS)
 BTF_ID_FLAGS(func, bpf_kfunc_rcu_task_test, KF_RCU)
+BTF_ID_FLAGS(func, bpf_kfunc_ret_rcu_test, KF_RET_NULL | KF_RCU_PROTECTED)
+BTF_ID_FLAGS(func, bpf_kfunc_ret_rcu_test_nostruct, KF_RET_NULL | KF_RCU_PROTECTED)
 BTF_ID_FLAGS(func, bpf_testmod_ctx_create, KF_ACQUIRE | KF_RET_NULL)
 BTF_ID_FLAGS(func, bpf_testmod_ctx_release, KF_RELEASE)
 BTF_ID_FLAGS(func, bpf_testmod_ops3_call_test_1)

tools/testing/selftests/bpf/test_kmods/bpf_testmod_kfunc.h

@@ -158,6 +158,8 @@ void bpf_kfunc_trusted_vma_test(struct vm_area_struct *ptr) __ksym;
 void bpf_kfunc_trusted_task_test(struct task_struct *ptr) __ksym;
 void bpf_kfunc_trusted_num_test(int *ptr) __ksym;
 void bpf_kfunc_rcu_task_test(struct task_struct *ptr) __ksym;
+struct task_struct *bpf_kfunc_ret_rcu_test(void) __ksym;
+int *bpf_kfunc_ret_rcu_test_nostruct(int rdonly_buf_size) __ksym;
 
 int bpf_kfunc_multi_st_ops_test_1(struct st_ops_args *args, u32 id) __ksym;