diff --git a/kernel/aster-nix/src/syscall/wait4.rs b/kernel/aster-nix/src/syscall/wait4.rs
index c1c763de2..c575acc7c 100644
--- a/kernel/aster-nix/src/syscall/wait4.rs
+++ b/kernel/aster-nix/src/syscall/wait4.rs
@@ -11,7 +11,8 @@ use crate::{
 
 pub fn sys_wait4(wait_pid: u64, exit_status_ptr: u64, wait_options: u32) -> Result<SyscallReturn> {
     log_syscall_entry!(SYS_WAIT4);
-    let wait_options = WaitOptions::from_bits(wait_options).expect("Unknown wait options");
+    let wait_options = WaitOptions::from_bits(wait_options)
+        .ok_or_else(|| Error::with_message(Errno::EINVAL, "unknown wait option"))?;
     debug!(
         "pid = {}, exit_status_ptr = {}, wait_options: {:?}",
         wait_pid as i32, exit_status_ptr, wait_options
diff --git a/regression/apps/fork/fork.s b/regression/apps/fork/fork.S
similarity index 78%
rename from regression/apps/fork/fork.s
rename to regression/apps/fork/fork.S
index 3bdfa103d..547220731 100644
--- a/regression/apps/fork/fork.s
+++ b/regression/apps/fork/fork.S
@@ -1,5 +1,10 @@
 # SPDX-License-Identifier: MPL-2.0
 
+# FIXME: WNOHANG option currently does not work properly without preemption, so we have temporarily 
+# removed it. Once preemption is supported, the following macro can be uncommented to add the WNOHANG 
+# option back.
+# #define PREEMPTION_ENABLE
+
 .global _start
 
 .section .text
@@ -22,14 +27,22 @@ _child:
     call    exit
 wait_child:
     mov     %rax, %rdi                  # child process id
+#ifdef PREEMPTION_ENABLE
 _loop:
     mov     $61, %rax                   # syscall number of wait4
     mov     $0, %rsi                    # exit status address
-    mov     $1, %rdx                    # WNOHANG
+    mov     $1, %rdx                    # wait option: WNOHANG
     syscall
     cmp     %rdi, %rax                  # The return value is the pid of child 
     jne _loop
     ret
+#else
+    mov     $61, %rax                   # syscall number of wait4
+    mov     $0, %rsi                    # exit status address
+    mov     $0, %rdx                    # wait option
+    syscall
+    ret
+#endif
 exit:
     mov     $60, %rax                   # syscall number of exit
     mov     $0, %rdi                    # exit code
diff --git a/regression/apps/hello_world/hello_world.s b/regression/apps/hello_world/hello_world.S
similarity index 100%
rename from regression/apps/hello_world/hello_world.s
rename to regression/apps/hello_world/hello_world.S
diff --git a/regression/apps/test_common.mk b/regression/apps/test_common.mk
index b8f2a18d5..e3342924b 100644
--- a/regression/apps/test_common.mk
+++ b/regression/apps/test_common.mk
@@ -10,8 +10,8 @@ DEP_OUTPUT_DIR := $(BUILD_DIR)/dep/$(CUR_DIR_NAME)
 C_SRCS := $(wildcard *.c)
 C_OBJS := $(addprefix $(OBJ_OUTPUT_DIR)/,$(C_SRCS:%.c=%))
 C_DEPS := $(addprefix $(DEP_OUTPUT_DIR)/,$(C_SRCS:%.c=%.d))
-ASM_SRCS := $(wildcard *.s)
-ASM_OBJS := $(addprefix $(OBJ_OUTPUT_DIR)/,$(ASM_SRCS:%.s=%))
+ASM_SRCS := $(wildcard *.S)
+ASM_OBJS := $(addprefix $(OBJ_OUTPUT_DIR)/,$(ASM_SRCS:%.S=%))
 CC := gcc
 C_FLAGS := -Wall -Werror
 
@@ -28,6 +28,6 @@ $(OBJ_OUTPUT_DIR)/%: %.c | $(OBJ_OUTPUT_DIR) $(DEP_OUTPUT_DIR)
 
 -include $(C_DEPS)
 
-$(OBJ_OUTPUT_DIR)/%: %.s | $(OBJ_OUTPUT_DIR)
+$(OBJ_OUTPUT_DIR)/%: %.S | $(OBJ_OUTPUT_DIR)
 	@$(CC) $(C_FLAGS) $(EXTRA_C_FLAGS) $< -o $@
 	@echo "CC <= $@"