diff --git a/osdk/src/base_crate/main.rs.template b/osdk/src/base_crate/main.rs.template
index bc51792e1..7ef117ed9 100644
--- a/osdk/src/base_crate/main.rs.template
+++ b/osdk/src/base_crate/main.rs.template
@@ -7,7 +7,7 @@ extern crate #TARGET_NAME#;
 
 #[panic_handler]
 fn panic(info: &core::panic::PanicInfo) -> ! {
-    extern "Rust" {
+    unsafe extern "Rust" {
         pub fn __ostd_panic_handler(info: &core::panic::PanicInfo) -> !;
     }
     unsafe { __ostd_panic_handler(info); }
diff --git a/osdk/src/commands/test.rs b/osdk/src/commands/test.rs
index 1413f6930..812846707 100644
--- a/osdk/src/commands/test.rs
+++ b/osdk/src/commands/test.rs
@@ -102,7 +102,6 @@ pub static KTEST_CRATE_WHITELIST: Option<&[&str]> = Some(&{:#?});
         ActionChoice::Test,
         &["--cfg ktest", "-C panic=unwind"],
     );
-    std::env::remove_var("RUSTFLAGS");
     drop(dir_guard);
 
     bundle.run(config, ActionChoice::Test);
diff --git a/ostd/libs/linux-bzimage/setup/src/x86/amd64_efi/efi.rs b/ostd/libs/linux-bzimage/setup/src/x86/amd64_efi/efi.rs
index 1b5df67ed..e2779b7d5 100644
--- a/ostd/libs/linux-bzimage/setup/src/x86/amd64_efi/efi.rs
+++ b/ostd/libs/linux-bzimage/setup/src/x86/amd64_efi/efi.rs
@@ -14,7 +14,7 @@ pub(super) const PAGE_SIZE: u64 = 4096;
 
 /// SAFETY: The name does not collide with other symbols.
 #[unsafe(no_mangle)]
-extern "sysv64" fn main_efi_common64(
+unsafe extern "sysv64" fn main_efi_common64(
     handle: Handle,
     system_table: *const SystemTable,
     boot_params_ptr: *mut BootParams,
diff --git a/ostd/libs/linux-bzimage/setup/src/x86/legacy_i386/alloc.rs b/ostd/libs/linux-bzimage/setup/src/x86/legacy_i386/alloc.rs
index 8af67c3ba..8307b8661 100644
--- a/ostd/libs/linux-bzimage/setup/src/x86/legacy_i386/alloc.rs
+++ b/ostd/libs/linux-bzimage/setup/src/x86/legacy_i386/alloc.rs
@@ -24,7 +24,7 @@ pub(super) unsafe fn init(boot_params: &'static linux_boot_params::BootParams) {
 
     let mut used = core::array::from_fn(|_| 0..0);
 
-    extern "C" {
+    unsafe extern "C" {
         fn __executable_start();
         fn __executable_end();
     }
diff --git a/ostd/libs/linux-bzimage/setup/src/x86/mod.rs b/ostd/libs/linux-bzimage/setup/src/x86/mod.rs
index 0bead8a87..df2ff1352 100644
--- a/ostd/libs/linux-bzimage/setup/src/x86/mod.rs
+++ b/ostd/libs/linux-bzimage/setup/src/x86/mod.rs
@@ -30,7 +30,7 @@ pub fn image_load_offset() -> isize {
     /// The load address of the `entry_legacy32` symbol specified in the linker script.
     const CODE32_START: isize = 0x100000;
 
-    extern "C" {
+    unsafe extern "C" {
         fn entry_legacy32();
     }
 
@@ -44,7 +44,7 @@ global_asm!(
 
 /// Returns an immutable slice containing the payload (i.e., the kernel).
 fn payload() -> &'static [u8] {
-    extern "C" {
+    unsafe extern "C" {
         fn __payload_start();
         fn __payload_end();
     }
diff --git a/ostd/libs/ostd-test/src/lib.rs b/ostd/libs/ostd-test/src/lib.rs
index aa75f9270..7f84037d1 100644
--- a/ostd/libs/ostd-test/src/lib.rs
+++ b/ostd/libs/ostd-test/src/lib.rs
@@ -95,6 +95,7 @@ pub enum KtestError {
 
 /// The information of the unit test.
 #[derive(Clone, PartialEq, Debug)]
+#[repr(C)]
 pub struct KtestItemInfo {
     /// The path of the module, not including the function name.
     ///
@@ -112,7 +113,8 @@ pub struct KtestItemInfo {
     pub col: usize,
 }
 
-#[derive(Clone, PartialEq, Debug)]
+#[derive(Clone, Debug)]
+#[repr(C)]
 pub struct KtestItem {
     fn_: fn() -> (),
     should_panic: (bool, Option<&'static str>),
@@ -181,14 +183,18 @@ impl KtestItem {
 
 macro_rules! ktest_array {
     () => {{
-        extern "C" {
+        unsafe extern "C" {
             fn __ktest_array();
             fn __ktest_array_end();
         }
         let item_size = size_of::<KtestItem>();
-        let l = (__ktest_array_end as usize - __ktest_array as usize) / item_size;
-        // SAFETY: __ktest_array is a static section consisting of KtestItem.
-        unsafe { core::slice::from_raw_parts(__ktest_array as *const KtestItem, l) }
+        let array_ptr = __ktest_array as *const () as *const KtestItem;
+        let array_end_ptr = __ktest_array_end as *const () as *const KtestItem;
+        // SAFETY: The pointer arithmetic is valid since both pointers point to
+        // the same section.
+        let l = unsafe { array_end_ptr.offset_from(array_ptr) as usize } / item_size;
+        // SAFETY: `__ktest_array` is a static section consisting of `KtestItem`s.
+        unsafe { core::slice::from_raw_parts(array_ptr, l) }
     }};
 }
 
@@ -224,7 +230,7 @@ impl core::iter::Iterator for KtestIter {
 
 // The whitelists that will be generated by the OSDK as static consts.
 // They deliver the target tests that the user wants to run.
-extern "Rust" {
+unsafe extern "Rust" {
     static KTEST_TEST_WHITELIST: Option<&'static [&'static str]>;
     static KTEST_CRATE_WHITELIST: Option<&'static [&'static str]>;
 }
diff --git a/ostd/src/arch/loongarch/trap/mod.rs b/ostd/src/arch/loongarch/trap/mod.rs
index 2199db699..07fbc8b49 100644
--- a/ostd/src/arch/loongarch/trap/mod.rs
+++ b/ostd/src/arch/loongarch/trap/mod.rs
@@ -38,7 +38,7 @@ pub(crate) unsafe fn init_on_cpu() {
 /// Handle traps (only from kernel).
 // SAFETY: The name does not collide with other symbols.
 #[unsafe(no_mangle)]
-extern "C" fn trap_handler(f: &mut TrapFrame) {
+unsafe extern "C" fn trap_handler(f: &mut TrapFrame) {
     let cause = estat::read().cause();
     let badi = loongArch64::register::badi::read().raw();
     let badv = loongArch64::register::badv::read().vaddr();
diff --git a/ostd/src/arch/riscv/boot/smp.rs b/ostd/src/arch/riscv/boot/smp.rs
index 128714a00..c17c09757 100644
--- a/ostd/src/arch/riscv/boot/smp.rs
+++ b/ostd/src/arch/riscv/boot/smp.rs
@@ -116,7 +116,7 @@ unsafe fn bringup_ap(hart_id: u32) {
 ///
 /// The caller must ensure that `__ap_boot_info_array_pointer` is safe to write.
 unsafe fn fill_boot_info_ptr(info_ptr: *const PerApRawInfo) {
-    extern "C" {
+    unsafe extern "C" {
         static mut __ap_boot_info_array_pointer: *const PerApRawInfo;
     }
 
@@ -132,7 +132,7 @@ unsafe fn fill_boot_info_ptr(info_ptr: *const PerApRawInfo) {
 ///
 /// The caller must ensure that `__ap_boot_page_table_pointer` is safe to write.
 unsafe fn fill_boot_page_table_ptr(pt_ptr: Paddr) {
-    extern "C" {
+    unsafe extern "C" {
         static mut __ap_boot_page_table_pointer: Paddr;
     }
 
diff --git a/ostd/src/arch/riscv/mm/util.rs b/ostd/src/arch/riscv/mm/util.rs
index e86a18ec0..d4c729c84 100644
--- a/ostd/src/arch/riscv/mm/util.rs
+++ b/ostd/src/arch/riscv/mm/util.rs
@@ -8,7 +8,7 @@ core::arch::global_asm!(include_str!("memset_fallible.S"), SSTATUS_SUM = const S
 core::arch::global_asm!(include_str!("atomic_load_fallible.S"), SSTATUS_SUM = const SSTATUS_SUM);
 core::arch::global_asm!(include_str!("atomic_cmpxchg_fallible.S"), SSTATUS_SUM = const SSTATUS_SUM);
 
-extern "C" {
+unsafe extern "C" {
     /// Copies `size` bytes from `src` to `dst`. This function works with exception handling
     /// and can recover from page fault.
     /// Returns number of bytes that failed to copy.
diff --git a/ostd/src/arch/riscv/trap/mod.rs b/ostd/src/arch/riscv/trap/mod.rs
index 88d7e4e89..4dbef13dd 100644
--- a/ostd/src/arch/riscv/trap/mod.rs
+++ b/ostd/src/arch/riscv/trap/mod.rs
@@ -42,7 +42,7 @@ pub(crate) unsafe fn init_on_cpu() {
 /// Handle traps (only from kernel).
 // SAFETY: The name does not collide with other symbols.
 #[unsafe(no_mangle)]
-extern "C" fn trap_handler(f: &mut TrapFrame) {
+unsafe extern "C" fn trap_handler(f: &mut TrapFrame) {
     fn enable_local_if(cond: bool) {
         if cond {
             enable_local();
diff --git a/ostd/src/arch/x86/boot/smp.rs b/ostd/src/arch/x86/boot/smp.rs
index 553e64d3a..b6b284d96 100644
--- a/ostd/src/arch/x86/boot/smp.rs
+++ b/ostd/src/arch/x86/boot/smp.rs
@@ -174,7 +174,7 @@ unsafe fn copy_ap_boot_code() {
 ///
 /// The caller must ensure the pointer to be filled is valid to write.
 unsafe fn fill_boot_info_ptr(info_ptr: *const PerApRawInfo) {
-    extern "C" {
+    unsafe extern "C" {
         static mut __ap_boot_info_array_pointer: *const PerApRawInfo;
     }
 
@@ -188,7 +188,7 @@ unsafe fn fill_boot_info_ptr(info_ptr: *const PerApRawInfo) {
 ///
 /// The caller must ensure the pointer to be filled is valid to write.
 unsafe fn fill_boot_pt_ptr(pt_ptr: Paddr) {
-    extern "C" {
+    unsafe extern "C" {
         static mut __boot_page_table_pointer: u32;
     }
 
@@ -201,7 +201,7 @@ unsafe fn fill_boot_pt_ptr(pt_ptr: Paddr) {
 }
 
 // The symbols are defined in the linker script.
-extern "C" {
+unsafe extern "C" {
     fn __ap_boot_start();
     fn __ap_boot_end();
 }
@@ -218,7 +218,7 @@ unsafe fn wake_up_aps_via_mailbox(num_cpus: u32) {
     use crate::arch::kernel::acpi::AcpiMemoryHandler;
 
     // The symbols are defined in `ap_boot.S`.
-    extern "C" {
+    unsafe extern "C" {
         fn ap_boot_from_real_mode();
         fn ap_boot_from_long_mode();
     }
diff --git a/ostd/src/arch/x86/mm/util.rs b/ostd/src/arch/x86/mm/util.rs
index 951bafd9d..08777ad49 100644
--- a/ostd/src/arch/x86/mm/util.rs
+++ b/ostd/src/arch/x86/mm/util.rs
@@ -6,7 +6,7 @@ core::arch::global_asm!(include_str!("memset_fallible.S"));
 core::arch::global_asm!(include_str!("atomic_load_fallible.S"));
 core::arch::global_asm!(include_str!("atomic_cmpxchg_fallible.S"));
 
-extern "C" {
+unsafe extern "C" {
     /// Copies `size` bytes from `src` to `dst`. This function works with exception handling
     /// and can recover from page fault.
     /// Returns number of bytes that failed to copy.
diff --git a/ostd/src/arch/x86/trap/idt.rs b/ostd/src/arch/x86/trap/idt.rs
index 9abb780ee..b4b44422e 100644
--- a/ostd/src/arch/x86/trap/idt.rs
+++ b/ostd/src/arch/x86/trap/idt.rs
@@ -16,7 +16,7 @@ global_asm!(include_str!("trap.S"));
 
 const NUM_INTERRUPTS: usize = 256;
 
-extern "C" {
+unsafe extern "C" {
     #[link_name = "trap_handler_table"]
     static VECTORS: [usize; NUM_INTERRUPTS];
 }
diff --git a/ostd/src/arch/x86/trap/mod.rs b/ostd/src/arch/x86/trap/mod.rs
index eb40d0b9a..47d938296 100644
--- a/ostd/src/arch/x86/trap/mod.rs
+++ b/ostd/src/arch/x86/trap/mod.rs
@@ -133,7 +133,7 @@ pub(super) struct RawUserContext {
 /// Handle traps (only from kernel).
 // SAFETY: The name does not collide with other symbols.
 #[unsafe(no_mangle)]
-extern "sysv64" fn trap_handler(f: &mut TrapFrame) {
+unsafe extern "sysv64" fn trap_handler(f: &mut TrapFrame) {
     fn enable_local_if(cond: bool) {
         if cond {
             enable_local();
diff --git a/ostd/src/boot/memory_region.rs b/ostd/src/boot/memory_region.rs
index 2211d7d4f..fd4b21bf0 100644
--- a/ostd/src/boot/memory_region.rs
+++ b/ostd/src/boot/memory_region.rs
@@ -78,7 +78,7 @@ impl MemoryRegion {
     /// we need to explicitly construct and append this memory region.
     pub fn kernel() -> Self {
         // These are physical addresses provided by the linker script.
-        extern "C" {
+        unsafe extern "C" {
             fn __kernel_start();
             fn __kernel_end();
         }
diff --git a/ostd/src/boot/mod.rs b/ostd/src/boot/mod.rs
index dbb1622b8..91dd9b81d 100644
--- a/ostd/src/boot/mod.rs
+++ b/ostd/src/boot/mod.rs
@@ -122,7 +122,7 @@ pub(crate) fn init_after_heap() {
 pub(crate) fn call_ostd_main() -> ! {
     // The entry point of kernel code, which should be defined by the package that
     // uses OSTD.
-    extern "Rust" {
+    unsafe extern "Rust" {
         fn __ostd_main() -> !;
     }
 
diff --git a/ostd/src/cpu/local/mod.rs b/ostd/src/cpu/local/mod.rs
index 081199a54..f25b3d361 100644
--- a/ostd/src/cpu/local/mod.rs
+++ b/ostd/src/cpu/local/mod.rs
@@ -67,7 +67,7 @@ pub type DynamicCpuLocal<T> = CpuLocal<T, DynamicStorage<T>>;
 pub type StaticCpuLocal<T> = CpuLocal<T, static_cpu_local::StaticStorage<T>>;
 
 // These symbols are provided by the linker script.
-extern "C" {
+unsafe extern "C" {
     fn __cpu_local_start();
     fn __cpu_local_end();
 }
diff --git a/ostd/src/ex_table.rs b/ostd/src/ex_table.rs
index cebbfbf2d..c2bf0e42d 100644
--- a/ostd/src/ex_table.rs
+++ b/ostd/src/ex_table.rs
@@ -8,7 +8,7 @@ struct ExTableItem {
     recovery_inst_addr: Vaddr,
 }
 
-extern "C" {
+unsafe extern "C" {
     fn __ex_table();
     fn __ex_table_end();
 }
diff --git a/ostd/src/io/io_port/allocator.rs b/ostd/src/io/io_port/allocator.rs
index f43600b47..b316c61fb 100644
--- a/ostd/src/io/io_port/allocator.rs
+++ b/ostd/src/io/io_port/allocator.rs
@@ -80,7 +80,7 @@ pub(in crate::io) unsafe fn init() {
     // SAFETY: `MAX_IO_PORT` is guaranteed not to exceed the maximum value specified by architecture.
     let mut allocator = IdAlloc::with_capacity(crate::arch::io::MAX_IO_PORT as usize);
 
-    extern "C" {
+    unsafe extern "C" {
         fn __sensitive_io_ports_start();
         fn __sensitive_io_ports_end();
     }
diff --git a/ostd/src/lib.rs b/ostd/src/lib.rs
index d61e5cd4a..770c6b7b7 100644
--- a/ostd/src/lib.rs
+++ b/ostd/src/lib.rs
@@ -143,7 +143,7 @@ pub(crate) static IN_BOOTSTRAP_CONTEXT: AtomicBool = AtomicBool::new(true);
 /// The component system uses this function to call the initialization functions of
 /// the components.
 fn invoke_ffi_init_funcs() {
-    extern "C" {
+    unsafe extern "C" {
         fn __sinit_array();
         fn __einit_array();
     }
diff --git a/ostd/src/mm/frame/allocator.rs b/ostd/src/mm/frame/allocator.rs
index 916917ee2..c5033712c 100644
--- a/ostd/src/mm/frame/allocator.rs
+++ b/ostd/src/mm/frame/allocator.rs
@@ -175,7 +175,7 @@ pub trait GlobalFrameAllocator: Sync {
     fn add_free_memory(&self, addr: Paddr, size: usize);
 }
 
-extern "Rust" {
+unsafe extern "Rust" {
     /// The global frame allocator's reference exported by
     /// [`crate::global_frame_allocator`].
     static __GLOBAL_FRAME_ALLOCATOR_REF: &'static dyn GlobalFrameAllocator;
diff --git a/ostd/src/mm/heap/mod.rs b/ostd/src/mm/heap/mod.rs
index 6718f67f3..0afbeedd0 100644
--- a/ostd/src/mm/heap/mod.rs
+++ b/ostd/src/mm/heap/mod.rs
@@ -51,7 +51,7 @@ pub trait GlobalHeapAllocator: Sync {
     fn dealloc(&self, slot: HeapSlot) -> Result<(), AllocError>;
 }
 
-extern "Rust" {
+unsafe extern "Rust" {
     /// The reference to the global heap allocator generated by the
     /// [`crate::global_heap_allocator`] attribute.
     static __GLOBAL_HEAP_ALLOCATOR_REF: &'static dyn GlobalHeapAllocator;