update ch4:add some exception support in trap_handler function, add two *_fault.rs apps

os/src/trap/mod.rs

@@ -48,7 +48,8 @@ pub fn trap_handler() -> ! {
             cx.sepc += 4;
             cx.x[10] = syscall(cx.x[17], [cx.x[10], cx.x[11], cx.x[12]]) as usize;
         }
-        Trap::Exception(Exception::StoreFault) | Trap::Exception(Exception::StorePageFault) => {
+        Trap::Exception(Exception::StoreFault) | Trap::Exception(Exception::StorePageFault) |
+        Trap::Exception(Exception::LoadFault)  | Trap::Exception(Exception::LoadPageFault) => {
             println!("[kernel] PageFault in application, bad addr = {:#x}, bad instruction = {:#x}, kernel killed it.", stval, cx.sepc);
             exit_current_and_run_next();
         }

user/src/bin/04load_fault.rs

@@ -0,0 +1,18 @@
+#![no_std]
+#![no_main]
+
+#[macro_use]
+extern crate user_lib;
+
+use core::ptr::{read_volatile,null_mut};
+
+#[no_mangle]
+fn main() -> i32 {
+    println!("\nload_fault APP running...\n");
+    println!("Into Test load_fault, we will insert an invalid load operation...");
+    println!("Kernel should kill this application!");
+    unsafe {
+        let _i=read_volatile(null_mut::<u8>());
+    }
+    0
+}

user/src/bin/05store_fault.rs

@@ -0,0 +1,18 @@
+#![no_std]
+#![no_main]
+
+#[macro_use]
+extern crate user_lib;
+
+use core::ptr::null_mut;
+
+#[no_mangle]
+fn main() -> i32 {
+    println!("\nstore_fault APP running...\n");
+    println!("Into Test store_fault, we will insert an invalid store operation...");
+    println!("Kernel should kill this application!");
+    unsafe {
+       null_mut::<u8>().write_volatile(1);
+    }
+    0
+}