Fetching buffer arguments from user space.

os/src/task/context.rs

@@ -1,3 +1,5 @@
+use crate::trap::trap_return;
+
 #[repr(C)]
 pub struct TaskContext {
     ra: usize,
@@ -5,10 +7,9 @@ pub struct TaskContext {
 }
 
 impl TaskContext {
-    pub fn goto_restore() -> Self {
-        extern "C" { fn __restore(); }
+    pub fn goto_trap_return() -> Self {
         Self {
-            ra: __restore as usize,
+            ra: trap_return as usize,
             s: [0; 12],
         }
     }