add sbrk and a usertest

os/src/syscall/process.rs

@@ -1,6 +1,6 @@
 //! Process management syscalls
 
-use crate::task::{exit_current_and_run_next, suspend_current_and_run_next};
+use crate::task::{exit_current_and_run_next, suspend_current_and_run_next, change_program_brk};
 use crate::timer::get_time_ms;
 
 /// task exits and submit an exit code
@@ -20,3 +20,12 @@ pub fn sys_yield() -> isize {
 pub fn sys_get_time() -> isize {
     get_time_ms() as isize
 }
+
+/// change data segment size
+pub fn sys_sbrk(size: i32) -> isize {
+    if let Some(old_brk) = change_program_brk(size) {
+        old_brk as isize
+    } else {
+        -1
+    }
+}