实现SYS_LINK和SYS_LINKAT (#611)

* 实现do_linkat及SYS_LINK和SYS_LINKAT

* 未在riscv上测试，添加target_arch

* 将c字符串检查移动到vfs/syscall.rs，修改do_linkat()逻辑

* 修改部分注释

kernel/src/syscall/mod.rs

@@ -345,6 +345,21 @@ impl Syscall {
                 Self::rmdir(path)
             }
 
+            SYS_LINK => {
+                let old = args[0] as *const u8;
+                let new = args[1] as *const u8;
+                return Self::link(old, new);
+            }
+
+            SYS_LINKAT => {
+                let oldfd = args[0] as i32;
+                let old = args[1] as *const u8;
+                let newfd = args[2] as i32;
+                let new = args[3] as *const u8;
+                let flags = args[4] as i32;
+                return Self::linkat(oldfd, old, newfd, new, flags);
+            }
+
             #[cfg(target_arch = "x86_64")]
             SYS_UNLINK => {
                 let path = args[0] as *const u8;

kernel/src/syscall/syscall_num.h

@@ -65,9 +65,10 @@
 #define SYS_CHDIR 80
 
 #define SYS_MKDIR 83
-
 #define SYS_RMDIR 84
 
+#define SYS_LINK 86
+
 #define SYS_GETTIMEOFDAY 96
 
 #define SYS_ARCH_PRCTL 158
@@ -86,6 +87,8 @@
 
 #define SYS_UNLINK_AT 263
 
+#define SYS_LINKAT 265
+
 #define SYS_PIPE 293
 
 #define SYS_WRITEV 20