fix kappl and read

kernel/syscall/skeleton.cc

@@ -66,16 +66,16 @@ size_t read(Vault &vault, uint32_t id, void *buf, size_t len) {
     (void)id;
 
     size_t read_cnt = 0;
-    while(read_cnt <= len){
+    while(read_cnt < len){
         Key key;
         vault.keys_sem.p(vault);
         vault.keys.consume(key);
         if(key.valid())
             ((char*)buf)[read_cnt++] = key.ascii();
         else
-            return read_cnt;
+            break;
     }
-    return len;
+    return read_cnt;
 }
 
 void sleep(Vault &vault, size_t ms) {