Merge branch 'exec-missing-validation' into 'master'

Add validation to check for missing files in exec() args

See merge request lab2425_autumn/pintos_22!36

src/userprog/process.c

@@ -81,6 +81,10 @@ process_execute (const char *cmd)
      of the process. */
   char *file_name = strtok_r (cmd_copy, " ", &data->cmd_saveptr);
 
+/* Validates that the current file to be executed is a valid file */
+if (filesys_open (file_name) == NULL)
+    return TID_ERROR;
+
   /* Create a new thread to execute the command, by initializing
      it running the function 'start_process' with the appropriate
      arguments. For details of arguments, see 'start_process'. */