fix: add check to mmap to ensure file isn't mapped over stack segment

src/userprog/syscall.c

@@ -426,6 +426,10 @@ syscall_mmap (int fd, void *addr)
   if (file_size == 0)
     return MMAP_FAILURE;
 
+  /* ensures the page for mmap does not overlap with the stack */
+  if (addr >= (thread_current ()->curr_esp - PGSIZE))
+    return MMAP_FAILURE;
+
   /* Check and ensure that there is enough space in the user virtual memory to
      hold the entire file. */
   for (off_t ofs = 0; ofs < file_size; ofs += PGSIZE)