remount issue on android 7.0

最近在新版本的android 7.0上，发现filesystem的remount老是报“ Device or resource busy”的错误。
最终发现，android 7.0上，从原来的toolbox切换到toybox。

:/ $ ls -al /system/bin/mount
lrwxr-xr-x 1 root shell 6 2016-09-02 16:23 /system/bin/mount -> toybox

然后仔细查看了一下toybox中关于mount的相关代码，果然是一个坑。

// For remount we need _last_ match (in case of overmounts), so traverse
  // in reverse order. (Yes I'm using remount as a boolean for a bit here,
  // the double cast is to get gcc to shut up about it.)
  remount = (void *)(long)comma_scan(opts, "remount", 1);
  if (((toys.optflags & FLAG_a) && !access("/proc/mounts", R_OK)) || remount) {
    mm = dlist_terminate(mtl = mtl2 = xgetmountlist(0));
    if (remount) remount = mm;
  }

关键就是这个comma_scan的最后一个参数clean

// check all instances of opt and "no"opt in optlist, return true if opt
// found and last instance wasn't no. If clean, remove each instance from list.
int comma_scan(char *optlist, char *opt, int clean)
{
  int optlen = strlen(opt), len, no, got = 0;

  if (optlist) for (;;) {
    char *s = comma_iterate(&optlist, &len);

    if (!s) break;
    no = 2*(*s == 'n' && s[1] == 'o');
    if (optlen == len-no && !strncmp(opt, s+no, optlen)) {
      got = !no;
      if (clean && optlist) memmove(s, optlist, strlen(optlist)+1);
    }
  }

  return got;
}

如果remount不是-o的最后一个参数（_last_ match ），那么就会被清除掉。最终调用syscall mount的时候，这个remount的flag就没了。
把命令重新改为 mount -t vfat -o rw,remount /firmware 就好了。
关于remount的说明在help里面根本没有，真是坑！