remount issue on android 7.0

http://blog.chinaunix.net/uid-23141914-id-5754416.html

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。

1. :/ $ ls -al
   /system/bin/mount
2. 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。

1. // For remount
   we need _last_ match (in case of
   overmounts), so traverse
2. //
   in reverse order. (Yes I'm using remount as a boolean for a bit here,
3. //
   the double cast is to get gcc
   to shut up about it.)
4. remount = (void *)(long)comma_scan(opts, "remount",
   1);
5. if (((toys.optflags
   & FLAG_a) && !access("/proc/mounts", R_OK)) || remount)
   {
6. mm = dlist_terminate(mtl = mtl2
   = xgetmountlist(0));
7. if (remount)
   remount = mm;
8. }

关键就是这个comma_scan的最后一个参数clean

1. // check all instances of opt and "no"opt
   in optlist, return true
   if opt
2. //
   found and last instance wasn't no. If clean,
   remove each instance from list.
3. int comma_scan(char *optlist, char
   *opt,
   int clean)
4. {
5. int optlen = strlen(opt), len, no, got
   = 0;
6. if (optlist)
   for (;;) {
7. char *s = comma_iterate(&optlist,
   &len);
8. if (!s) break;
9. no = 2*(*s == 'n' && s[1] == 'o');
10. if (optlen == len-no && !strncmp(opt, s+no,
    optlen))
    {
11. got = !no;
12. if (clean &&
    optlist) memmove(s,
    optlist, strlen(optlist)+1);
13. }
14. }
15. return got;
16. }

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！