remount issue on android 7.0

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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。

:/ $ 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！