The background is like this:
I want to implement a sandbox, which is a system-level sandbox to isolate the data of a certain App.
I need to isolate his one
[/data/data/package_name]and [/sdcard/]
Place these directories arbitrarily, such as [system / data / mydir]
(Because I want these configurations to be managed through system_server)

There are now some problems in achieving isolation / sdcard /,
My command is
mount –bind [/storage/emulated/0] [system/data/mydir/sdcard]
When I finish the execution, I find it in writing. The extended target, the ExternalFiles
(/storage/emulated/0/Android/data/com.example.testfp/files/myfileasfsd.txt)
It tells me that the error is java.nio.file. AccessDeniedException: /storage/emulated/0/Android

I have given this directory [system / data / mydi r/sdcard] and [/ storage / emulated / 0]
Same uid, same selinux rules, same read and write permission.

Why is this about?

I tried many options and I don t know why