In order to protect the '/android_secure' directory on VFAT removable media
from being mucked with by 3rd party applications on the device, we hide the
directory with a read-only, zero-sized tmpfs mounted on-top. A reference to the
hidden directory is kept by a bind-mount which is mounted at a location which
only root can access.
Staging consists of:
1. Mount checked media at a secure location (/mnt/secure/staging)
2. Ensure /android_secure exists on the media, (creating if it doesnt)
3. Bind-mount /mnt/secure/staging/android_secure -> /mnt/secure/asec
(where only root can access it)
4. Mount an RDONLY zero-sized tmpfs over /mnt/secure/staging/android_secure
5. Atomically move /mnt/secure/staging to the publicly accessable storage
directory (/mnt/sdcard)
Signed-off-by: San Mehat <san@google.com>
3bb6020e46