How to bind mount a folder inside /sdcard with correct permissions?

I have been using two different approaches (in fact many with small differences) on my older Android versions to mount whole /sdcard/WhatsApp directory from external SD card. I have tested, it works on Android 9 too, but the storage things have changed on Android 10.

Before getting into practical details, we need to keep in mind a few points:

• /sdcard isn't an actual but emulated filesystem. Android uses sdcardfs (or FUSE) to emulate actual storage on /sdcard. See What is /storage/emulated/0/?
• Both of the above filesystems have a fixed SELinux context: u:object_r:sdcardfs:s0 (or u:object_r:fuse:s0).
• Normally /data/media is emulated over /sdcard, but in case of Adoptable Storage when data is migrated, /mnt/expand/[UUID]/media is emulated. See How to free Internal Storage by moving data or using symlink / bind-mount with Adoptable Storage?

• Files and directories on /sdcard have fixed ownership and permissions which depend on if the app has android.permission.[READ|WRITE]_EXTERNAL_STORAGE permission granted or not. Files have never executable permission. Apps' data directories in /sdcard/Android/data/ have ownership set to respective app's UID. For details see What is the “u#_everybody” UID?

  We assume here that every app is allowed to write to /sdcard by setting ownership 0/9997 (user/group) and permissions 0771/0660 (directories/files).

• To achieve above said behavior, since Android 6 every app is run in an isolated mount namespace and /storage/emulated is bind mounted to a different VIEW: /mnt/runtime/[default|read|write]/emulated with private/slave mount propagation. So mounting directly to /storage/emulated won't appear in apps' mount namespaces unless you enter every app's mount namespace explicitly. The same is true if you mount from some app's isolated mount namespace. See Partition gets unmounted automatically in Android Oreo.

  We are going to mount from root mount namespace to /mnt/runtime/write/emulated which is propagated to all apps' mount namespaces.

• read and default views have different permissions than write ⁽¹⁾, but mounting to both is usually unnecessary. Permissions READ_EXTERNAL_STORAGE and WRITE_EXTERNAL_STORAGE belong to same permission group. So granting one to an app through GUI also grants the other, and all apps with Storage permission will only see write view. default is only to let apps (which don't have READ/WRITE Storage permission) traverse /sdcard/Android/data directories. So mounting to default will let such apps just pass through subdirectories on /sdcard/, no files will be visible.

  Also at least with sdcardfs emulation, read and write are bind-mounted ⁽²⁾ from default and mounting to one also mounts to other two. So it's not possible to mount all of three with different permissions.

• /sdcard does not support Extended Attributes (xattr) and Access Time (atime). Other mount options include nosuid, nodev and noexec. See mount manpage for details.

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First of all make sure you are in root mount namespace as explained in the link given above. Or use nsenter to get a root shell in global namespace:

~# [ $(readlink /proc/1/ns/mnt) = $(readlink /proc/self/ns/mnt) ] || busybox nsenter -t 1 -m /system/bin/sh

1. MOUNT SD CARD MANUALLY:

The straightforward way is to format external SD card as portable storage with exFAT or FAT32. Since these filesystems aren't native to Linux, their in-kernel driver implementations support uid, gid, fmask and dmask mount options. You can use exfat or sdfat drivers with exFAT and vfat with FAT32. Former has also a userspace implementation mount.exfat-fuse which requires only FUSE support from kernel. Check with grep fuse /proc/filesystems.

Let's say /dev/block/sda1 is your exFAT partition:

~# mount -t exfat -o nosuid,nodev,noexec,noatime,context=u:object_r:sdcardfs:s0,uid=0,gid=9997,fmask=0117,dmask=0006 /dev/block/sda1 /mnt/runtime/write/emulated/0/WhatsApp
~# mv /data/media/0/WhatsApp/* /sdcard/WhatsApp/

^{* Replace u:object_r:sdcardfs:s0 with u:object_r:fuse:s0 or whatever label your /sdcard has.}

You can also create multiple partitions on SD card. Or after mounting with required mount options you may also bind mount a directory instead of whole partition. Let's say you first mount SD card to /mnt/my_sdcard, then bind mount WhatsApp directory:

~# mount -o bind /mnt/my_sdcard/WhatsApp /mnt/runtime/write/emulated/0/WhatsApp

Downside with this approach is that vold mounts external SD card on boot, so you need to un-mount first.
Secondly the data on SD card isn't encrypted, though there are multiple ways to encrypt manually. See Decrypting microSD card on another Android device or desktop computer.

2. ADOPTABLE STORAGE:

An easy way to counter the above said downsides is to make use of kernel's built-in FDE by formatting the SD card as Adoptable Storage, but only if you don't want to migrate all /data/media/ to external SD card. Once formatted, external SD card will be mounted at /mnt/expand/[UUID] (filesystem UUID is a 16 bytes number). But we can't simply bind mount a directory from there to /sdcard because the filesystem on Adoptable Storage is ext4, which follows UNIX permissions model but the apps can't handle those as explained above. Even if you make it work somehow (using chown, chmod etc.), every app will create files with its own UID which won't be accessible to other apps e.g. Gallery may not be able see pictures downloaded by WhatsApp.

For this method to work your kernel must support sdcardfs (check with grep sdcardfs /proc/filesystems). Create a directory on Adopted SD card and emulate it to /sdcard/WhatsApp:

~# mkdir /mnt/expand/[UUID]/media/0/WhatsApp
~# mv /sdcard/WhatsApp/* /mnt/expand/[UUID]/media/0/WhatsApp/
~# restorecon -rv /mnt/expand/[UUID]/media/
~# mount -t sdcardfs -o nosuid,nodev,noexec,noatime,mask=7,gid=9997 /mnt/expand/[UUID]/media/0/WhatsApp /mnt/runtime/write/emulated/0/Whatsapp

Please note that we necessarily need to use path /mnt/expand/[UUID]/media/ because it's labeled as media_rw_data_file ⁽³⁾ (like /data/media ⁽⁴⁾) which is allowed by SELinux policy to be accessed by apps ⁽⁵⁾. If you use a different path, you need to modify SELinux policy. Unlike other filesystems sdcardfs itself doesn't change SELinux context when accessing underlying filesystem.

3. PORTABLE / ADOPTABLE STORAGE:

This method is the most flexible, it works if you want to use:

• Portable storage but don't want to mount whole partition, instead a directory.
• Adoptable storage but your kernel doesn't have sdcardfs support.
• Adoptable storage but don't want to use /mnt/expand/[UUID]/media/ path necessarily.

Use a third party tool named bindfs (which uses FUSE) to simulate the behavior of emulated filesystem. Actually Android's built-in tool /system/bin/sdcard does exactly this on pre-sdcardfs releases but it has some fixed paths and other unnecessary things, so its source code needs to be modified to achieve what we want. You can build bindfs yourself or try this this one.

~# DIR=/mnt/media_rw/[UUID]    # for Portable Storage
~# DIR=/mnt/expand/[UUID]      # for Adoptable Storage
~# mkdir $DIR/WhatsApp
~# mv /sdcard/WhatsApp/* $DIR/WhatsApp/
~# bindfs -o nosuid,nodev,noexec,noatime,context=u:object_r:sdcardfs:s0 -u 0 -g 9997 -p a-rwx,ug+rw,ugo+X --create-with-perms=a-rwx,ug+rw,ugo+X --xattr-none --chown-ignore --chgrp-ignore --chmod-ignore $DIR/WhatsApp /mnt/runtime/write/emulated/0/WhatsApp

Side notes:

• sdcardfs also works partially with this method except it doesn't support SELinux context= option (so far). So it depends on what is the SELinux label of backing directory on SD card.
• Other tools like rclone, encfs, sshfs etc. which make use of FUSE can also be mounted inside /sdcard the same way. Related: How to mount rclone on Android?
• -u and -g options require /etc/passwd and /etc/group to exist on bindfs < v1.14.2.

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So you may go with whatever method suits you. Usually in-kernel drivers perform better than userspace solutions, and methods native to Linux kernel are more robust and stable. FUSE (over FUSE) may exert performance penalty sometimes e.g. if SD card itself supports high speed data transfers.

You can place the required mount commands in an init.d script or define and init service. See How to run an executable on boot and keep it running?

Note:

• Apps which don't scan /sdcard filesystem themselves but rely on Android's MediaProvider for any changes may need a forced media scan for new files on mounted filesystem to appear immediately.
• If you use multiple users or profiles, you need to mount new filesystem for every User_ID. For device owner it's 0.

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RELATED:

• How to bind-mount “data” and “obb” folders from external SD card in “/sdcard/Android”?