Using mount - bind safely (with rollback)

Purpose: Present one reliable pattern for bind-mounting a directory elsewhere, then make it read-only, persist it in /etc/fstab, and roll back cleanly.

1) Basic bind mount

# Example: expose /srv/app/upload at /var/www/app/upload
sudo mkdir -p /var/www/app/upload
sudo mount --bind /srv/app/upload /var/www/app/upload

2) Make it read-only

Bind mounts ignore -o ro on first mount; you must remount:

sudo mount -o remount,bind,ro /var/www/app/upload

3) Persist in /etc/fstab

# <source>              <target>                 type  options         dump pass
/srv/app/upload        /var/www/app/upload       none  bind            0    0
/var/www/app/upload    /var/www/app/upload       none  remount,bind,ro 0    0

Test reload with sudo mount -a. If anything fails, revert the lines and re-run.

4) Rollback

sudo umount /var/www/app/upload
# remove the two fstab lines and run:
sudo mount -a

5) Pitfalls

Backups: avoid recursing into both source and target (exclude one of them).
Permissions: the target inherits the source; fix ownership on the source, not the target.
Use --rbind only when you truly want to include submounts.

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