17.3.3. rw
By default, the Linux kernel mounts the root filesystem as readonly. TODO rationale?
This cannot be observed in the default BusyBox init, because by default our rootfs_overlay/etc/inittab does:
/bin/mount -o remount,rw /
Analogously, Ubuntu 18.04 does in its fstab something like:
UUID=/dev/sda1 / ext4 errors=remount-ro 0 1
which uses default mount rw flags.
We have however removed those setups init setups to keep things more minimal, and replaced them with the rw kernel boot parameter makes the root mounted as writable.
To observe the default readonly behaviour, hack the run script to remove replace init, and then run on a raw shell:
./run --kernel-cli 'init=/bin/sh'
Now try to do:
touch a
which fails with:
touch: a: Read-only file system
We can also observe the read-onlyness with:
mount -t proc /proc mount
which contains:
/dev/root on / type ext2 (ro,relatime,block_validity,barrier,user_xattr)
and so it is Read Only as shown by ro.