Filesystem overlays

Source

This article assumes your TV is rooted.

Most of webOS filesystem is read-only. All read-only partitions are stored as signed squashfs images, and thus can't be "rw remounted" like it is common on Android devices.

In order to apply system config-level changes on read-only partitions we need to apply these in runtime.

Replacing files/whole directories

A simple method of replacing/modifying single files is using mount --bind, like so:

# Create sample override file on writable partition (/tmp is cleaned on boot so
# good for random tests...)
echo 'Hello world!' > /tmp/motd

# Replace read-only file with our customized one
mount --bind /tmp/motd /etc/motd

This also works for replacing whole directories, just use:

mount --bind /tmp/my-example-directory /etc/ssl/certs

An example of an app that uses this method is custom screensaver.

Adding files to an existing directory

If one needs to add a file into a directory overlayfs may be used. Little example here will add an additional service to SSAP server:

# Create config overrides directory on writable partition
mkdir -p /home/root/extra-interfaces

# Create config override/addition manifest file
echo '{"service": "com.webos.service.acb","methods": [{"path": "/getForegroundAppInfo","description": "get foreground lol","requiredPermissions": ["LAUNCH"]}]}' > /home/root/extra-interfaces/com.webos.service.acb.interface

# Enable config override - this will make files present in /home/root/extra-interfaces
# show up together with existing files in /usr/palm/services/com.webos.service.secondscreen.gateway/interfaces
mount -t overlay overlay -olowerdir=/usr/palm/services/com.webos.service.secondscreen.gateway/interfaces:/home/root/extra-interfaces /usr/palm/services/com.webos.service.secondscreen.gateway/interfaces

# For this specific example - restart ssap service (it is restarted
# automatically when killed) - the name changes randomly because of [reasons]
pkill -9 -f ss.apiadapter ; pkill -9 -f ss.gateway

Making a whole directory writable

It is possible to make a whole directory writable by creating a writable overlay. This is generally discouraged for homebrew use, since underlying files may change unexpectedly on system update. However, for quick hacking/testing this is feasible method, and, currently, the only one that allows removal of files.

An example on how to do this, together with a handy indempotent "make sure this directory is overlaid/writable" helper is available here: https://gist.github.com/Informatic/d7bcdd59eac16ffbffd3a5b5c24b4195

Keeping overlays persistent

Any changes we make using methods listed above are not persistent across reboots by default. In order to make them such, we need to apply these on every boot. This can be achieved by putting these commands in a script in /var/lib/webosbrew/init.d directory. Executable script from that directory are ran on startup by run-parts shell tool.

Filenames of executable scripts there may only contain a-zA-Z0-9-_ characters. Scripts are always ran in an ordered manner. (sorted alphabetically/ lexicographically)

Symlinks are also supported. If you are a homebrew developer it is highly advised to only symlink (and not copy) your scripts from application/service directory into /var/lib/webosbrew/init.d, to properly handle situation when user removes said homebrew application. Symlink filename shall consist of a two digit number prefix (used for script ordering) and a unique app name/identifier.

Additionally /var/lib/webosbrew/init.d itself is not guaranteed to exist, since it was not created by default on some Root exploits (namely RootMyTV v1).

In order to indempotently ensure a startup script symlink exists, this snippet can be used:

mkdir -p /var/lib/webosbrew && ln -sf /media/developer/apps/usr/palm/applications/your.app.id/our-startup-script.sh /var/lib/webosbrew/init.d/50-yourappid

In order to remove it indempotently:

rm -rf /var/lib/webosbrew/init.d/50-yourappid

A practical example of how this should be handled in apps can be seen in custom screensaver.

Links