30 Creating a Read-Only Root Filesystem
Suppose, for security reasons, you need to disable your target device’s root filesystem’s write permissions (i.e. you need a read-only root filesystem). Or, perhaps you are running the device’s operating system from a read-only storage device. For either case, you can customize your image for that behavior.
Supporting a read-only root filesystem requires that the system and applications do not try to write to the root filesystem. You must configure all parts of the target system to write elsewhere, or to gracefully fail in the event of attempting to write to the root filesystem.
30.1 Creating the Root Filesystem
To create the read-only root filesystem, simply add the “read-only-rootfs” feature to your image, normally in one of two ways. The first way is to add the “read-only-rootfs” image feature in the image’s recipe file via the IMAGE_FEATURES variable:
IMAGE_FEATURES += "read-only-rootfs"
EXTRA_IMAGE_FEATURES = "read-only-rootfs"
For more information on how to use these variables, see the “Customizing Images Using Custom IMAGE_FEATURES and EXTRA_IMAGE_FEATURES” section. For information on the variables, see IMAGE_FEATURES and EXTRA_IMAGE_FEATURES.
30.2 Post-Installation Scripts and Read-Only Root Filesystem
It is very important that you make sure all post-Installation
pkg_postinst) scripts for packages that are installed into the
image can be run at the time when the root filesystem is created during
the build on the host system. These scripts cannot attempt to run during
the first boot on the target device. With the “read-only-rootfs” feature
enabled, the build system makes sure that all post-installation scripts
succeed at file system creation time. If any of these scripts
still need to be run after the root filesystem is created, the build
immediately fails. These build-time checks ensure that the build fails
rather than the target device fails later during its initial boot
Most of the common post-installation scripts generated by the build system for the out-of-the-box Yocto Project are engineered so that they can run during root filesystem creation (e.g. post-installation scripts for caching fonts). However, if you create and add custom scripts, you need to be sure they can be run during this file system creation.
Here are some common problems that prevent post-installation scripts from running during root filesystem creation:
Not using $D in front of absolute paths: The build system defines
$D when the root filesystem is created. Furthermore,
$Dis blank when the script is run on the target device. This implies two purposes for
$D: ensuring paths are valid in both the host and target environments, and checking to determine which environment is being used as a method for taking appropriate actions.
Attempting to run processes that are specific to or dependent on the target architecture: You can work around these attempts by using native tools, which run on the host system, to accomplish the same tasks, or by alternatively running the processes under QEMU, which has the
qemu_run_binaryfunction. For more information, see the qemu class.
30.3 Areas With Write Access
With the “read-only-rootfs” feature enabled, any attempt by the target
to write to the root filesystem at runtime fails. Consequently, you must
make sure that you configure processes and applications that attempt
these types of writes do so to directories with write access (e.g.