Home>Posts>Tech Explained>How To: Build A Read-Only Linux System

How To: Build A Read-Only Linux System

By ·Categories: Tech Explained·Published On: July 14th, 2022·11.2 min read·

From time to time, we receive questions from customers looking to make their Linux platforms read-only in order to maximize the longevity of their flash and solid state storage devices. We thought we’d take the opportunity to create a blog post detailing how to build a read-only linux system.

There are a couple of different approaches to making a Linux system read-only. Unfortunately, it’s usually not as simple as using a conventional filesystem mounted with the read-only option. Many programs assume that at least some parts of the system are writable. In some cases, these programs will fail to run correctly if this turns out not to be the case.

I’ll outline here what I think is the best approach for most applications. It is similar to that taken by one-prominent live CD distributions. 

Live CDs typically had read-only access to a root filesystem, which was often compressed into a single file to be mounted later using a loopback device. Knoppix broke new ground with its use of the cloop filesystem for this purpose.

Other live distributions take this a step further by using a union filesystem to make the root filesystem writable. This approach is quite useful for our purposes, as well.

Union filesystems

Generally speaking, a union filesystem combines multiple filesystems into a single virtual filesystem. We generally reference two popular union fileystems: unionfs and aufs. Both have the same basic model. The following is a dramatic simplification:

  • Filesystems are stacked vertically.
  • Read accesses are attempted on each filesystem in turn from top to bottom. The first filesystem that contains the file being read is used for the read operation.
  • Write accesses are performed similarly, but files that are written to are stored in the top-most writable filesystem. This usually means there is a single writable layer in the union. If files that exist in a read-only layer are written to, they are first copied to the next highest writable layer.

Obviously, there are a lot of subtleties and corner cases that I am not presenting here. What’s important is that we can use a read-only filesystem (which may be a compressed filesystem image or a flash device containing a more conventional filesystem like ext3) and build a writable system on top of it. All we need is a writable filesystem to union with the read-only layer.

The writable layer

What kind of writable filesystem you use depends on what you are trying to achieve. If you don’t need any persistence between boots, it is pretty easy to use tmpfs. Writes to the system will be preserved in RAM while the system is up, but will disappear when the system is shutdown or rebooted.

 If you want persistence over the whole system directory structure, you’ll need to use a persistent writable layer. This is likely a conventional filesystem on some other media (a second disk, perhaps). This is probably most useful for live systems or thin clients where using a read-only base is not done so much for longevity as it is to minimize local storage requirements.

In many cases, when you do need persistence, you only need it for specific files. For instance, if you have a kiosk that stores user input in a local database, the database must persist on disk, but you probably don’t want to persist temporary files or other transient runtime data. The best approach for dealing with this common use case is to have a tmpfs read/write layer and then mount some writable media on an arbitrary mount point like /var/local/data (for example).

Implementation

Implementing a read-only system requires hooking into the boot process. How this is done varies from distribution to distribution, and can probably be done in a variety of ways on a single distribution. In this article, I’ll demonstrate an approach that works with Ubuntu 8.04.

By default, Ubuntu 8.04 uses an initramfs. This is the best place to make our modification, as we can make sure that the union filesystem is mounted early on in the boot process.

initramfs-tools

Ubuntu has an extensible system for building the initramfs called “initramfs-tools”. We can use this to plug some scripts into the initramfs. There are a few different ways that initramfs-tools can be extended: “hooks” and “scripts.”

Hooks are run when the initramfs is being built, and are useful for adding kernel modules or executables to the initramfs image. Hooks that are distributed with packages are usually installed in

/usr/share/initramfs-tools/hooks, and they make use of the functions defined in

/usr/share/initramfs-tools/hook-functions. Local hooks should be placed in

/etc/initramfs-tools/hooks.

Scripts are run within the initramfs environment at boot time. These can be used to modify the early boot process. As with hooks, scripts that are distributed with packages are usually installed into

/usr/share/initramfs-tools/scripts. Local scripts should go into

/etc/initramfs-tools/scripts.

initramfs generation is controlled by the configuration files found in

/etc/initramfs-tools. /etc/initramfs-tools/initramfs.conf is the primary configuration file, but files can also be placed in

/etc/initramfs-tools/conf.d. The primary boot method can be configured in initramfs.conf by changing the value of variable “BOOT.” By default, it is “local,” a boot method that mounts the root filesystem on local media like a hard disk.

For each boot method, there is a script with that name that controls how that boot method works. For instance, there is a script called “local” that defines how a local boot is performed. Many such scripts also provide hooks for other shell scripts to be executed at certain points during the boot process. For instance, any scripts placed in /usr/share/initramfs-tools/local-premount will be executed by the “local” script just prior to mounting the root filesystem. The init script itself (which acts as process #1 up until the point where the real init daemon is launched, after mounting the root filesystem) provides similar hooks. See the contents of /usr/share/initramfs-tools/scripts to get an idea of what other hooks are available.

Finally, both hooks and scripts must be written such that, if they are run with a single argument “prereqs,” they print a space-separated list of the names of other scripts or hooks that should be run before running this particular script or hook. This provides a simple system of dependencies between hooks and scripts. I find that I very rarely make use of this feature, but it is available should your application require it.

Hooks and scripts

We’ll implement our read-only system by introducing one hook and one script. Our script will actually be an init-bottom script, run after the real root device is already mounted. Our goal will be to take the already-mounted root filesystem and shuffle it around as the base for an aufs union with a tmpfs writable layer. This allows us to continue to use the standard Ubuntu configuration mechanisms for specifying the device that contains the real root filesystem.

We need a hook to tell initramfs-tools that we need a few kernel modules (aufs and tmpfs, both of which are included with Ubuntu 8.04) and an executable (chmod). We’ll see why we need chmod shortly. Our hook is quite simple (as most of them are). We’ll call this hooks/ro_root:

#!/bin/sh

PREREQ=”

prereqs() {

  echo “$PREREQ”

}

case $1 in

prereqs)

  prereqs

  exit 0

  ;;

esac

. /usr/share/initramfs-tools/hook-functions

manual_add_modules aufs

manual_add_modules tmpfs

copy_exec /bin/chmod /bin

The script does the real work of making sure the filesystems are all mounted in the right places. At this point in the boot process, the real root device has been mounted on $rootmnt, and /sbin/init on that mount point is about to be executed. At this point, we’ll be looking to move the root device mount to a different mount point, and build our union mount in its place.

Here’s how we’ll do this:

  1. Move $rootmnt to /${rootmnt}.ro (this is the read-only layer).
  2. Mount our writable layer as tmpfs on /${rootmnt}.rw.
  3. Mount the union on ${rootmnt}.

Additionally, we may want to have access to the read-only and read/write layers independently from the union. In order to maintain access to these mounts, we’ll have to bind them to a new mount point under ${rootmnt}. We’ll do this with “mount –bind”.

The union is still able to access the original read-only and read/write mounts even after the root is rotated and init is launched, causing those mount points to fall outside of the new root filesystem. I assume that aufs opens these directories at mount time and the filesystems continue to be accessible as long as processes have open file handles. The kernel seems to be pretty smart about dealing with these kinds of interesting situations.

Getting back to things, here is the init-bottom script we’ll be using

(scripts/init-bottom/ro_root):

#!/bin/sh

PREREQ=”

prereqs() {

  echo “$PREREQ”

}

case $1 in

prereqs)

  prereqs

  exit 0

  ;;

esac

ro_mount_point=”${rootmnt%/}.ro”

rw_mount_point=”${rootmnt%/}.rw”

# Create mount points for the read-only and read/write layers:

mkdir “${ro_mount_point}” “${rw_mount_point}”

# Move the already-mounted root filesystem to the ro mount point:

mount –move “${rootmnt}” “${ro_mount_point}”

# Mount the read/write filesystem:

mount -t tmpfs root.rw “${rw_mount_point}”

# Mount the union:

mount -t aufs -o “dirs=${rw_mount_point}=rw:${ro_mount_point}=ro” root.union “${rootmnt}”

# Correct the permissions of /:

chmod 755 “${rootmnt}”

# Make sure the individual ro and rw mounts are accessible from within the root

# once the union is assumed as /.  This makes it possible to access the

# component filesystems individually.

mkdir “${rootmnt}/ro” “${rootmnt}/rw”

mount –bind “${ro_mount_point}” “${rootmnt}/ro”

mount –bind “${rw_mount_point}” “${rootmnt}/rw”

Rebuilding the initramfs

The hook and init-bottom script that we wrote above can be installed in the following locations, respectively:

  • /etc/initramfs-tools/hooks/ro_root.
  • /etc/initramfs-tools/scripts/init-bottom/ro_root.

They should both have the execute permission bit set. 

After copying the files into place, regenerate your initramfs with update-initramfs:

update-initramfs -u

The -u switch tells update-initramfs to update the initramfs for the most recent kernel on the system. I assume that that is the kernel that you are running. For most embedded or other single-purpose machines, there is typically only one kernel installed.

Booting

The system should appear to boot as it would without the changes we made. However, once it is finished booting, you can confirm that:

  • /ro contains the read-only base filesystem.
  • /rw contains the read/write layer, and will usually have some new files
    there immediately following boot (/var/run, etc.).
  • If you create a file and then reboot, the file will be gone.

Of course, a system like this has a few caveats:

  • The contents of /etc/mtab are likely not correct, so the output of the mount command is probably missing some information. There are steps we can take to correct /etc/mtab, but I won’t cover those in detail here.
  • No runtime state is preserved. Don’t forget that and save a file, expecting it to be around after a reboot!
  • Subtle semantic differences between aufs, tmpfs, and traditional filesystems may cause problems with some applications. Most applications won’t notice, but those that leverage more advanced filesystem features or rely on filesystem implementation details might run into errors or, worse, fail subtly. I believe most of these kinds of issues are now a thing of the past, but if you find yourself troubleshooting mysterious failures, keep it in mind.

This kind of system customization really demonstrates the power and flexibility of the initramfs-tools configuration infrastructure. This architectural style is common in Debian and Ubuntu, making these distributions ideal choices for embedded and applied computing projects.

I hope this has been helpful. If you have any additional questions on how to build a read-only linux system, contact our team today.

Improvements

[Section added February 23rd, 2009, updated July 14th, 2022.]

The following updated script incorporates some improvements that helped with some problems that commenters ran into:

  • Boot with normally-mounted read/write root filesystem when the user requests single user mode (a.k.a. recovery mode).
  • Prevent /etc/init.d/checkroot.sh from running when booting into the read-only system.
  • Use mount –move instead of mount –bind when moving the ro and rw mount points into the new root.

#!/bin/sh

PREREQ=”

prereqs() {

  echo “$PREREQ”

}

case $1 in

prereqs)

  prereqs

  exit 0

  ;;

esac

# Boot normally when the user selects single user mode.

if grep single /proc/cmdline >/dev/null; then

  exit 0

fi

ro_mount_point=”${rootmnt%/}.ro”

rw_mount_point=”${rootmnt%/}.rw”

# Create mount points for the read-only and read/write layers:

mkdir “${ro_mount_point}” “${rw_mount_point}”

# Move the already-mounted root filesystem to the ro mount point:

mount –move “${rootmnt}” “${ro_mount_point}”

# Mount the read/write filesystem:

mount -t tmpfs root.rw “${rw_mount_point}”

# Mount the union:

mount -t aufs -o “dirs=${rw_mount_point}=rw:${ro_mount_point}=ro” root.union “${rootmnt}”

# Correct the permissions of /:

chmod 755 “${rootmnt}”

# Make sure the individual ro and rw mounts are accessible from within the root

# once the union is assumed as /.  This makes it possible to access the

# component filesystems individually.

mkdir “${rootmnt}/ro” “${rootmnt}/rw”

mount –move “${ro_mount_point}” “${rootmnt}/ro”

mount –move “${rw_mount_point}” “${rootmnt}/rw”

# Make sure checkroot.sh doesn’t run.  It might fail or erroneously remount /.

rm -f “${rootmnt}/etc/rcS.d”/S[0-9][0-9]checkroot.sh

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