Read only Initramfs Linux system

2
votes

I have an embedded board which I am running Linux on it. Linux is booted via uboot. The full Linux image is build as Initramfs. When the system is booted I find that the root FS is read write. My understanding is that the Initramfs should be by default read only. Right?

I have tried to change boot parameter in uboot as well to have it read only, but it had no effect. I wonder what to do to make the Initramfs read only by default?

1
linuxlinux-kernelu-bootrootfs
I think changes made in the Initramfs are not persist b/w reboot. so it is ultimately read only - vkumar
Yes I know. But I would still want the rootfs to be read only in Ram as well. So no one depend on it being writable at some point and wonder why his changes not working afterwards. ! So I would want it to be ro, unless I explicitly remount it to rw. - silentnights
Did u try root option in uboot, you can use root=/dev/ram ro option. - vkumar
Yes I did. This is why it is driving me nuts. I can even see the above options when inspecting the uboot environment variables after the Linux boots, but still. ! - silentnights

1 Answers

3
votes

The full Linux image is build as Initramfs.

That's a poorly worded, if not inaccurate, description.
An initramfs uses a cpio archive file, which is not an image file.
The cpio archive file can be linked into the Linux kernel image.

An initramfs "image" is always a cpio archive file, and not a filesystem image.

My understanding is that the Initramfs should be by default read only. Right?

Wrong.
It makes no sense to mount the initramfs as a read-only filesystem.
When the initramfs is created and mounted, it's an empty filesystem.
If it was mounted read-only, then the initramfs could not be populated with its cpio archive, and the initramfs would remain an empty filesystem.

The suggestion to use a kernel command line parameter such as root=/dev/ram ro indicates a confusion between (deprecated) ramdisk and ramfs.
See Linux kernel Documentation/filesystems/ramfs-rootfs-initramfs.txt

I wonder what to do to make the Initramfs read only by default?

You can remount it to be read-only.
See http://lkml.iu.edu/hypermail/linux/kernel/0908.1/01693.html
and
https://serverfault.com/questions/463828/linux-initramfs-read-only