Using prebuilt kernels

From postmarketOS
Jump to: navigation, search

Some vendors do not release the kernel source code, which is very bad for freedom and goes against the Linux kernel license (see My device runs the Linux kernel (e.g. Android), but the vendor does not provide the source in the Troubleshooting page.

But you can still extract the kernel image from a device that is running Android and use it to run postmarketOS. Of course this is totally discouraged because we want to make sure that we know the code our devices are running but you might find it useful in order to start hacking on postmarketOS without having the kernel source code.

Extracting boot.img from an Android device

You can extract the kernel that is running your Android device, it doesn't matter if it's the stock ROM, LineageOS or any custom ROM. For this approach you need the adb tool and root permissions.

NOTE: in this example the boot partition is named msm_sdcc.1 but it might have a different name in your device

# Connect to your Android device
adb shell

# Find the partition name
ls -l /dev/block/platform/
ls -l /dev/block/platform/msm_sdcc.1/by-name

# From there we know that the boot partition is /dev/block/mmcblk0p14
# Now we just dump the partition to a file (we need to root permissions)
su
cat /dev/block/mmcblk0p14 > /sdcard/boot.img
chmod 0666 /sdcard/boot.img

# Exit from adb and copy boot.img to your computer
adb pull /sdcard/boot.img .

Running postmarketOS with a pre-built kernel

You can use unpackbootimg (usage is described in here) to extract the kernel from your recently extracted boot.img or from a LineageOS release and run it together with postmarketOS initramfs.

If you extracted the kernel from you Stock ROM, it might not have encryption support enabled and in this case you would not be able to boot from an encrypted partition which is how postmarketOS works by default. To disable encryption in postmarketOS use pmbootstrap install --no-fde. You might need other modules as well for pmOS to work properly.

# fastboot boot <kernel> [ <ramdisk> ]
fastboot boot \
uncompressed_boot/boot.img-zImage \
~/.local/var/pmboostrap/chroot_rootfs-XXX-XXXX/boot/initramfs-lg-hammerhead

Maybe we can chainload the pmOS kernel?

Linux kernels can load other kernels via the kexec syscall. The deal is, that signed bootloaders only allow signed kernels. But what if we repackaged the signed kernel with our own initramfs, that loads our own kernel via kexecboot from the boot subpartition? Not sure if this will work because of SELinux etc, but if it did, it would be a nice way to get around the bootloader's signed kernel enforcement! If you have an usecase for this, please open a ticket to discuss this further.

More research showed, that the kexec syscall is usually disabled in such kernels. However, it could be possible to do kexec without it being enabled in the kernel (such as it has been done in the PS4 hack, where a kexec gets done on the PS4's BSD kernel, which does obviously not support this feature, to load a Linux kernel). This is a lot of work, but it would be an impressive hack!

kexec is also used by MultiROM in order to allow users to choose which ROM to boot up. For MultiROM support in pmbootstrap, see issue #421.

Useful links