Hello everyone, I will be going over how to compile a kernel from beginning to end!
- A Linux environment (preferably 64-bit)
- Knowledge of how to navigate the command line
- Common sense
- A learning spirit, there will be no spoonfeeding here
What this guide will cover:
- Downloading the source
- Downloading a cross compiler
- Building the kernel
- Flashing the kernel
What this guide will NOT cover:
- Setting up a build environment (plenty of existing Linux installation guides)
- Adding features to the kernel (plenty of git cherry-picking guides)
I know this has been done before but on a cursory search, I have not seen a guide that was recently updated at all.
1. Downloading the source
If you have a custom kernel you want to build, move along after cloning the kernel using the git clone command below.
If you are compiling your stock kernel, it is ultimately up to you to know where to get your kernel source from but here are some common places:
Google: https://android.googlesource.com/kernel/msm/ (pick your architecture and look at the branches)
To download the kernel, you can either use git clone or download the tarball and extract it:
git clone -b <branch_to_checkout> <url> <desired_folder_name> OR tar -xvf <filename>
git clone -b android-msm-angler-3.10-nougat-mr2 https://android.googlesource.com/kernel/msm/ angler
I can try and help you locate your source if necessary.
2. Downloading a cross compiler
Since most Android devices are ARM based, we need a compiler that is targeting ARM devices. A host (or native) compiler will not work unless you are compiling on another ARM device.
You can either compile one yourself if you know how (crosstool-NG is a great tool for this) or download a prebuilt one. Luckily Google provides a high quality toolchain for this, in both an arm (32-bit) and arm64 (64-bit). It's up to you to know the architecture of your device. Typically speaking, most devices in the past two-three years are 64-bit.
Another popular toolchain is UberTC, which can be found here: https://bitbucket.org/matthewdalex/. Most kernels will need patches for anything higher than 4.9 and while I don't mind assisting with finding them, you should compile with Google's toolchain first.
Once you have decided, clone the toolchain:
git clone <url>
3. Compile the kernel
1. Point the Makefile to your compiler (run this from within the toolchain folder!!)
export ARCH=<arch> && export SUBARCH=<arch>
export ARCH=arm64 && export SUBARCH=arm64
Navigate to the arch/<arch>/configs folder within the kernel source (e.g. arch/arm64/configs) and locate your device's or custom kernel developer's proper config file. For example, it will often be in the form of <codename>_defconfig or <kernel_name>_defconfig. Generic Qualcomm configs may be used as well (msm-perf_defconfig, msmcortex-perf_defconfig). When in doubt, ask here if you are confused. A defconfig tells the compiler what options to add to the kernel.
4. Build the kernel
make clean make mrproper make <defconfig_name> make -j$(nproc --all)
If it failed, as was pointed out to me by @flar2 while making a complete idiot of myself, you may need to specify an output directory while making new CAF based kernels, like so:
mkdir -p out make O=out clean make O=out mrproper make O=out <defconfig_name> make O=out -j$(nproc --all)
If it's an OEM, it's up to you to try and fix it, which we can assist with.
4. Flash the kernel
Assuming you were able to compile the kernel successfully, you now need to flash it! I will be covering two different ways to flash a compiled kernel: unpacking and repacking the boot image by hand using Android Image Kitchen or AnyKernel2, both by the brilliant @osm0sis. If there are any per-device nuances, please let me know and I'll add them here! Additionally, this section can vary drastically by device, you may need to consult developers of your device for assistance if necessary.
Android Image Kitchen
- Pull your device's boot image from the latest image available for your device (whether it be a ROM or stock)
- Download the latest Android Image Kitchen from this thread.
- Run the following with the boot image:
- Locate the zImage file and replace it with your kernel image (rename it to what came out of the boot image)
- Run the following to repack:
- Flash the new boot image with fastboot or TWRP!
- Download the latest AnyKernel2 zip: https://github.com/osm0sis/AnyKernel...ive/master.zip
- Apply this patch to clean out all of the demo files:
wget https://github.com/nathanchance/AnyKernel2/commit/addb6ea860aab14f0ef684f6956d17418f95f29a.diff patch -p1 < addb6ea860aab14f0ef684f6956d17418f95f29a.diff rm addb6ea860aab14f0ef684f6956d17418f95f29a.diff
- Place your kernel image in the root of the file.
- Open the anykernel.sh file and modify the following values:
- kernel.string: your kernel name
- device.name#: List all of your device's codenames (from the /system/build.prop: ro.product.device, ro.build.product)
- block: Your boot image's path in your fstab. The fstab can be opened from the root of your device and it will look something like this:
The first column is the value you want to set block to.
- After that, zip up the kernel and flash it!
zip -r9 kernel.zip * -x README.md kernel.zip
Tips and tricks
1. Remove GCC wrapper
A lot of kernels from CAF include a Python script that will essentially turn on -Werror, causing your build to error at the most benign stuff. This is necessary with higher GCC versions as there are a lot more warnings.
Here is the diff of what you need to change in the Makefile:
diff --git a/Makefile b/Makefile index 1aaa760f255f..bfccd5594630 100644 --- a/Makefile +++ b/Makefile @@ -326,7 +326,7 @@ include $(srctree)/scripts/Kbuild.include AS = $(CROSS_COMPILE)as LD = $(CROSS_COMPILE)ld -REAL_CC = $(CROSS_COMPILE)gcc +CC = $(CROSS_COMPILE)gcc CPP = $(CC) -E AR = $(CROSS_COMPILE)ar NM = $(CROSS_COMPILE)nm @@ -340,10 +340,6 @@ DEPMOD = /sbin/depmod PERL = perl CHECK = sparse -# Use the wrapper for the compiler. This wrapper scans for new -# warnings and causes the build to stop upon encountering them. -CC = $(srctree)/scripts/gcc-wrapper.py $(REAL_CC) - CHECKFLAGS := -D__linux__ -Dlinux -D__STDC__ -Dunix -D__unix__ \ -Wbitwise -Wno-return-void $(CF) CFLAGS_MODULE =
2. Using a higher level GCC toolchain
Using a higher GCC toolchain (5.x, 6.x, 7.x or even 8.x) will require you to nuke the GCC wrapper script as above and use a unified GCC header file (pick the following if you have an include/linux/compiler-gcc#.h file):
You may get a lot of warnings but they are not entirely necessary to fix.
3. Adding upstream Linux to kernel source
Once you have gotten familiar with git and the compilation process, you should consider upstreaming your kernel. This will allow you to stay on top of CVE and bug fixes by staying up to date with the latest work of the Linux kernel developers.
I am happy to answer anything that I touched on in this guide. I may point you to another thread if it's better suited but I don't mind off topic (within reason) within the thread. I also want this to be a collaborative effort; other developers, if you have something to add, correct, or improve upon, please let me know!
I am particular in how people ask for help. I do NOT respond to posts asking for a hand out ("How do I fix this?", "Please fix this!", etc.). I only respond to posts with clear logs and steps that you have tried. Basically, show me that you have read this guide and have a specific issue. I am not here to hold your hand through this, this is a developers' forum.