clang-kernel-build

Steps to build the Linux kernel using Clang

0. Start with an empty dir

   git clone https://github.com/ramosian-glider/clang-kernel-build.git
   cd clang-kernel-build
   export WORLD=`pwd`
   

1. Install Clang from Chromium:

   cd $WORLD
   # Instruction taken from http://llvm.org/docs/LibFuzzer.html
   mkdir TMP_CLANG
   cd TMP_CLANG
   git clone https://chromium.googlesource.com/chromium/src/tools/clang
   cd ..
   TMP_CLANG/clang/scripts/update.py
   

2. Clone the Linux source tree

   cd $WORLD
   git clone git://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable.git
   cd linux-stable
   git reset --hard v4.6-rc5
   

3. Download and apply the patches

   cd $WORLD
   wget http://buildbot.llvm.linuxfoundation.org/configs/x86_64/kernel-patches.tar.bz2
   tar -jxf kernel-patches.tar.bz2
   cd linux-stable
   patch -p1 -i ../clang-flags.patch 
   patch -p1 -i ../clang-uaccess.patch
   patch -p1 -i ../patches/boot-workaround-PR18415.patch
   

4. Configure and build the kernel

   cd $WORLD
   export CLANG_PATH=`pwd`/third_party/llvm-build/Release+Asserts/bin/
   cd linux-stable
   make CC=$CLANG_PATH/clang defconfig
   make CC=$CLANG_PATH/clang -j64 2>&1 | tee build.log
   

5. Set up the VM:

   cd $WORLD
   wget https://raw.githubusercontent.com/google/sanitizers/master/address-sanitizer/kernel_buildbot/create_os_image.sh
   # create_os_image.sh requires sudo
   sh create_os_image.sh
   

6. Run the VM

   cd $WORLD
   ./run_qemu.sh
   # in a separate console:
   ssh -i ssh/id_rsa -p 10023 root@localhost
   

Known problems

1. The kernel doesn't boot if configured with CONFIG_KVM (e.g. make kvmconfig)

Debugging

```
# Start the VM before running GDB
cd $WORLD
gdb -x gdb.script
(gdb) br dump_stack
```

Hacking

```
# Perform these steps instead of Step 4 above.
cd $WORLD/linux-stable
make CC=$CLANG_PATH/clang defconfig
make CC=`pwd`/../clang_wrapper.py 2>&1 | tee build.log
```

The handy clang_wrapper.py lets you add arguments to Clang invocation, measure compilation times or fall back to GCC for certain files.