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Generate Linux Kernel LLVM Bitcode with Custom Optimization

Sun, 08 Mar 2020 22:42:22 +0000

In Compile Linux Kernel to LLVM Bitcode we use wllvm to generate the LLVM bitcode of Linux kernel. when analyzing the kernel, we sometimes need to disable some optimization passes. However, it’s hard to do with clang. First compiling it to -O0 IR and then applying some optimization passes on it seems easier.

The optimization level of the LLVM IR is -O2 by default. If we need an -O0 LLVM IR, we need to modify the kernel compilation flag.

Compile Linux Kernel to LLVM Bitcode

Fri, 27 Dec 2019 18:32:22 +0000

Tools

wllvm 1.2.7
clang 9.0.1
LLVM 9.0.1
linux 5.3.6

Compile Kernel with WLLVM

make CC=wllvm defconfig
make CC=wllvm -j$(nproc)

Extract Bitcode

extract-bc vmlinux

Then vmlinux.bc is the bitcode.

Cross-compile

BINUTILS_TARGET_PREFIX=aarch64-linux-gnu make ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- HOSTCC=clang CC=wllvm defconfig
BINUTILS_TARGET_PREFIX=aarch64-linux-gnu make ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- HOSTCC=clang CC=wllvm -j$(nproc)

Then, extract bitcode from vmlinux.

extract-bc vmlinux

Problems

symbol multiply defined during bitcode extraction

When extracting bitcode from vmlinux (arm64), an error occurs:

error: Linking globals named ‘sort’: symbol multiply defined!

Minix IPC

Mon, 09 Dec 2019 14:16:22 +0000

Minix kernel, services and user tasks

Minix services:

rs: Reincarnation Server
ds: Data Store
ipc: shared memory and semaphore
sched: scheduler
vfs: Virtual File Systems
vm: Virtual Memory
is: System Information Service
mib: Manage Information Base
pm: Process Manager
drivers, filesystems, …

kernel IPC

Minix kernel provides interface for IPC and kernel call. System services can do kernel IPC and kernel calls, but user process can only do kernel IPC. Kernel IPC delivers 64 bytes message between processes.