How to use the split utility

Overview

The split utility operates on an LLVM bitcode file, splitting all of the functions in it into separate (.bc) files.

Usage:

./split-llvm-extract <program-to-split>.bc -o <output-dir>

Documentation

You can read about the LLVM splitter project in the CapableVMs documentation repository and in this Google Doc.

Building the splitter

Prerequisites

The minimum supported LLVM version is 13.

Building

To build the split utility, run make split-llvm-extract. By default, this will build the splitter using the clang and LLVM libraries installed on your system.

If you built LLVM from source, set CXX, LLVM_CONFIG, LLVM_LINK and LLVM_EXTRACT, and then run:

make CXX=$CXX \
    LLVM_CONFIG=$LLVM_CONFIG \
    LLVM_LINK=$LLVM_LINK \
    LLVM_EXTRACT=$LLVM_EXTRACT \
    split-llvm-extract

For example, if you built CHERI LLVM using cheribuild, set CHERI to your cheribuild installation directory (which is ~/cheri/output/<SDK> by default) and run:

make CXX=$CHERI/bin/clang++ \
    LLVM_CONFIG=$CHERI/bin/llvm-config \
    LLVM_LINK=$CHERI/bin/llvm-link \
    LLVM_EXTRACT=$CHERI/bin/llvm-extract \
    split-llvm-extract

CHERI cross-compilation

Steps specific to CHERI cross-compilation can be found in .buildbot.sh.

Using the splitter

Once you have built the split utility, you will need to build the programs you wish to apply it to. Each input program needs to be compiled to a single LLVM bitcode file.

Obtaining the final "fat" LLVM bitcode file can, theoretically, be done in multiple ways. The recommended way is to use CapableVMs fork of "Go Whole Program LLVM", gllvm. For example, to get the bitcode from a generic Makefile project:

• Install gllvm: go get github.com/capablevms/gllvm/cmd/...

• Build with gllvm:

GCLANG=$HOME/go/bin/gclang
export GET_BC=$HOME/go/bin/get-bc
CC=$GCLANG LLVM_COMPILER_PATH=<path_to_llvm>/bin make

• Extract the bitcode:

Finally, the split utility can be launched with:

./split-llvm-extract path/to/binary.bc -o outdir

To run the version of the binary just split, it is necessary to join the parts (in outdir) together as shared libraries. A "joiner" Makefile can be found here.

Other (old and not maintained) variants

There are 2 other utilities that reside in this repository:

• manual-split.cpp, which shows how splitting can be done using the LLVM API instead of using llvm-extract.
• find-and-split-static.cpp, which focuses on finding functions which are not public and should be. It also tries to find ways to split the bitcode files while preserving the linkage of the functions.

These splitters are not meant for general-purpose use, and are kept as a showcase of what is possible.