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Developer's Guide

Working with KLEE source code

Contents

GitHub Environment

KLEE’s codebase is currently hosted on GitHub. For those unfamiliar with GitHub, a good starting point is here.

We are using a fork & pull model in KLEE, based on pull requests. For those of you unfamiliar with the process, you can find more information here.

When submitting patches to KLEE, please open a separate pull request for each independent feature or bug fix. This makes it easier to review and approve patches.

Build System

KLEE uses cmake as build system. The very basic build setup similar to what KLEE uses is presented in LLVM’s Writing an LLVM pass tutorial.

LLVM’s build system supports out-of-source builds and so does KLEE. It is highly recommended you take advantage of this. For example, you could create three builds (Release, Release with debug symbols, Debug) that all use the same source tree. This allows you to keep your source tree clean and to build with multiple configurations from a single source tree.

Setting up a debug build of KLEE

Setting up a debug build of KLEE (we’ll assume it is an out-of-source build) is very similar to the build process described in Getting Started. KLEE uses standard LLVM build type idioms, so building a Debug build means just setting the CMAKE_BUILD_TYPE variable to Debug for example:

   cmake \
     -DCMAKE_BUILD_TYPE=Debug \
     -DENABLE_SOLVER_STP=ON \
     -DENABLE_POSIX_RUNTIME=ON \
     -DENABLE_KLEE_UCLIBC=ON \
     -DKLEE_UCLIBC_PATH=<KLEE_UCLIBC_SOURCE_DIR> \
     -DGTEST_SRC_DIR=<GTEST_SOURCE_DIR> \
     -DENABLE_SYSTEM_TESTS=ON \
     -DENABLE_UNIT_TESTS=ON \
     -DLLVM_CONFIG_BINARY=<PATH_TO_LLVM_llvm-config> \
     -DLLVMCC=<PATH_TO_clang> \
     -DLLVMCXX=<PATH_TO_clang++>
     <KLEE_SRC_DIRECTORY>

The rest of the build process is exactly the same as in our build guides. Note that we only provide build guides for some popular LLVM versions, however KLEE builds with many more (at the time of writing LLVM 3.4 - LLVM 10). The build process is exactly the same, cmake only needs LLVM_CONFIG_BINARY, LLVMCC and LLVMCXX to point to versions of LLVM you want to build with.

Note that KLEE depends on LLVM and STP (and optionally Z3). If you need to debug KLEE’s calls to that code, then you will need to build LLVM/STP/Z3 with debug support too.

Source overview

This section gives a brief overview of how KLEE’s source code is structured:

Adding a class

Because KLEE uses LLVM’s build system, adding a class to an existing library in KLEE is very simple. For example, to add a class to libkleaverExpr (lib/Expr), the following steps would be followed:

  1. Create the header file (.h) for the class and place it somewhere inside include/ (the location isn’t really important except that #include is relative to the include/ directory). Note that if you only require the header in the same directory you can also put it next to the source file (ie. lib/Expr/).

  2. Create the source file (.cpp) for the class place it in lib/Expr/.

  3. Add your .cpp file to the lib/Expr/CMakeLists.txt as an argument to the klee_add_component function.

That’s it!

Building code documentation

KLEE uses Doxygen to generate code documentation. To generate it yourself you can run the following from KLEE’s build directory root.

$ make docs

This will generate the documentation in path/to/build-dir/docs/doxygen/.

Regression Testing Framework

KLEE uses LLVM’s testing infrastructure for its regression tests. In KLEE these are…

External tests

llvm-lit is used to test KLEE’s tools by invoking them as shell commands.

KLEE’s tests are currently divided into categories, each of which is a subdirectory in test/ in the source tree (e.g. test/Feature).

llvm-lit is passed one or more paths to test files or directories which it will search recursively for tests. The llvm-lit tool needs to know what test-suite the tests belong to and how to run them. This information is in the lit.site.cfg (generated by the Makefile). This file itself refers to lit.cfg which tells llvm-lit how to run the test suite. At the time of writing the lit.cfg instructs llvm-lit to treat files with the following file extensions as tests:.ll .c .cpp .kquery.

It is desirable to disable some tests (e.g. disable klee-uclibc tests if support was not compiled in) or change which file extensions to look for. This is done by adding a lit.local.cfg file to a directory which can be used to customise how tests in that directory are executed. For example to change the file extensions searched for to .cxx and .txt the following could be used in a lit.local.cfg file:

config.suffixes = ['.cxx', '.txt' ]

To disable execution of tests in a directory based on a condition you can mark them as unsupported by placing the following inside a lit.local.cfg file in that directory (where some_condition is any Python expression):

if some_condition:  config.unsupported = True

All llvm-lit configuration files are Python scripts loaded by llvm-lit so you have the full power of Python at your disposal.

The actions performed in each test are specified by special comments in the file. For example, in test/Feature/ByteSwap.c the first two lines are:

// RUN: %clang %s -emit-llvm %O0opt -c -o %t1.bc
// RUN: %klee --libc=klee --exit-on-error %t1.bc

This first runs clang on the source file (%s) and generates a temporary file (%t1.bc). Then, KLEE symbolically executes this bitcode file with one of its runtimes (here --libc=klee). If either program returns a non-zero exit code (or crashes), the test is considered to have failed. More information on the available substitution variables (such as %s) can be found here.

For LLVM versions greater than 5.0 programs that are to be analysed with KLEE should not be compiled with -O0, since it disables KLEE’s ability to run optimisation passes. Therefore, we have the %O0opt variable, which substitutes to appropriate flags. At the time of writing these are: -O0 -Xclang -disable-O0-optnone. For more details see this issue.

To run the entire test suite run:

$ cd path/to/klee/build/test
$ make check

or you can use llvm-lit directly

$ cd path/to/klee/build/test
$ llvm-lit .

If you want to run a subset of the test suite simply pass the filenames of the tests or directories to search for tests to llvm-lit. For example the commands below will execute the Feature/DoubleFree.c and CXX/ArrayNew.cpp test and all tests nested in the regression/ folder.

$ cd path/to/klee/build/test
$ llvm-lit Feature/DoubleFree.c CXX/ArrayNew.cpp  regression/

Sometimes it can be useful to pass extra command line options to klee or kleaver when running tests. This could be used for example to quickly see if changing the default value for a command line option changes the passing/failing of a test. This is not a substitute for writing new tests for klee or kleaver! If you add a new feature that is exposed by a new command line option a new test should be added that tests this behaviour.

Here is an example:

$ cd test/
$ llvm-lit --param klee_opts=-use-forked-solver=0 --param kleaver_opts="-use-forked-solver=0 -use-query-log=all:smt2" .

In this example when running klee in tests an extra option -use-forked-solver=0 is passed to klee and when running kleaver the -use-forked-solver=0 and -use-query-log=all:smt2 options will be passed to kleaver. It is important to realise if the test already invokes klee or kleaver with a particular option you will not be able to override that option because the klee_opts and kleaver_opts are substituted just after the tool name so subsequent options used in the test will override these.

For more information on llvm-lit tests see LLVM’s testing infrastructure documentation and the llvm-lit tool documentation.

Internal tests

These tests are located in unittests/ and can be executed by running:

$ cd path/to/klee/build
$ make unittests

These test use Google’s C++ testing framework and is well documented.

Miscellaneous

Writing messages to standard error

The kleeCore library (lib/Core) provides several functions that can be used similarly to printf() in C. See include/klee/Internal/Support/ErrorHandling.h for more information.

Adding a command line option to a tool

KLEE uses LLVM’s CommandLine library for adding options to tools in KLEE, which is well documented here. See lib/Core/Executor.cpp for examples.

Run-time libraries

KLEE searches for run-time libraries in install and build paths. These are hard-coded to the binary, so if the filesystem tree changes, KLEE will not find them until recompiled. This behaviour can be overridden by setting KLEE_RUNTIME_LIBRARY_PATH environment variable to the path to the libraries.