KLEE

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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.

Pull Requests

When making a pull request, please ensure the following:

  1. PR addresses a single issue. In other words, if some parts of a PR could form another independent PR, you should break this PR into multiple smaller PRs. This makes it easier to review and approve patches.

  2. There are no unnecessary commits. For instance, commits that fix issues with a previous commit in this PR are unnecessary and should be removed (you can find documentation on squashing commits here).

  3. Larger PRs are divided into a logical sequence of commits. Large PRs are much easier to review if they are broken down into a sequence of commits.

  4. Each commit has a meaningful message documenting what it does. The commit message should add as a summary of its changes. Generic commits such as “More work” are obviously unhelpful.

  5. The code is commented. In particular, newly added classes and functions should be documented. New files should also include the standard KLEE header.

  6. The patch is formatted via clang-format. KLEE uses the LLVM style guide, which can be applied via clang-format. You might also want to use git-clang-format for Git integration. Please only format the patch itself and code surrounding the patch, not entire files. However, if the patch touches most of the code in a function, or most of the code in a file, it is fine (and recommended) to format the entire file. Divergences from clang-formatting are only rarely accepted, and only if they clearly improve code readability.

  7. Add test cases exercising the code you added or modified. We expect system and/or unit test cases for all non-trivial changes. After you submit your PR, you will be able to see a Codecov report telling you which parts of your patch are not covered by the regression test suite. You will also be able to see if the Github Actions CI and Cirrus CI tests have passed. If they don’t, you should examine the failures and address them before the PR can be reviewed.

  8. Spellcheck all messages added to the codebase, all comments, as well as commit messages. Make sure all messages added to the code (e.g. error messages, command-line options), all comments, and commit messages are spellchecked.

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!

Tracing

KLEE provides macros that aid with tracing or “print debugging”.

KLEE_DEBUG_WITH_TYPE(type, code); which can be used as follows:

#include "klee/Support/Debug.h"
...
KLEE_DEBUG_WITH_TYPE("my-feature",
  llvm::errs() << "Currently there are " << states.size() << "states\n";
);

This code will be a noop unless -debug-only=my-feature flag is passed to KLEE, for example klee -debug-only=my-feature myprog.bc. Multiple types can be passed in a comma separated list: -debug-only=my-feature1,my-feature2.

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.

Run-time libraries

KLEE provides a set of different run-time libraries that can be selected to augment the software under test. To support this, runtime libraries are meant to be independent of the actual architecture KLEE is running on. Therefore, when KLEE is built, a set of differently configured runtime libraries is built as well. Currently, every combination of the supported architectures (32bit x86, 64bit x86) and different optimisations: Release, Release+Debug, Release+Debug+Asserts, Debug+Asserts, and Debug is pre-built.

These pre-built libraries can be found in the build directory (build/runtime/lib) and have the following pattern: libklee+MyLibrary+architecture+optimisation+.bca.

During runtime, KLEE detects for which platform the software under test was compiled and the user can select the level of optimisation to link with and override the default with klee --runtime-build=Release+Debug. During startup, KLEE links against this specific setup.

Adding new run-time libraries

To add a new library (e.g, MyLibrary), place the directory containing the library below the runtime/ directory, e.g. runtime/MyLibrary.

Add the following code to runtime/CMakeLists.txt to make CMake aware of additional subdirectories:

add_subdirectory(MyLibrary)

and add the same library to the list of RUNTIME_LIBRARIES, i.e.: list(APPEND RUNTIME_LIBRARIES MyLibrary)

Add a file CMakeLists.txt to runtime/MyLibrary that follows a similar structure as CMakeLists.txt in other sub-directories of runtime/MyLibrary, e.g.:

Add new architectures

Modify runtime/CMakeLists.txt to add a new architecture at bc_architecture and add specific compiler flags int the following foreach loop.

Modify tools/klee/main.cpp to add appropriate architecture detection as well.

Add new optimizations

Fancy a new way the provided runtime libraries should be pre-compiled (e.g., MyFancyOptimization)?

Add the name for the new optimization at the beginning of runtime/CMakeLists.txt:

list(APPEND available_klee_runtime_build_types MyFancyOptimization)

And add the specific optimization inside of the foreach loop at the beginning of the file:

elseif (bc_optimization STREQUAL "MyFancyOptimization")
            list(APPEND local_flags -O42 -g)

External bitcode libraries: uclibc, libc++

KLEE utilizes bitcode libraries that are not generating while compiling KLEE, e.g. libc++ or uclibc. 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.

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/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.