SymCC: efficient compiler-based symbolic execution

SymCC is a compiler wrapper which embeds symbolic execution into the program during compilation, and an associated run-time support library. In essence, the compiler inserts code that computes symbolic expressions for each value in the program. The actual computation happens through calls to the support library at run time.

To build the pass and the support library, make sure that LLVM 8, 9 or 10 and Z3 version 4.5 or later, as well as a C++ compiler with support for C++17 are installed. (Alternatively, see below for using the provided Dockerfile.) Make sure to pull the QSYM code:

$ git submodule init
$ git submodule update

Note that it is not necessary or recommended to build the QSYM submodule - our build system will automatically extract the right source files and include them in the build.

Create a build directory somewhere, and execute the following commands inside it:

$ cmake -G Ninja -DQSYM_BACKEND=ON /path/to/compiler/sources
$ ninja check

If LLVM is installed in a non-standard location, add the CMake parameter -DLLVM_DIR=/path/to/llvm/cmake/module. Similarly, you can point to a non-standard Z3 installation with -DZ3_DIR=/path/to/z3/cmake/module (which requires Z3 to be built with CMake).

The main build artifact from the user's point of view is symcc, a wrapper script around clang that sets the right options to load our pass and link against the run-time library. (See below for additional C++ support.)

To try the compiler, take some simple C code like the following:

#include <stdio.h>
#include <stdint.h>
#include <unistd.h>

int foo(int a, int b) {
    if (2 * a < b)
        return a;
    else if (a % b)
        return b;
    else
        return a + b;
}

int main(int argc, char* argv[]) {
    int x;
    if (read(STDIN_FILENO, &x, sizeof(x)) != sizeof(x)) {
        printf("Failed to read x\n");
        return -1;
    }
    printf("%d\n", foo(x, 7));
    return 0;
}

Save the code as test.c. To compile it with symbolic execution built in, we call symcc as we would normally call clang:

$ ./symcc test.c -o test

Before starting the analysis, create a directory for the results and tell SymCC about it:

$ mkdir results
$ export SYMCC_OUTPUT_DIR=`pwd`/results

Then run the program like any other binary, providing arbitrary input:

$ echo 'aaaa' | ./test

The program will execute the same computations as an uninstrumented version would, but additionally the injected code will track computations symbolically and attempt to compute diverging inputs at each branch point. All data that the program reads from standard input is treated as symbolic; alternatively, you can set the environment variable SYMCC_INPUT_FILE to the name of a file whose contents will be treated as symbolic when read.

Note that due how the QSYM backend is implemented, all input has to be available from the start. In particular, when providing symbolic data on standard input interactively, you need to terminate your input by pressing Ctrl+D before the program starts to execute.

When execution is finished, the result directory will contain the new test cases generated during program execution. Try running the program again on one of those or combine it with a fuzzer to automate this process (see docs/Fuzzing.txt).

Documentation

The directory docs contains documentation on several internal aspects of SymCC, as well as building C++ code, compiling 32-bit binaries on a 64-bit host, and running SymCC with a fuzzer. There is also a list of all configuration options.

If you're interested in the research paper that we wrote about SymCC, have a look at our group's website. It also contains detailed instructions to replicate our experiments, as well as the raw results that we obtained.

Building a Docker image

If you prefer a Docker container over building SymCC natively, just tell Docker to build the image after pulling the QSYM code as above. (Be warned though: the Docker image enables optional C++ support and builds Z3 from source, so creating the image can take quite some time!)

$ git submodule init
$ git submodule update
$ docker build -t symcc .
$ docker run -it --rm symcc

This will build a Docker image and run an ephemeral container to try out SymCC. Inside the container, symcc is available as a drop-in replacement for clang, using the QSYM backend; similarly, sym++ can be used instead of clang++. Now try something like the following inside the container:

container$ cat sample.cpp
(Note that "root" is the input we're looking for.)
container$ sym++ -o sample sample.cpp
container$ echo test | ./sample
...
container$ cat /tmp/output/000008-optimistic
root

The Docker image also has AFL and symcc_fuzzing_helper preinstalled, so you can use it to run SymCC with a fuzzer as described in the docs. (The AFL binaries are located in /afl.)

While the Docker image is very convenient for using SymCC, I recommend a local build outside Docker for development. Docker will rebuild most of the image on every change to SymCC (which is, in principle the right thing to do), whereas in many cases it is sufficient to let the build system figure out what to rebuild (and recompile, e.g., libc++ only when necessary).

Contact

Feel free to use GitHub issues and pull requests for improvements, bug reports, etc. Alternatively, you can send an email to Sebastian Poeplau ([email protected]) and Aurélien Francillon ([email protected]).

Reference

To cite SymCC in scientific work, please use the following BibTeX:

@inproceedings {poeplau2020symcc,
  author =       {Sebastian Poeplau and Aurélien Francillon},
  title =        {Symbolic execution with {SymCC}: Don't interpret, compile!},
  booktitle =    {29th {USENIX} Security Symposium ({USENIX} Security 20)},
  year =         2020,
  address =      {Boston, MA},
  url =          {https://www.usenix.org/conference/usenixsecurity20/presentation/poeplau},
  publisher =    {{USENIX} Association},
  month =        August,
}

More information on the paper is available here.

License

SymCC is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.

SymCC is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with SymCC. If not, see https://www.gnu.org/licenses/.

The following pieces of software have additional or alternate copyrights, licenses, and/or restrictions:

Program	Directory
QSYM	runtime/qsym_backend/qsym