C11Tester: A Fuzzer for C11 and C++11 Atomics ===================================================== C11Tester is a fuzzer for C11/C++11 which randomly explores the behaviors of code under the C/C++ memory model. C11Tester is constructed as a dynamically-linked shared library which implements the C and C++ atomic types and portions of the other thread-support libraries of C/C++ (e.g., std::atomic, std::mutex, etc.). C11Tester should compile on Linux OSX. Instrumenting programs requires using our LLVM pass. It likely can be ported to other \*NIX flavors. Getting Started --------------- If you haven't done so already, you may download C11Tester using git: git clone git://demsky.eecs.uci.edu/c11fuzzer.git Get the benchmarks (not required; distributed separately): git clone git://demsky.eecs.uci.edu/concurrency-benchmarks.git Compile the fuzzer: make To see the help message on how to run C11Tester, execute: ./run.sh -h Useful Options -------------- `-v` > Verbose: show all executions and not just buggy ones. `-u num` > Value to provide to atomics loads from uninitialized memory locations. The > default is 0, but this may cause some programs to throw exceptions > (segfault) before the model checker prints a trace. Benchmarks ------------------- Many simple tests are located in the `tests/` directory. You may also want to try the larger benchmarks (distributed separately), which can be placed under the `benchmarks/` directory. After building C11Tester, you can build and run the benchmarks as follows: > make benchmarks > cd benchmarks > > # run barrier test with fairness/memory liveness > ./run.sh barrier/barrier -y -m 2 > > # Linux reader/write lock test with fairness/memory liveness > ./run.sh linuxrwlocks/linuxrwlocks -y -m 2 > > # run all benchmarks and provide timing results > ./bench.sh Running your own code --------------------- You likely want to test your own code, not just our simple tests. To do so, you need to perform a few steps. First, because C11Tester executes your program dozens (if not hundreds or thousands) of times, you will have the most success if your code is written as a unit test and not as a full-blown program. Second, because C11Tester must be able to manage your program for you, your program should declare its main entry point as `user_main(int, char**)` rather than `main(int, char**)`. Third, test programs must use the standard C11/C++11 library headers (see below for supported APIs) and must compile against the versions provided in C11Tester's `include/` directory. Notably, we only support C11 thread syntax (`thrd_t`, etc. from ``). Test programs may also use our included happens-before race detector by including and utilizing the appropriate functions (`store_{8,16,32,64}()` and `load_{8,16,32,64}()`) for storing/loading data to/from non-atomic shared memory. C11Tester can also check boolean assertions in your test programs. Just include `` and use the `MODEL_ASSERT()` macro in your test program. C11Tester will report a bug in any possible execution in which the argument to `MODEL_ASSERT()` evaluates to false (that is, 0). Test programs should be compiled against our shared library (libmodel.so) using the headers in the `include/` directory. Then the shared library must be made available to the dynamic linker, using the `LD_LIBRARY_PATH` environment variable, for instance. ### Supported C11/C++11 APIs ### To model-check multithreaded code properly, C11Tester needs to instrument any concurrency-related API calls made in your code. Currently, we support parts of the following thread-support libraries. The C versions can be used in either C or C++. * ``, ``, `` * `` * `` * `` Because we want to extend support to legacy (i.e., non-C++11) compilers, we do not support some new C++11 features that can't be implemented in C++03 (e.g., C++ ``). Reading an execution trace -------------------------- When C11Tester detects a bug in your program (or when run with the `--verbose` flag), it prints the output of the program run (STDOUT) along with some summary trace information for the execution in question. The trace is given as a sequence of lines, where each line represents an operation in the execution trace. These lines are ordered by the order in which they were run by C11Tester (i.e., the "execution order"), which does not necessarily align with the "order" of the values observed (i.e., the modification order or the reads-from relation). The following list describes each of the columns in the execution trace output: * \#: The sequence number within the execution. That is, sequence number "9" means the operation was the 9th operation executed by C11Tester. Note that this represents the execution order, not necessarily any other order (e.g., modification order or reads-from). * t: The thread number * Action type: The type of operation performed * MO: The memory-order for this operation (i.e., `memory_order_XXX`, where `XXX` is `relaxed`, `release`, `acquire`, `rel_acq`, or `seq_cst`) * Location: The memory location on which this operation is operating. This is well-defined for atomic write/read/RMW, but other operations are subject to C11Tester implementation details. * Value: For reads/writes/RMW, the value returned by the operation. Note that for RMW, this is the value that is *read*, not the value that was *written*. For other operations, 'value' may have some C11Tester-internal meaning, or it may simply be a don't-care (such as `0xdeadbeef`). * Rf: For reads, the sequence number of the operation from which it reads. [Note: If the execution is a partial, infeasible trace (labeled INFEASIBLE), as printed during `--verbose` execution, reads may not be resolved and so may have Rf=? or Rf=Px, where x is a promised future value.] * CV: The clock vector, encapsulating the happens-before relation (see our paper, or the C/C++ memory model itself). We use a Lamport-style clock vector similar to [1]. The "clock" is just the sequence number (#). The clock vector can be read as follows: Each entry is indexed as CV[i], where i = 0, 1, 2, ..., So for any thread i, we say CV[i] is the sequence number of the most recent operation in thread i such that operation i happens-before this operation. Notably, thread 0 is reserved as a dummy thread for certain C11Tester operations. See the following example trace: ------------------------------------------------------------------------------------ # t Action type MO Location Value Rf CV ------------------------------------------------------------------------------------ 1 1 thread start seq_cst 0x7f68ff11e7c0 0xdeadbeef ( 0, 1) 2 1 init atomic relaxed 0x601068 0 ( 0, 2) 3 1 init atomic relaxed 0x60106c 0 ( 0, 3) 4 1 thread create seq_cst 0x7f68fe51c710 0x7f68fe51c6e0 ( 0, 4) 5 2 thread start seq_cst 0x7f68ff11ebc0 0xdeadbeef ( 0, 4, 5) 6 2 atomic read relaxed 0x60106c 0 3 ( 0, 4, 6) 7 1 thread create seq_cst 0x7f68fe51c720 0x7f68fe51c6e0 ( 0, 7) 8 3 thread start seq_cst 0x7f68ff11efc0 0xdeadbeef ( 0, 7, 0, 8) 9 2 atomic write relaxed 0x601068 0 ( 0, 4, 9) 10 3 atomic read relaxed 0x601068 0 2 ( 0, 7, 0, 10) 11 2 thread finish seq_cst 0x7f68ff11ebc0 0xdeadbeef ( 0, 4, 11) 12 3 atomic write relaxed 0x60106c 0x2a ( 0, 7, 0, 12) 13 1 thread join seq_cst 0x7f68ff11ebc0 0x2 ( 0, 13, 11) 14 3 thread finish seq_cst 0x7f68ff11efc0 0xdeadbeef ( 0, 7, 0, 14) 15 1 thread join seq_cst 0x7f68ff11efc0 0x3 ( 0, 15, 11, 14) 16 1 thread finish seq_cst 0x7f68ff11e7c0 0xdeadbeef ( 0, 16, 11, 14) HASH 4073708854 ------------------------------------------------------------------------------------ Now consider, for example, operation 10: This is the 10th operation in the execution order. It is an atomic read-relaxed operation performed by thread 3 at memory address `0x601068`. It reads the value "0", which was written by the 2nd operation in the execution order. Its clock vector consists of the following values: CV[0] = 0, CV[1] = 7, CV[2] = 0, CV[3] = 10 End of Execution Summary ------------------------ C11Tester prints summary statistics at the end of each execution. These summaries are based off of a few different properties of an execution, which we will break down here: * An _infeasible_ execution is an execution which is not consistent with the memory model. Such an execution can be considered overhead for the model-checker, since it should never appear in practice. * A _buggy_ execution is an execution in which C11Tester has found a real bug: a data race, a deadlock, failure of a user-provided assertion, or an uninitialized load, for instance. C11Tester will only report bugs in feasible executions. * A _redundant_ execution is a feasible execution that is exploring the same state space explored by a previous feasible execution. Such exploration is another instance of overhead, so C11Tester terminates these executions as soon as they are detected. C11Tester is mostly able to avoid such executions but may encounter them if a fairness option is enabled. Now, we can examine the end-of-execution summary of one test program: $ ./run.sh test/rmwprog.o + test/rmwprog.o ******* Model-checking complete: ******* Number of complete, bug-free executions: 6 Number of redundant executions: 0 Number of buggy executions: 0 Number of infeasible executions: 29 Total executions: 35 * _Number of complete, bug-free executions:_ these are feasible, non-buggy, and non-redundant executions. They each represent different, legal behaviors you can expect to see in practice. * _Number of redundant executions:_ these are feasible but redundant executions that were terminated as soon as C11Tester noticed the redundancy. * _Number of buggy executions:_ these are feasible, buggy executions. These are the trouble spots where your program is triggering a bug or assertion. Ideally, this number should be 0. * _Number of infeasible executions:_ these are infeasible executions, representing some of the overhead of model-checking. * _Total executions:_ the total number of executions explored by C11Tester. Should be the sum of the above categories, since they are mutually exclusive. Other Notes and Pitfalls ------------------------ * Many programs require some form of fairness in order to terminate in a finite amount of time. C11Tester supports the `-y num` and `-f num` flags for these cases. The `-y` option (yield-based fairness) is preferable, but it requires careful usage of yields (i.e., `thrd_yield()`) in the test program. For programs without proper `thrd_yield()`, you may consider using `-f` instead. * Deadlock detection: C11Tester can detect deadlocks. For instance, try the following test program. > ./run.sh test/deadlock.o Deadlock detection currently detects when a thread is about to step into a deadlock, without actually including the final step in the trace. But you can examine the program to see the next step. * C11Tester has to speculatively explore many execution behaviors due to the relaxed memory model, and many of these turn out to be infeasible (that is, they cannot be legally produced by the memory model). C11Tester discards these executions as soon as it identifies them (see the "Number of infeasible executions" statistic); however, the speculation can occasionally cause C11Tester to hit unexpected parts of the unit test program (causing a division by 0, for instance). In such programs, you might consider running C11Tester with the `-u num` option. * Related to the previous point, C11Tester may report more than one bug for a particular candidate execution. This is because some bugs may not be reportable until C11Tester has explored more of the program, and in the time between initial discovery and final assessment of the bug, C11Tester may discover another bug. * Data races may be reported as multiple bugs, one for each byte-address of the data race in question. See, for example, this run: $ ./run.sh test/releaseseq.o ... Bug report: 4 bugs detected [BUG] Data race detected @ address 0x601078: Access 1: write in thread 2 @ clock 4 Access 2: read in thread 3 @ clock 9 [BUG] Data race detected @ address 0x601079: Access 1: write in thread 2 @ clock 4 Access 2: read in thread 3 @ clock 9 [BUG] Data race detected @ address 0x60107a: Access 1: write in thread 2 @ clock 4 Access 2: read in thread 3 @ clock 9 [BUG] Data race detected @ address 0x60107b: Access 1: write in thread 2 @ clock 4 Access 2: read in thread 3 @ clock 9 See Also -------- The C11Tester project page: > The C11Tester source and accompanying benchmarks on Gitweb: > > > Contact ------- Please feel free to contact us for more information. Bug reports are welcome, and we are happy to hear from our users. We are also very interested to know if C11Tester catches bugs in your programs. Contact Weiyu Luo at or Brian Demsky at . Copyright --------- Copyright © 2013 and 2019 Regents of the University of California. All rights reserved. C11Tester is distributed under the GPL v2. See the LICENSE file for details. References ---------- [1] L. Lamport. Time, clocks, and the ordering of events in a distributed system. CACM, 21(7):558-565, July 1978.