Overview
The crux-llvm tool (and corresponding C library) are intended for
verifying C programs containing inline specifications (in the form of
function calls to create non-deterministic values and assert
properties). For more information about Crux itself, refer to the
official website.
crux-llvm is licensed under the 3-Clause BSD license. For more details, refer
to the LICENSE file. Some binary distributions of crux-llvm come bundled
with external compilers and SMT solvers:
clang and llvm-link (Apache v2.0 licensed)
- CVC4 (BSD-3 licensed)
- Yices (GPLv3 licensed)
- Z3 (MIT licensed)
These tools are only invoked by crux-llvm as subprocesses. That is,
crux-llvm does not link against any of these tools directly. As a result,
crux-llvm adheres to the GPLv3 licensing terms in Yices.
Prerequisites
Before running crux-llvm, you'll need to install the following
software:
We have tested crux-llvm most heavily with GHC 9.4, GHC 9.6, GHC 9.8,
and cabal version 3.10. We recommend Yices 2.6.x, and Z3
4.8.x. Technically, only one of Yices or Z3 is required, and CVC4 is
also supported. However, in practice, having both tends to be
convenient. Finally, LLVM versions from 3.6 through 16 are likely to
work well, and any failures with versions in that range should be
reported as bugs.
Building
The crux-llvm tool can be built by doing the following:
-
Clone (incl. the submodules) the crucible repository:
git clone --recurse-submodules https://github.com/GaloisInc/crucible.git
-
Build the crux-llvm package:
cabal build crux-llvm
This will compile crux-llvm and supporting libraries such that they
can be executed with cabal run. To install the binaries in the
standard Cabal binary path (usually $HOME/.cabal/bin), run the
following:
cabal install exe:crux-llvm --overwrite-policy=always
You can also use the --installdir flag to install binaries in a
different location.
Invocation
In the crux-llvm directory (either in the repository or the root of
the directory extracted from a distribution tarball), to analyze
file.c, run
cabal run exe:crux-llvm file.c
If you've installed crux-llvm somewhere on your PATH, you can
instead run
crux-llvm file.c
You'll see output indicating the progress of analysis, how many proof
goals are generated, and how many were successfully proved. In addition,
the results directory will contain a subdirectory for the file you
provided. This directory will contain an index.html file that shows a
visualization of proof results overlaid on the C source code. If
crux-llvm found a counter-example to any of the attempted proofs, the
values of that counter-example will be overlaid on the source code (at
the location of calls to create non-deterministic values), and the
following two executable files will also exist in the results
directory:
-
debug-NNN: an executable file that runs the program and provides it
with the counter-example values. The number NNN indicates the line
of the source on which the error occurred (and where it may make sense
to set a breakpoint in a debugger to examine the state of the
program).
-
print-model-NNN: an executable file that prints out the values
associated with the counter-example.
To define properties and assumptions about the code to analyze, you can
annotate the source code with inline properties. When crux-llvm
compiles your code, it defines the CPP macro CRUCIBLE to help specify
Crucible-specific functionality such as inline properties. The following
simple example includes on version of main when running with
crux-llvm and different one when compiled normally.
#include <stdint.h>
#ifdef CRUCIBLE
#include <crucible.h>
#endif
int8_t f(int8_t x) {
return x + 1;
}
#ifdef CRUCIBLE
int main() {
int8_t x = crucible_int8_t("x");
assuming(x < 100);
check(f(x) < 100);
return 0;
}
#else
int main(int argc, char **argv) {
return f(argc);
}
#endif
When running under crux-llvm, this file includes the crucible.h
header file that declares functions and macros such as
crucible_int8_t, assuming, and check. The call to
crucible_int8_t marks variable x as a symbolic variable whose value
can be any 8-bit signed integer. The C expression within the assuming
statement states that x must be less than 100. The expression within
the check statement is a proof goal: crux-llvm will attempt to prove
that property f(x) < 100 holds whenever the assumption on x is
satisfied. The proof will fail in this case and crux-llvm will produce
a counterexample describing the case where x is 99.
Counterexample limitations
All counterexamples assume that the entrypoint function is main, even
if entry-point option is used to specify a different entrypoint during
simulation. Counterexamples also do not respect the supply-main-arguments
option. That is, if you simulate a main(int argc, char *argv[]) function
and use supply-main-arguments: empty to pass argc=0 and argv={} to
main, these arguments will not be passed automatically to the
counterexample executables.
API
The crucible.h header file contains
declarations of several functions that can be used to describe the
properties of a program that you would like to prove.
-
The crucible_assume function states an assumption as a C expression.
Any proofs after this point will assume this expression is true. The
macro assuming will automatically fill in its location arguments.
-
The crucible_assert function states an property to check as a C
expression. Every call to this function will create an additional
proof goal. The check macro will automatically fill in its location
arguments.
-
The crucible_*_t functions create fresh (non-deterministic) values
of the corresponding type. The verification process ensures that
whatever results are returned by these functions, out of all possible
values for the corresponding type, all crucible_assert calls will
succeed.
For programs that have been written for the SV-COMP
competition, the __VERIFIER_nondet_*
functions create non-deterministic values of the corresponding type.
These symbolic values all have the name x. To supply distinct names,
use the crucible_*_t functions, instead.
Note that support for the SV-COMP API exists primarily for backward
compatibility, since a large number of benchmarks already exist in that
form. The crucible.h API allows for better explanations by a) allowing
user-specified names for non-deterministic variables, and b) ensuring
that the conditions used in assertions are directly available and not
obscured by a conditional wrapper around an error function.
Standard C and C++ Libraries
The code supplied to crux-llvm should be largely self-contained,
without calls to external code. However, some standard library functions
have built-in support. For C code, the following functions are understood:
__assert_rtn
calloc
free
getenv (always returns NULL)
malloc
memcpy
__memcpy_chk
memmove
memset
__memset_chk
posix_memalign
printf (supports a subset of standard printf formatting codes)
__printf_chk
putchar
puts
realloc
strlen
open
read
write
close
In addition, the following LLVM intrinsic functions are supported:
llvm.assume
llvm.bitreverse.*
llvm.bswap.*
llvm.ctlz.*
llvm.ctpop.*
llvm.cttz.*
llvm.expect.*
llvm.invariant.end.*
llvm.invariant.start.*
llvm.lifetime.end.*
llvm.lifetime.start.*
llvm.memcpy.*
llvm.memmove.*
llvm.memset.*
llvm.objectsize.*
llvm.sadd.with.overflow.*
llvm.smul.with.overflow.*
llvm.ssub.with.overflow.*
llvm.stackrestore
llvm.stacksave
llvm.uadd.with.overflow.*
llvm.umul.with.overflow.*
llvm.usub.with.overflow.*
llvm.x86.pclmulqdq
llvm.x86.sse2.storeu.dq
For C++ code, several core functions have built-in support, but
crux-llvm will also link with a precompiled LLVM bitcode file
containing the libc++ library included with the clang compiler, so
most C++ code that doesn't use third-party libraries (or that includes
those libraries linked into a single bitcode file) should work.
Command-line Flags
The most important and only required argument to crux-llvm is the
source file or list of source files to analyze. In the case that
multiple files are provided, they will be compiled independently and
linked together with llvm-link.
In addition, the following flags can optionally be provided:
-
--help, -h, -?: Print all options with brief descriptions.
-
--include-dirs=DIRS, -I DIRS: Set directories to search for C/C++
include files. This will be passed along to clang.
-
-O NUM: Set the optimization level for clang.
-
--version, -V: Show the version of the tool.
-
--sim-verbose=NUM, -d NUM: Set the verbosity level of the symbolic
simulator to N.
-
--floating-point=FPREP, -f FPREP: Select the floating point
representation to use. The value of FPREP can be one of real,
ieee, uninterpreted, or default. Default representation is
solver specific: real for CVC4 and Yices, and ieee for Z3.
-
--iteration-bound=N, -i N: Set a bound on the number of times a
single loop can iterate. This can also make it more likely to get at
least a partial verification result for complex programs, and can be
more clearly connected to the execution of the program than a
time-based bound.
-
--profiling-period=N, -p N: Set how many seconds to wait between
each dump of profiling data (default: 5). Intermediate profiling data
can be helpful for diagnosing a run that does not terminate in a
reasonable amount of time.
-
--quiet, -q: Quiet mode; produce minimal output.
-
--recursion-bound=N, -i N: Set a bound on the number of times a
single function can recur. This can also make it more likely to get at
least a partial verification result for complex programs, and can be
more clearly connected to the execution of the program than a
time-based bound.
-
--solver=NAME, -s NAME: Use the given SMT solver to discharge
proof obligations. Valid values for NAME are cvc4, cvc5, yices, and
z3.
-
--timeout=N, -t N: Set the timeout for the first phase of analysis
(symbolic execution) which happens before sending the main goals to an
SMT solver. Setting this to a low value can give you a result more
quickly, but the result is more likely to be "Unknown" (default: 60).
-
--no-execs, -x: Do not create executables to demonstrate
counter-examples.
-
--branch-coverage: Record branch coverage information.
-
--config=FILE: Load configuration from FILE. A configuration file
can specify the same settings as command-line flags. Details of the
format for configuration files appear in the next section.
-
--entry-point=SYMBOL: Start symbolic execution at SYMBOL.
-
--fail-fast: Stop attempting to prove goals as soon as one of them
is disproved.
-
--force-offline-goal-solving: Force goals to be solved using an
offline solver, even if the selected solver could have been used in
online mode.
-
--goal-timeout=N: Set the timeout for each call to the SMT solver to
N seconds.
-
--hash-consing: Enable hash-consing in the symbolic expression
backend.
-
--lax-arithmetic: Allow arithmetic overflow.
-
--lax-pointers: Allow order comparisons between pointers from
different allocation blocks.
-
--lazy-compile: Avoid compiling bitcode from source if intermediate
files already exist.
-
--mcsat: Enable the MC-SAT engine when using the Yices SMT solver.
This disables the use of UNSAT cores, so the HTML rendering of proved
goals won't include highlighting a set of the assumptions that were
necessary for proving the goal.
-
--no-unsat-cores: Disable computing unsat cores for successful
proofs.
-
--online-solver-output=FILE: Store a log of interaction with the
online goal solver in FILE.
-
--opt-loop-merge: Insert merge blocks in loops with early exits.
-
--output-directory=DIR: Set the directory to use to store output
files (default: results).
-
--path-sat: Enable path satisfiability checking, which can help
programs terminate, particularly in the case where the bounds on loops
are complex.
-
--path-sat-solver=SOLVER: Select the solver to use for path
satisfiability checking, in order to use a different solver than for
discharging goals.
-
--path-sat-solver-output: Store a log of interaction with the path
satisfiability solver in FILE.
-
--path-strategy=STRATEGY: Set the strategy to use for exploring
paths during symbolic execution. A STRATEGY of always-merge (the
default) causes all paths being explored to be merged into a single
symbolic state at every post-dominator node in the control flow graph.
The split-dfs strategy explores each path independently in
depth-first order. The former is typically more appropriate for full
verification whereas the latter can be more effective for bug finding.
Sometimes, however split-dfs can lead to faster full verification
times.
-
--profile-crucible: Enable profiling of the symbolic execution
process. Produces an additional HTML file in the output directory that
provides a graphical and tabular depiction of the execution time
profile.
-
--profile-solver: Include profiling of SMT solver calls in the
symbolic execution profile.
-
--skip-incomplete-reports: Skip reporting on proof obligations that
arise from timeouts and resource exhaustion.
-
--skip-print-failures: Skip printing messages related to failed
verification goals.
-
--skip-report: Skip producing the HTML report following
verification.
-
--skip-success-reports: Skip reporting on successful proof
obligations.
-
--target=ARCH: Pass ARCH as the target architecture to LLVM build
operations.
-
--no-compile: Assume the input file is an LLVM bitcode module, rather than a
C program.
-
--symbolic-fs-root=DIR: Specify a directory containing the initial contents of
a symbolic filesystem to use during symbolic execution. See the Symbolic I/O
documentation for the format of this directory. [Experimental]
Environment Variables
The following environment variables are supported:
-
BLDDIR: Specify the directory for writing build files generated from
the input files.
-
CLANG: Specify the name of the clang compiler command used to
translate from C/C++ to LLVM.
-
CLANG_OPTS: Specify additional options to pass to clang.
-
LLVM_LINK: Specify the name of the llvm-link command used to
combine multiple LLVM bitcode files.
Configuration Files
In addition to command-line flags and environment variables, crux-llvm
can be configured with a key-value input file. The file consists of a
set of KEY: VALUE entries, each on a separate line, where each KEY
generally corresponds to the textual part of the long version of a
command-line flag. For example, one can set the iteration bound to 10 as
follows:
iteration-bound: 10
Options that take a list of arguments can be written with either a
single value (for a list of length one) or with multiple values on
successive lines, each starting with *. For example, the following is
a valid input file:
llvm-link: "llvm-link-6.0"
clang: "clang-6.0"
make-executables: no
files:
* "a.c"
* "b.c"
This specifies the name of the command to run for clang and
llvm-link, instructs crux-llvm not to create counter-example
demonstration executables, and provides a list of input files.
Symbolic I/O [Experimental]
Note that Symbolic I/O is currently experimental. We expect that the API (both
internal and command line) will change.
crux-llvm supports symbolic I/O operations via the POSIX open, read,
write, and close functions. These operations are symbolic in the sense that
the contents of files in the symbolic filesystem can be a mix of concrete and
symbolic values. The original motivation of the work was to support checking
assertions for programs with configuration files; by supporting symbolic file
contents, crux-llvm can check entire families of configuration at once.
Symbolic Filesystem Contents
The --symbolic-fs-root option (and corresponding configuration file option)
enable users to specify the initial contents of the symbolic filesystem. The
directory pointed to by this options contains:
- A sub-directory named
root that contains concrete files that will exist in
the initial filesystem. The directory layout within root is preserved. For
example, a file named root/etc/fstab will be mapped to /etc/fstab in the
initial symbolic filesystem.
- An optional file named
stdin, which contains the standard input of the program.
- A manifest named
system-manifest.json that describes the contents of symbolic files.
The manifest maps absolute file paths to specifications of what parts of the
corresponding file are symbolic. Note that the specification is currently
coarse and only supports fully-symbolic files. The format of the system
manifest is:
{ "symbolicFiles": [<SymFilePair>],
"useStdout": bool,
"useStderr": bool
}
SymFilePair := [ FilePath, <SymbolicFileContents> ]
FilePath := string (an absolute file path)
SymbolicFileContents := { "symbolicContentSize": Word64 }
If no initial filesystem is specified, crux-llvm defaults to an empty standard
input, while allowing output to standard output and standard error. Note that
if a program attempts to use the symbolic I/O primitives without being backed by
appropriate files (e.g., opening a file that does not exist), the symbolic I/O
backend will simply report that those functions fail in the expected way (i.e.,
returning -1).
Interaction with Standard I/O
If the program being verified writes to standard output or standard error (via
the POSIX STDOUT_FILENO or STDERR_FILENO handles, which correspond to file
descriptors 1 and 2), the symbolic I/O backend reflects as much of the output as
it can to the actual standard output and standard error of crux-llvm. Note
that only concrete writes are mirrored to the real world; symbolic writes
still take effect in the symbolic files representing standard output and
standard error.
Note that, due to the branching structure of symbolic execution, the same output
may appear more than once if a call to write occurs on a symbolic branch.
Current Limitations
- Filenames must be concrete
- Filenames must be absolute paths (relative paths require additionally modeling the current working directory)
- Many file operations are not yet modeled
- Files can currently only be entirely concrete or entirely symbolic
- The
open function does not accept a mode (i.e., file permissions are not yet modeled)
- The
open function accepts flags (but currently ignores them)
- The
open function cannot create new files that do not exist in the filesystem
- The special handling of
printf does not yet interact with the symbolic standard output file descriptor
It is intended that the symbolic I/O facility will be extended over time to
support more operations.
Also note that the order in which file descriptor numbers are handed out to
client code can be subtly different than in the real program. In particular, on
a symbolic branch where both branches open a new file, the two branches will get
sequential file descriptors. In contrast, the real program would allocate the
same file descriptor to both (as only one branch would be taken).
Test suite
The crux-llvm test suite is implemented in test/Test.hs, and the
accompanying test case data can be found under test-data. Each test case
comprises a C or LLVM bitcode file that is provided as input to crux-llvm,
along with an expected output file. For most test cases, this expected output
file will be named <test-case>.z3.good; this picks Z3 as a default solver to
use when simulating the test case. There are also a handful of tests that
require other solvers, e.g., abd-test-file-32.cvc5.good.
Some of the test cases have slightly different output depending on which Clang
version is used. These test cases will have accompanying
<test-case>.pre-clang<version>.<...>.good files, where pre-clang<version>
indicates that this test output is used for all Clang versions up to (but not
including) <version>. Note that if a test case has multiple
pre-clang<version> .good files, then the <version> that is closest to the
current Clang version (without going over) is picked.
To illustrate this with a concrete example, consider suppose we have a test
case foo with the following .good files
foo.pre-clang11.z3.good
foo.pre-clang13.z3.good
foo.z3.good
The following .good files would be used for the following Clang versions:
- Clang 10:
foo.pre-clang11.z3.good
- Clang 11:
foo.pre-clang13.z3.good
- Clang 12:
foo.pre-clang13.z3.good
- Clang 13 or later:
foo.z3.good
There are some test cases that require a sufficiently recent Clang version to
run. To indicate that a test should not be run on Clangs older than
<version>, create a pre-clang<version> .good file with SKIP_TEST as the
first line. The use of SKIP_TEST signals that this test should be skipped
when using Clangs older than <version>. Note that the test suite will not
read anything past SKIP_TEST, so the rest of the file can be used to document
why the test is skipped on that particular configuration.
Acknowledgements
Crux is partly based upon work supported by the Defense Advanced
Research Projects Agency (DARPA) under Contract No. N66001-18-C-4011.
Any opinions, findings and conclusions or recommendations expressed in
this material are those of the author(s) and do not necessarily reflect
the views of the Defense Advanced Research Projects Agency (DARPA).
