afl-cov - AFL Fuzzing Code Coverage
- Directory and File Structure
- Usage Information
afl-cov uses test case files produced by the
afl-fuzz to generate gcov code
coverage results for a targeted binary. Code coverage is interpreted from one
case to the next by
afl-cov in order to determine which new functions and
lines are hit by AFL with each new test case. Further,
afl-cov allows for
specific lines or functions to be searched for within coverage results, and
when a match is found the corresponding test case file is displayed. This
allows the user to discover which AFL test case is the first to exercise a
particular function. In addition,
afl-cov produces a "zero coverage" report
of functions and lines that were never executed during any AFL fuzzing run.
Although of no use to AFL itself, the main application of
afl-cov is to wrap
some automation around gcov together with AFL test cases and thereby provide
data on how to maximize code coverage with AFL fuzzing runs. Manual
interpretation of cumulative gcov results from AFL test cases is usually still
required, but the "fiddly" steps of iterating over all test cases and
generating code coverage reports (along with the "zero coverage" report) is
Producing code coverage data for AFL test cases is an important step to try
and maximize code coverage, and thereby help to maximize the effectiveness of
AFL. For example, some binaries have code that is reachable only after a
complicated (or even cryptographic) test is passed, and AFL may not be able to
exercise this code without taking special measures. These measures commonly
include patching the project code to bypass such tests. (For example, there is
a patch to solve this problem for a CRC test in libpng included in the AFL
When a project implements a patch to assist AFL in reaching code that would
otherwise be inaccessible, a natural question to ask is whether the patch is
effective. Code coverage results can help to verify this.
afl-cov requires the following software:
- gcov, lcov, genhtml
afl-cov can parse files created by
afl-fuzz from a different
system, so technically
afl-fuzz does not need to be installed on the same
afl-cov. This supports scenarios where fuzzing output is collected,
say, within a git repository on one system, and coverage results are produced
on a different system. However, most workflows typically focus on producing
afl-cov results simultaneously for current fuzzing runs on the same system.
At a high level, the general workflow for
afl-cov against a targeted project
- Have a target project compiled and known to work with AFL.
- Create a spare copy of the project sources, and compile this copy with gcov profiling support.
afl-covagainst the copy either while
afl-fuzzis building test cases against the original sources, or after
afl-fuzzhas been stopped.
- Review the cumulative code coverage results in the final web report.
- Iterate to achieve higher coverage results. This might involve building better initial test cases for AFL, or sometimes changing project sources themselves.
Now, in more detail:
Copy the project sources to a new directory,
/path/to/project-gcov/. This directory should contain the project binaries compiled for gcov profiling support (gcc
--livemode before also starting the
afl-fuzzfuzzing cycle. The command line arguments to
afl-covmust specify the path to the output directory used by
afl-fuzz, and the command to execute along with associated arguments. This command and arguments should closely resemble the manner in which
afl-fuzzexecutes the targeted binary during the fuzzing cycle. If there is already an existing directory of AFL fuzzing results, then just omit the
--liveargument to process the existing results. Here is an example:
$ cd /path/to/project-gcov/ $ afl-cov -d /path/to/afl-fuzz-output/ --live --coverage-cmd \ "cat AFL_FILE | LD_LIBRARY_PATH=./lib/.libs ./bin/.libs/somebin -a -b -c" \ --code-dir .
/path/to/afl-fuzz-output/ is the output directory of afl-fuzz.
AFL_FILE string above refers to the test case file that AFL will
build in the
queue/ directory under
/path/to/afl-fuzz-output. Just leave this
string as-is since
afl-cov will automatically substitute it with each AFL
queue/id:NNNNNN* in succession as it builds the code coverage reports.
Also, in the above command, this handles the case where the AFL fuzzing cycle
is fuzzing the targeted binary via stdin. This explains the
cat AFL_FILE | ... ./bin/.lib/somebin ... invocation. For the other style of
fuzzing with AFL where a file is read from the filesystem, here is an example:
$ cd /path/to/project-gcov/ $ afl-cov -d /path/to/afl-fuzz-output/ --live --coverage-cmd \ "LD_LIBRARY_PATH=./lib/.libs ./bin/.libs/somebin -f AFL_FILE -a -b -c" \ --code-dir .
afl-covrunning, open a separate terminal/shell, and launch
$ LD_LIBRARY_PATH=./lib/.libs afl-fuzz -T somebin -t 1000 \ -i /path/to/test-cases/ -o /path/to/afl-fuzz-output/ ./bin/.libs/somebin -a -b -c
The familiar AFL status screen will be displayed, and
afl-cov will start
generating code coverage data.
Note that by default
afl-cov does not direct
lcov to include branch
coverage results. This is because there are commonly many hundreds of AFL
test cases in the
queue/ directory, and generating branch coverage across all
of these cases may slow
afl-cov down significantly. If branch coverage is
desired, just add the
--enable-branch-coverage argument to
Here is a sample of what the
afl-cov output looks like (note this includes
--enable-branch-coverage argument as described above):
$ afl-cov -d /path/to/afl-fuzz-output/ --live --coverage-cmd \ "LD_LIBRARY_PATH=./lib/.libs ./bin/.libs/somebin -f AFL_FILE -a -b -c" \ --code-dir . --enable-branch-coverage [+] Imported 184 files from: /path/to/afl-fuzz-output/queue [+] AFL file: id:000000,orig:somestr.start (1 / 184), cycle: 0 lines......: 18.6% (1122 of 6032 lines) functions..: 30.7% (100 of 326 functions) branches...: 14.0% (570 of 4065 branches) [+] AFL file: id:000001,orig:somestr256.start (2 / 184), cycle: 2 lines......: 18.7% (1127 of 6032 lines) functions..: 30.7% (100 of 326 functions) branches...: 14.1% (572 of 4065 branches) [+] Coverage diff id:000000,orig:somestr.start id:000001,orig:somestr256.start Src file: /path/to/project-gcov/lib/proj_decode.c New 'line' coverage: 140 New 'line' coverage: 141 New 'line' coverage: 142 Src file: /path/to/project-gcov/lib/proj_util.c New 'line' coverage: 217 New 'line' coverage: 218 [+] AFL file: id:000002,orig:somestr384.start (3 / 184), cycle: 10 lines......: 18.8% (1132 of 6032 lines) functions..: 30.7% (100 of 326 functions) branches...: 14.1% (574 of 4065 branches) [+] Coverage diff id:000001,orig:somestr256.start id:000002,orig:somestr384.start Src file: /path/to/project-gcov/lib/proj_decode.c New 'line' coverage: 145 New 'line' coverage: 146 New 'line' coverage: 147 Src file: /path/to/project-gcov/lib/proj_util.c New 'line' coverage: 220 New 'line' coverage: 221 [+] AFL file: id:000003,orig:somestr.start (4 / 184), cycle: 5 lines......: 18.9% (1141 of 6032 lines) functions..: 31.0% (101 of 326 functions) branches...: 14.3% (581 of 4065 branches) [+] Coverage diff id:000002,orig:somestr384.start id:000003,orig:somestr.start Src file: /path/to/project-gcov/lib/proj_message.c New 'function' coverage: validate_cmd_msg() New 'line' coverage: 244 New 'line' coverage: 247 New 'line' coverage: 248 New 'line' coverage: 250 New 'line' coverage: 255 New 'line' coverage: 262 New 'line' coverage: 263 New 'line' coverage: 266 . . . [+] Coverage diff id:000182,src:000000,op:havoc,rep:64 id:000184,src:000000,op:havoc,rep:4 [+] Processed 184 / 184 files [+] Final zero coverage report: /path/to/afl-fuzz-output/cov/zero-cov [+] Final positive coverage report: /path/to/afl-fuzz-output/cov/pos-cov [+] Final lcov web report: /path/to/afl-fuzz-output/cov/web/lcov-web-final.html
In the last few lines above, the locations of the final web coverage and zero
coverage reports are shown. The zero coverage reports contains function names
that were never executed across the entire
The code coverage results in
represent cumulative code coverage across all AFL test cases. This data can then
be reviewed to ensure that all expected functions are indeed exercised by AFL -
just point a web browser at
Below is a sample of what this report looks like for a cumulative AFL fuzzing
run - this is against the fwknop project, and the full report is
Note that even though fwknop has a dedicated set of
AFL wrappers, it is still
difficult to achieve high percentages of code coverage. This provides evidence
that measuring code coverage under AFL fuzzing runs is an important aspect of
trying to achieve maximal fuzzing results. Every branch/line/function that is
not exercised by AFL represents a location for which AFL has not been given the
opportunity to find bugs.
Parallelized AFL Execution
With the 0.4 release,
afl-cov supports parallelized execution runs of
afl-fuzz. All that is required is to point
afl-cov -d sync_dir at the top
level sync directory that is used by all
afl-fuzz -o sync_dir). The coverage results are calculated globally
across all fuzzing instances, and in
--live mode new instances will be added
to the coverage results as they are created.
The workflow above is probably the main strategy for using
additional use cases are supported such as:
Suppose there are a set of wrapper scripts around
afl-fuzzto run fuzzing cycles against various aspects of a project. By building a set of corresponding
afl-covwrappers, and then using the
--disable-coverage-initoption on all but the first of these wrappers, it is possible to generate code coverage results across the entire set of
afl-fuzzfuzzing runs. (By default,
afl-covresets gcov counters to zero at start time, but the
--disable-coverage-initargument stops this behavior.) The end result is a global picture of code coverage across all invocations of
Specific functions can be searched for in the code coverage results, and
afl-covwill return the first
afl-fuzztest case where a given function is executed. This allows
afl-covto be used as a validation tool by other scripts and testing infrastructure. For example, a test case could be written around whether an important function is executed by
afl-fuzzto validate a patching strategy mentioned in the introduction.
Here is an example where the first test case that executes the function
validate_cmd_msg() is returned (this is after all
afl-cov results have been
produced in the main workflow above):
$ ./afl-cov -d /path/to/afl-fuzz-output --func-search "validate_cmd_msg" [+] Function 'validate_cmd_mag()' executed by: id:000002,orig:somestr384.start
An equivalent way of searching the coverage results is to just
function from the
cov/id-delta-cov file described below. The number "3" in
the output below is the AFL cycle number where the function is first executed:
$ grep validate_cmd_msg /path/to/afl-fuzz-output/cov/id-delta-cov id:000002,orig:somestr384.start, 3, /path/to/project-gcov/file.c, function, validate_cmd_msg()
Directory and File Structure
afl-cov creates a few files and directories for coverage results within the
afl-fuzz directory (
-d). These files and directories are
displayed below, and all are contained within the main
/path/to/afl-fuzz-output/cov/ directory and
<dirname> refers to the
top level directory name for the fuzzing instance. When AFL is parallelized,
there will be one
<dirname> directory path for each
cov/diff/<dirname>- contains new code coverage results when a
afl-fuzzto execute new code.
cov/lcov/<dirname>- contains raw code coverage data produced by the lcov front-end to gcov.
cov/web/<dirname>- contains code coverage results in web format produced by
cov/zero-cov- file that globally lists all functions (and optionally lines) that are never executed by any
cov/pos-cov- file that globally lists all functions (and optionally lines) that are executed at least once by an
cov/id-delta-cov- lists the functions (and optionally lines) that are executed by the first
id:000000*test case, and then lists all new functions/lines executed in subsequent test cases.
cov/afl-cov.log- log file for
cov/afl-cov-status- status file for
afl-covPID, version number , and command line arguments.
--help output appears below:
usage: afl-cov [-h] [-e COVERAGE_CMD] [-d AFL_FUZZING_DIR] [-c CODE_DIR] [-O] [--disable-cmd-redirection] [--disable-lcov-web] [--disable-coverage-init] [--coverage-include-lines] [--enable-branch-coverage] [--live] [--cover-corpus] [--coverage-at-exit] [--sleep SLEEP] [--gcov-check] [--gcov-check-bin GCOV_CHECK_BIN] [--background] [--lcov-web-all] [--disable-lcov-exclude-pattern] [--lcov-exclude-pattern LCOV_EXCLUDE_PATTERN] [--func-search FUNC_SEARCH] [--line-search LINE_SEARCH] [--src-file SRC_FILE] [--afl-queue-id-limit AFL_QUEUE_ID_LIMIT] [--ignore-core-pattern] [--lcov-path LCOV_PATH] [--genhtml-path GENHTML_PATH] [--readelf-path READELF_PATH] [--stop-afl] [--validate-args] [-v] [-V] [-q] optional arguments: -h, --help show this help message and exit -e COVERAGE_CMD, --coverage-cmd COVERAGE_CMD Set command to exec (including args, and assumes code coverage support) -d AFL_FUZZING_DIR, --afl-fuzzing-dir AFL_FUZZING_DIR top level AFL fuzzing directory -c CODE_DIR, --code-dir CODE_DIR Directory where the code lives (compiled with code coverage support) -O, --overwrite Overwrite existing coverage results --disable-cmd-redirection Disable redirection of command results to /dev/null --disable-lcov-web Disable generation of all lcov web code coverage reports --disable-coverage-init Disable initialization of code coverage counters at afl-cov startup --coverage-include-lines Include lines in zero-coverage status files --enable-branch-coverage Include branch coverage in code coverage reports (may be slow) --live Process a live AFL directory, and afl-cov will exit when it appears afl-fuzz has been stopped --cover-corpus Measure coverage after running all available tests instead of individually per queue file --coverage-at-exit Only calculate coverage just before afl-cov exit. --sleep SLEEP In --live mode, # of seconds to sleep between checking for new queue files --gcov-check Check to see if there is a binary in --coverage-cmd (or in --gcov-check-bin) has coverage support --gcov-check-bin GCOV_CHECK_BIN Test a specific binary for code coverage support --background Background mode - if also in --live mode, will exit when the alf-fuzz process is finished --lcov-web-all Generate lcov web reports for all id:NNNNNN* files instead of just the last one --disable-lcov-exclude-pattern Allow default /usr/include/* pattern to be included in lcov results --lcov-exclude-pattern LCOV_EXCLUDE_PATTERN Set exclude pattern for lcov results --func-search FUNC_SEARCH Search for coverage of a specific function --line-search LINE_SEARCH Search for coverage of a specific line number (requires --src-file) --src-file SRC_FILE Restrict function or line search to a specific source file --afl-queue-id-limit AFL_QUEUE_ID_LIMIT Limit the number of id:NNNNNN* files processed in the AFL queue/ directory --ignore-core-pattern Ignore the /proc/sys/kernel/core_pattern setting in --live mode --lcov-path LCOV_PATH Path to lcov command --genhtml-path GENHTML_PATH Path to genhtml command --readelf-path READELF_PATH Path to readelf command --stop-afl Stop all running afl-fuzz instances associated with --afl-fuzzing-dir <dir> --validate-args Validate args and exit -v, --verbose Verbose mode -V, --version Print version and exit -q, --quiet Quiet mode
afl-cov is released as open source software under the terms of
the GNU General Public License (GPL v2+). The latest release can be found
All feature requests and bug fixes are managed through github issues tracking. However, you can also email me (michael.rash_AT_gmail.com), or reach me through Twitter (@michaelrash).