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Environmental variables
This document discusses the environment variables used by American Fuzzy Lop
to expose various exotic functions that may be (rarely) useful for power
users or for some types of custom fuzzing setups. See README for the general
instruction manual.
1) Settings for afl-gcc, afl-clang, and afl-as
Because they can't directly accept command-line options, the compile-time
tools make fairly broad use of environmental variables:
- Setting AFL_HARDEN automatically adds code hardening options when invoking
the downstream compiler. This currently includes -D_FORTIFY_SOURCE=2 and
-fstack-protector-all. The setting is useful for catching non-crashing
memory bugs at the expense of a very slight (sub-5%) performance loss.
- By default, the wrapper appends -O3 to optimize builds. Very rarely, this
will cause problems in programs built with -Werror, simply because -O3
enables more thorough code analysis and can spew out additional warnings.
To disable optimizations, set AFL_DONT_OPTIMIZE.
- Setting AFL_USE_ASAN automatically enables ASAN, provided that your
compiler supports that. Note that fuzzing with ASAN is mildly challenging
- see notes_for_asan.txt.
(You can also enable MSAN via AFL_USE_MSAN; ASAN and MSAN come with the
same gotchas; the modes are mutually exclusive. UBSAN and other exotic
sanitizers are not officially supported yet, but are easy to get to work
by hand.)
- Setting AFL_CC, AFL_CXX, and AFL_AS lets you use alternate downstream
compilation tools, rather than the default 'clang', 'gcc', or 'as' binaries
in your $PATH.
- AFL_PATH can be used to point afl-gcc to an alternate location of afl-as.
One possible use of this is experimental/clang_asm_normalize/, which lets
you instrument hand-written assembly when compiling clang code by plugging
a normalizer into the chain. (There is no equivalent feature for GCC.)
- Setting AFL_INST_RATIO to a percentage between 0 and 100% controls the
probability of instrumenting every branch. This is (very rarely) useful
when dealing with exceptionally complex programs that saturate the output
bitmap. Examples include v8, ffmpeg, and perl.
(If this ever happens, afl-fuzz will warn you ahead of the time by
displaying the "bitmap density" field in fiery red.)
Setting AFL_INST_RATIO to 0 is a valid choice. This will instrument only
the transitions between function entry points, but not individual branches.
- AFL_NO_BUILTIN causes the compiler to generate code suitable for use with (but perhaps running a bit slower than without the flag).
- TMPDIR is used by afl-as for temporary files; if this variable is not set,
the tool defaults to /tmp.
- Setting AFL_KEEP_ASSEMBLY prevents afl-as from deleting instrumented
assembly files. Useful for troubleshooting problems or understanding how
the tool works. To get them in a predictable place, try something like:
mkdir assembly_here
TMPDIR=$PWD/assembly_here AFL_KEEP_ASSEMBLY=1 make clean all
- Setting AFL_QUIET will prevent afl-cc and afl-as banners from being
displayed during compilation, in case you find them distracting.
2) Settings for afl-clang-fast
The native LLVM instrumentation helper accepts a subset of the settings
discussed in section #1, with the exception of:
- AFL_AS, since this toolchain does not directly invoke GNU as.
- TMPDIR and AFL_KEEP_ASSEMBLY, since no temporary assembly files are
Note that AFL_INST_RATIO will behave a bit differently than for afl-gcc,
because functions are *not* instrumented unconditionally - so low values
will have a more striking effect. For this tool, 0 is not a valid choice.
3) Settings for afl-fuzz
The main fuzzer binary accepts several options that disable a couple of sanity
checks or alter some of the more exotic semantics of the tool:
- Setting AFL_SKIP_CPUFREQ skips the check for CPU scaling policy. This is
useful if you can't change the defaults (e.g., no root access to the
system) and are OK with some performance loss.
- Setting AFL_NO_FORKSRV disables the forkserver optimization, reverting to
fork + execve() call for every tested input. This is useful mostly when
working with unruly libraries that create threads or do other crazy
things when initializing (before the instrumentation has a chance to run).
Note that this setting inhibits some of the user-friendly diagnostics
normally done when starting up the forkserver and causes a pretty
significant performance drop.
- AFL_EXIT_WHEN_DONE causes afl-fuzz to terminate when all existing paths
have been fuzzed and there were no new finds for a while. This would be
normally indicated by the cycle counter in the UI turning green. May be
convenient for some types of automated jobs.
- Setting AFL_NO_AFFINITY disables attempts to bind to a specific CPU core
on Linux systems. This slows things down, but lets you run more instances
of afl-fuzz than would be prudent (if you really want to).
- AFL_SKIP_CRASHES causes AFL to tolerate crashing files in the input
queue. This can help with rare situations where a program crashes only
intermittently, but it's not really recommended under normal operating
- Setting AFL_HANG_TMOUT allows you to specify a different timeout for
deciding if a particular test case is a "hang". The default is 1 second
or the value of the -t parameter, whichever is larger. Dialing the value
down can be useful if you are very concerned about slow inputs, or if you
don't want AFL to spend too much time classifying that stuff and just
rapidly put all timeouts in that bin.
- AFL_NO_ARITH causes AFL to skip most of the deterministic arithmetics.
This can be useful to speed up the fuzzing of text-based file formats.
- AFL_SHUFFLE_QUEUE randomly reorders the input queue on startup. Requested
by some users for unorthodox parallelized fuzzing setups, but not
advisable otherwise.
- When developing custom instrumentation on top of afl-fuzz, you can use
AFL_SKIP_BIN_CHECK to inhibit the checks for non-instrumented binaries
and shell scripts; and AFL_DUMB_FORKSRV in conjunction with the -n
setting to instruct afl-fuzz to still follow the fork server protocol
without expecting any instrumentation data in return.
- When running in the -M or -S mode, setting AFL_IMPORT_FIRST causes the
fuzzer to import test cases from other instances before doing anything
else. This makes the "own finds" counter in the UI more accurate.
Beyond counter aesthetics, not much else should change.
- Setting AFL_POST_LIBRARY allows you to configure a postprocessor for
mutated files - say, to fix up checksums. See experimental/post_library/
for more.
- AFL_FAST_CAL keeps the calibration stage about 2.5x faster (albeit less
precise), which can help when starting a session against a slow target.
- The CPU widget shown at the bottom of the screen is fairly simplistic and
may complain of high load prematurely, especially on systems with low core
counts. To avoid the alarming red color, you can set AFL_NO_CPU_RED.
- In QEMU mode (-Q), AFL_PATH will be searched for afl-qemu-trace.
- Setting AFL_PRELOAD causes AFL to set LD_PRELOAD for the target binary
without disrupting the afl-fuzz process itself. This is useful, among other
things, for bootstrapping
- Setting AFL_NO_UI inhibits the UI altogether, and just periodically prints
some basic stats. This behavior is also automatically triggered when the
output from afl-fuzz is redirected to a file or to a pipe.
- If you are Jakub, you may need AFL_I_DONT_CARE_ABOUT_MISSING_CRASHES.
Others need not apply.
- Benchmarking only: AFL_BENCH_JUST_ONE causes the fuzzer to exit after
processing the first queue entry; and AFL_BENCH_UNTIL_CRASH causes it to
exit soon after the first crash is found.
4) Settings for afl-qemu-trace
The QEMU wrapper used to instrument binary-only code supports several settings:
- It is possible to set AFL_INST_RATIO to skip the instrumentation on some
of the basic blocks, which can be useful when dealing with very complex
- Setting AFL_INST_LIBS causes the translator to also instrument the code
inside any dynamically linked libraries (notably including glibc).
- The underlying QEMU binary will recognize any standard "user space
emulation" variables (e.g., QEMU_STACK_SIZE), but there should be no
reason to touch them.
5) Settings for afl-cmin
The corpus minimization script offers very little customization:
- Setting AFL_PATH offers a way to specify the location of afl-showmap
and afl-qemu-trace (the latter only in -Q mode).
- AFL_KEEP_TRACES makes the tool keep traces and other metadata used for
minimization and normally deleted at exit. The files can be found in the
- AFL_ALLOW_TMP permits this and some other scripts to run in /tmp. This is
a modest security risk on multi-user systems with rogue users, but should
be safe on dedicated fuzzing boxes.
6) Settings for afl-tmin
Virtually nothing to play with. Well, in QEMU mode (-Q), AFL_PATH will be
searched for afl-qemu-trace. In addition to this, TMPDIR may be used if a
temporary file can't be created in the current working directory.
You can specify AFL_TMIN_EXACT if you want afl-tmin to require execution paths
to match when minimizing crashes. This will make minimization less useful, but
may prevent the tool from "jumping" from one crashing condition to another in
very buggy software. You probably want to combine it with the -e flag.
7) Settings for afl-analyze
You can set AFL_ANALYZE_HEX to get file offsets printed as hexadecimal instead
of decimal.
8) Settings for
The library honors three environmental variables:
- AFL_LD_LIMIT_MB caps the size of the maximum heap usage permitted by the
library, in megabytes. The default value is 1 GB. Once this is exceeded,
allocations will return NULL.
- AFL_LD_HARD_FAIL alters the behavior by calling abort() on excessive
allocations, thus causing what AFL would perceive as a crash. Useful for
programs that are supposed to maintain a specific memory footprint.
- AFL_LD_VERBOSE causes the library to output some diagnostic messages
that may be useful for pinpointing the cause of any observed issues.
- AFL_LD_NO_CALLOC_OVER inhibits abort() on calloc() overflows. Most
of the common allocators check for that internally and return NULL, so
it's a security risk only in more exotic setups.
9) Settings for
This library accepts AFL_TOKEN_FILE to indicate the location to which the
discovered tokens should be written.
10) Third-party variables set by afl-fuzz & other tools
Several variables are not directly interpreted by afl-fuzz, but are set to
optimal values if not already present in the environment:
- By default, LD_BIND_NOW is set to speed up fuzzing by forcing the
linker to do all the work before the fork server kicks in. You can
override this by setting LD_BIND_LAZY beforehand, but it is almost
certainly pointless.
- By default, ASAN_OPTIONS are set to:
If you want to set your own options, be sure to include abort_on_error=1 -
otherwise, the fuzzer will not be able to detect crashes in the tested
app. Similarly, include symbolize=0, since without it, AFL may have
difficulty telling crashes and hangs apart.
- In the same vein, by default, MSAN_OPTIONS are set to:
exit_code=86 (required for legacy reasons)
Be sure to include the first one when customizing anything, since some
MSAN versions don't call abort() on error, and we need a way to detect
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