path: root/docs/env_variables.md

# Environment variables

  This document discusses the environment variables used by AFL++ to expose
  various exotic functions that may be (rarely) useful for power users or for
  some types of custom fuzzing setups. For general information about AFL++, see
  [README.md](../README.md).

  Note: Most tools will warn on any unknown AFL++ environment variables; for
  example, because of typos. If you want to disable this check, then set the
  `AFL_IGNORE_UNKNOWN_ENVS` environment variable.

## 1) Settings for all compilers

Starting with AFL++ 3.0, there is only one compiler: afl-cc.

To select the different instrumentation modes, use one of the following options:

  - Pass the --afl-MODE command-line option to the compiler. Only this option
    accepts further AFL-specific command-line options.
  - Use a symlink to afl-cc: afl-clang, afl-clang++, afl-clang-fast,
    afl-clang-fast++, afl-clang-lto, afl-clang-lto++, afl-g++, afl-g++-fast,
    afl-gcc, afl-gcc-fast. This option does not accept AFL-specific command-line
    options. Instead, use environment variables.
  - Use the `AFL_CC_COMPILER` environment variable with `MODE`. To select
    `MODE`, use one of the following values:

    - `GCC` (afl-gcc/afl-g++)
    - `GCC_PLUGIN` (afl-g*-fast)
    - `LLVM` (afl-clang-fast*)
    - `LTO` (afl-clang-lto*).

The compile-time tools do not accept AFL-specific command-line options. The
--afl-MODE command line option is the only exception. The other options make
fairly broad use of environment variables instead:

  - Some build/configure scripts break with AFL++ compilers. To be able to pass
    them, do:

    ```
          export CC=afl-cc
          export CXX=afl-c++
          export AFL_NOOPT=1
          ./configure --disable-shared --disabler-werror
          unset AFL_NOOPT
          make
    ```

  - Setting `AFL_AS`, `AFL_CC`, and `AFL_CXX` lets you use alternate downstream
    compilation tools, rather than the default 'as', 'clang', or 'gcc' binaries
    in your `$PATH`.

  - If you are a weird person that wants to compile and instrument asm text
    files, then use the `AFL_AS_FORCE_INSTRUMENT` variable:
    `AFL_AS_FORCE_INSTRUMENT=1 afl-gcc foo.s -o foo`

  - Most AFL tools do not print any output if stdout/stderr are redirected. If
    you want to get the output into a file, then set the `AFL_DEBUG` environment
    variable. This is sadly necessary for various build processes which fail
    otherwise.

  - By default, the wrapper appends `-O3` to optimize builds. Very rarely, this
    will cause problems in programs built with -Werror, because `-O3` enables
    more thorough code analysis and can spew out additional warnings. To disable
    optimizations, set `AFL_DONT_OPTIMIZE`. However, if `-O...` and/or
    `-fno-unroll-loops` are set, these are not overridden.

  - 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.

  - 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 ffmpeg, perl, and v8.

    (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.

    Note that this is an outdated variable. A few instances (e.g., afl-gcc)
    still support these, but state-of-the-art (e.g., LLVM LTO and LLVM PCGUARD)
    do not need this.

  - `AFL_NO_BUILTIN` causes the compiler to generate code suitable for use with
    libtokencap.so (but perhaps running a bit slower than without the flag).

  - `AFL_PATH` can be used to point afl-gcc to an alternate location of afl-as.
    One possible use of this is utils/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_QUIET` will prevent afl-as and afl-cc banners from being
    displayed during compilation, in case you find them distracting.

  - Setting `AFL_USE_...` automatically enables supported sanitizers - provided
    that your compiler supports it. Available are:
    - `AFL_USE_ASAN=1` - activates the address sanitizer (memory corruption
      detection)
    - `AFL_USE_CFISAN=1` - activates the Control Flow Integrity sanitizer (e.g.
      type confusion vulnerabilities)
    - `AFL_USE_LSAN` - activates the leak sanitizer. To perform a leak check
      within your program at a certain point (such as at the end of an
      `__AFL_LOOP()`), you can run the macro  `__AFL_LEAK_CHECK();` which will
      cause an abort if any memory is leaked (you can combine this with the
      `__AFL_LSAN_OFF();` and `__AFL_LSAN_ON();` macros to avoid checking for
      memory leaks from memory allocated between these two calls.
    - `AFL_USE_MSAN=1` - activates the memory sanitizer (uninitialized memory)
    - `AFL_USE_TSAN=1` - activates the thread sanitizer to find thread race
      conditions
    - `AFL_USE_UBSAN=1` - activates the undefined behavior sanitizer

  - `TMPDIR` is used by afl-as for temporary files; if this variable is not set,
    the tool defaults to /tmp.

## 2) Settings for LLVM and LTO: afl-clang-fast / afl-clang-fast++ / afl-clang-lto / afl-clang-lto++

The native instrumentation helpers (instrumentation and gcc_plugin) accept a
subset of the settings discussed in section 1, with the exception of:

  - `AFL_AS`, since this toolchain does not directly invoke GNU `as`.

  - `AFL_INST_RATIO`, as we use collision free instrumentation by default. Not
    all passes support this option though as it is an outdated feature.

  - LLVM modes support `AFL_LLVM_DICT2FILE=/absolute/path/file.txt` which will
    write all constant string comparisons to this file to be used later with
    afl-fuzz' `-x` option.

  - An option to `AFL_LLVM_DICT2FILE` is `AFL_LLVM_DICT2FILE_NO_MAIN=1` which
    skill not parse `main()`.

  - `TMPDIR` and `AFL_KEEP_ASSEMBLY`, since no temporary assembly files are
    created.

  - LLVM modes compiling C++ will normally set rpath in the binary if LLVM is
    not in a usual location (/usr or /lib). Setting `AFL_LLVM_NO_RPATH=1`
    disables this behaviour in case it isn't desired. For example, the compiling
    toolchain might be in a custom location, but the target machine has LLVM
    runtime libs in the search path.

Then there are a few specific features that are only available in
instrumentation mode:

### Select the instrumentation mode

`AFL_LLVM_INSTRUMENT` - this configures the instrumentation mode.

Available options:

  - CLANG - outdated clang instrumentation
  - CLASSIC - classic AFL (map[cur_loc ^ prev_loc >> 1]++) (default)

    You can also specify CTX and/or NGRAM, separate the options with a comma ","
    then, e.g.: `AFL_LLVM_INSTRUMENT=CLASSIC,CTX,NGRAM-4`

    Note: It is actually not a good idea to use both CTX and NGRAM. :)
  - CTX - context sensitive instrumentation
  - GCC - outdated gcc instrumentation
  - LTO - LTO instrumentation
  - NATIVE - clang's original pcguard based instrumentation
  - NGRAM-x - deeper previous location coverage (from NGRAM-2 up to NGRAM-16)
  - PCGUARD - our own pcguard based instrumentation (default)

#### CMPLOG

Setting `AFL_LLVM_CMPLOG=1` during compilation will tell afl-clang-fast to
produce a CmpLog binary.

For afl-gcc-fast, set `AFL_GCC_CMPLOG=1` instead.

For more information, see
[instrumentation/README.cmplog.md](../instrumentation/README.cmplog.md).

#### CTX

Setting `AFL_LLVM_CTX` or `AFL_LLVM_INSTRUMENT=CTX` activates context sensitive
branch coverage - meaning that each edge is additionally combined with its
caller. It is highly recommended to increase the `MAP_SIZE_POW2` definition in
config.h to at least 18 and maybe up to 20 for this as otherwise too many map
collisions occur.

For more information, see
[instrumentation/README.llvm.md#6) AFL++ Context Sensitive Branch Coverage](../instrumentation/README.llvm.md#6-afl-context-sensitive-branch-coverage).

#### INSTRUMENT LIST (selectively instrument files and functions)

This feature allows selective instrumentation of the source.

Setting `AFL_LLVM_ALLOWLIST` or `AFL_LLVM_DENYLIST` with a file name and/or
function will only instrument (or skip) those files that match the names listed
in the specified file.

For more information, see
[instrumentation/README.instrument_list.md](../instrumentation/README.instrument_list.md).

#### INJECTIONS

This feature is able to find simple injection vulnerabilities in insecure
calls to mysql/mariadb/nosql/postgresql/ldap and XSS in libxml2.

  - Setting `AFL_LLVM_INJECTIONS_ALL` will enable all injection hooking

  - Setting `AFL_LLVM_INJECTIONS_SQL` will enable SQL injection hooking

  - Setting `AFL_LLVM_INJECTIONS_LDAP` will enable LDAP injection hooking

  - Setting `AFL_LLVM_INJECTIONS_XSS` will enable XSS injection hooking

#### LAF-INTEL

This great feature will split compares into series of single byte comparisons to
allow afl-fuzz to find otherwise rather impossible paths. It is not restricted
to Intel CPUs. ;-)

  - Setting `AFL_LLVM_LAF_TRANSFORM_COMPARES` will split string compare
    functions.

  - Setting `AFL_LLVM_LAF_SPLIT_COMPARES` will split all floating point and 64,
    32 and 16 bit integer CMP instructions.

  - Setting `AFL_LLVM_LAF_SPLIT_FLOATS` will split floating points, needs
    `AFL_LLVM_LAF_SPLIT_COMPARES` to be set.

  - Setting `AFL_LLVM_LAF_SPLIT_SWITCHES` will split all `switch` constructs.

  - Setting `AFL_LLVM_LAF_ALL` sets all of the above.

For more information, see
[instrumentation/README.laf-intel.md](../instrumentation/README.laf-intel.md).

#### LTO

This is a different way of instrumentation: first it compiles all code in LTO
(link time optimization) and then performs an edge inserting instrumentation
which is 100% collision free (collisions are a big issue in AFL and AFL-like
instrumentations). This is performed by using afl-clang-lto/afl-clang-lto++
instead of afl-clang-fast, but is only built if LLVM 11 or newer is used.

`AFL_LLVM_INSTRUMENT=CFG` will use Control Flow Graph instrumentation. (Not
recommended for afl-clang-fast, default for afl-clang-lto as there it is a
different and better kind of instrumentation.)

None of the following options are necessary to be used and are rather for manual
use (which only ever the author of this LTO implementation will use). These are
used if several separated instrumentations are performed which are then later
combined.

  - `AFL_LLVM_DOCUMENT_IDS=file` will document to a file which edge ID was given
    to which function. This helps to identify functions with variable bytes or
    which functions were touched by an input.
  - `AFL_LLVM_LTO_DONTWRITEID` prevents that the highest location ID written
    into the instrumentation is set in a global variable.
  - `AFL_LLVM_LTO_STARTID` sets the starting location ID for the
    instrumentation. This defaults to 1.
  - `AFL_LLVM_MAP_ADDR` sets the fixed map address to a different address than
    the default `0x10000`. A value of 0 or empty sets the map address to be
    dynamic (the original AFL way, which is slower).
  - `AFL_LLVM_MAP_DYNAMIC` sets the shared memory address to be dynamic.
  - `AFL_LLVM_LTO_SKIPINIT` skips adding initialization code. Some global vars
    (e.g. the highest location ID) are not injected. Needed to instrument with
    [WAFL](https://github.com/fgsect/WAFL.git).
  For more information, see
  [instrumentation/README.lto.md](../instrumentation/README.lto.md).

#### NGRAM

Setting `AFL_LLVM_INSTRUMENT=NGRAM-{value}` or `AFL_LLVM_NGRAM_SIZE` activates
ngram prev_loc coverage. Good values are 2, 4, or 8 (any value between 2 and 16
is valid). It is highly recommended to increase the `MAP_SIZE_POW2` definition
in config.h to at least 18 and maybe up to 20 for this as otherwise too many map
collisions occur.

For more information, see
[instrumentation/README.llvm.md#7) AFL++ N-Gram Branch Coverage](../instrumentation/README.llvm.md#7-afl-n-gram-branch-coverage).

#### NOT_ZERO

  - Setting `AFL_LLVM_NOT_ZERO=1` during compilation will use counters that skip
    zero on overflow. This is the default for llvm >= 9, however, for llvm
    versions below that this will increase an unnecessary slowdown due a
    performance issue that is only fixed in llvm 9+. This feature increases path
    discovery by a little bit.

  - Setting `AFL_LLVM_SKIP_NEVERZERO=1` will not implement the skip zero test.
    If the target performs only a few loops, then this will give a small
    performance boost.

#### Thread safe instrumentation counters (in all modes)

Setting `AFL_LLVM_THREADSAFE_INST` will inject code that implements thread safe
counters. The overhead is a little bit higher compared to the older non-thread
safe case. Note that this disables neverzero (see NOT_ZERO).

## 3) Settings for GCC / GCC_PLUGIN modes

There are a few specific features that are only available in GCC and GCC_PLUGIN
mode.

  - GCC mode only: 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
    ```

  - GCC_PLUGIN mode only: Setting `AFL_GCC_INSTRUMENT_FILE` or
    `AFL_GCC_ALLOWLIST` with a filename will only instrument those files that
    match the names listed in this file (one filename per line).

    Setting `AFL_GCC_DENYLIST` or `AFL_GCC_BLOCKLIST` with a file name and/or
    function will only skip those files that match the names listed in the
    specified file. See
    [instrumentation/README.instrument_list.md](../instrumentation/README.instrument_list.md)
    for more information.

    Setting `AFL_GCC_OUT_OF_LINE=1` will instruct afl-gcc-fast to instrument the
    code with calls to an injected subroutine instead of the much more efficient
    inline instrumentation.

    Setting `AFL_GCC_SKIP_NEVERZERO=1` will not implement the skip zero test. If
    the target performs only a few loops, then this will give a small
    performance boost.

## 4) 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_AUTORESUME` will resume a fuzz run (same as providing `-i -`)
    for an existing out folder, even if a different `-i` was provided. Without
    this setting, afl-fuzz will refuse execution for a long-fuzzed out dir.

  - 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.

  - `AFL_CMPLOG_ONLY_NEW` will only perform the expensive cmplog feature for
    newly found test cases and not for test cases that are loaded on startup
    (`-i in`). This is an important feature to set when resuming a fuzzing
    session.

  - `AFL_IGNORE_SEED_PROBLEMS` will skip over crashes and timeouts in the seeds
    instead of exiting.

  - Setting `AFL_CRASH_EXITCODE` sets the exit code AFL++ treats as crash. For
    example, if `AFL_CRASH_EXITCODE='-1'` is set, each input resulting in a `-1`
    return code (i.e. `exit(-1)` got called), will be treated as if a crash had
    occurred. This may be beneficial if you look for higher-level faulty
    conditions in which your target still exits gracefully.

  - Setting `AFL_CUSTOM_MUTATOR_LIBRARY` to a shared library with
    afl_custom_fuzz() creates additional mutations through this library. If
    afl-fuzz is compiled with Python (which is autodetected during building
    afl-fuzz), setting `AFL_PYTHON_MODULE` to a Python module can also provide
    additional mutations. If `AFL_CUSTOM_MUTATOR_ONLY` is also set, all
    mutations will solely be performed with the custom mutator. This feature
    allows to configure custom mutators which can be very helpful, e.g., fuzzing
    XML or other highly flexible structured input. For details, see
    [custom_mutators.md](custom_mutators.md).

  - Setting `AFL_CYCLE_SCHEDULES` will switch to a different schedule every time
    a cycle is finished.

  - Setting `AFL_DEBUG_CHILD` will not suppress the child output. This lets you
    see all output of the child, making setup issues obvious. For example, in an
    unicornafl harness, you might see python stacktraces. You may also see other
    logs that way, indicating why the forkserver won't start. Not pretty but
    good for debugging purposes. Note that `AFL_DEBUG_CHILD_OUTPUT` is
    deprecated.

  - Setting `AFL_DISABLE_TRIM` tells afl-fuzz not to trim test cases. This is
    usually a bad idea!

  - Setting `AFL_KEEP_TIMEOUTS` will keep longer running inputs if they reach
    new coverage

  - On the contrary, if you are not interested in any timeouts, you can set
    `AFL_IGNORE_TIMEOUTS` to get a bit of speed instead.

  - `AFL_EXIT_ON_SEED_ISSUES` will restore the vanilla afl-fuzz behavior which
    does not allow crashes or timeout seeds in the initial -i corpus.

  - `AFL_CRASHING_SEEDS_AS_NEW_CRASH` will treat crashing seeds as new crash. these 
    crashes will be written to crashes folder as op:dry_run, and orig:<seed_file_name>.

  - `AFL_EXIT_ON_TIME` causes afl-fuzz to terminate if no new paths were found
    within a specified period of time (in seconds). May be convenient for some
    types of automated jobs.

  - `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_EXPAND_HAVOC_NOW` will start in the extended havoc mode that
    includes costly mutations. afl-fuzz automatically enables this mode when
    deemed useful otherwise.

  - `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.
    `AFL_CAL_FAST` works too.

  - Setting `AFL_FORCE_UI` will force painting the UI on the screen even if no
    valid terminal was detected (for virtual consoles).

  - Setting `AFL_FORKSRV_INIT_TMOUT` allows you to specify a different timeout
    to wait for the forkserver to spin up. The specified value is the new timeout, in milliseconds.
    The default is the `-t` value times `FORK_WAIT_MULT` from `config.h` (usually 10), so for a `-t 100`, the default would wait for `1000` milliseconds.
    The `AFL_FORKSRV_INIT_TMOUT` value does not get multiplied. It overwrites the initial timeout afl-fuzz waits for the target to come up with a constant time.
    Setting a different time here is useful if the target has a very slow startup time, for example, when doing
    full-system fuzzing or emulation, but you don't want the actual runs to wait
    too long for timeouts.

  - 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.

  - If you are Jakub, you may need `AFL_I_DONT_CARE_ABOUT_MISSING_CRASHES`.
    Others need not apply, unless they also want to disable the
    `/proc/sys/kernel/core_pattern` check.

  - If afl-fuzz encounters an incorrect fuzzing setup during a fuzzing session
    (not at startup), it will terminate. If you do not want this, then you can
    set `AFL_IGNORE_PROBLEMS`. If you additionally want to also ignore coverage
    from late loaded libraries, you can set `AFL_IGNORE_PROBLEMS_COVERAGE`.

  - When running with multiple afl-fuzz or with `-F`,  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.

  - When running with multiple afl-fuzz or with `-F`,  setting `AFL_FINAL_SYNC`
    will cause the fuzzer to perform a final import of test cases when
    terminating. This is beneficial for `-M` main fuzzers to ensure it has all
    unique test cases and hence you only need to `afl-cmin` this single
    queue.

  - Setting `AFL_INPUT_LEN_MIN` and `AFL_INPUT_LEN_MAX` are an alternative to
    the afl-fuzz -g/-G command line option to control the minimum/maximum
    of fuzzing input generated.

  - `AFL_KILL_SIGNAL`: Set the signal ID to be delivered to child processes
    on timeout. Unless you implement your own targets or instrumentation, you
    likely don't have to set it. By default, on timeout and on exit, `SIGKILL`
    (`AFL_KILL_SIGNAL=9`) will be delivered to the child.

  - `AFL_FORK_SERVER_KILL_SIGNAL`: Set the signal ID to be delivered to the
    fork server when AFL++ is terminated. Unless you implement your
    fork server, you likely do not have to set it. By default, `SIGTERM`
    (`AFL_FORK_SERVER_KILL_SIGNAL=15`) will be delivered to the fork server.
    If only `AFL_KILL_SIGNAL` is provided, `AFL_FORK_SERVER_KILL_SIGNAL` will
    be set to same value as `AFL_KILL_SIGNAL` to provide backward compatibility.
    If `AFL_FORK_SERVER_KILL_SIGNAL` is also set, it takes precedence.

    NOTE: Uncatchable signals, such as `SIGKILL`, cause child processes of
    the fork server to be orphaned and leaves them in a zombie state.

  - `AFL_MAP_SIZE` sets the size of the shared map that afl-analyze, afl-fuzz,
    afl-showmap, and afl-tmin create to gather instrumentation data from the
    target. This must be equal or larger than the size the target was compiled
    with.

  - Setting `AFL_MAX_DET_EXTRAS` will change the threshold at what number of
    elements in the `-x` dictionary and LTO autodict (combined) the
    probabilistic mode will kick off. In probabilistic mode, not all dictionary
    entries will be used all of the time for fuzzing mutations to not slow down
    fuzzing. The default count is `200` elements. So for the 200 + 1st element,
    there is a 1 in 201 chance, that one of the dictionary entries will not be
    used directly.

  - 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_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.

  - Setting `AFL_NO_AUTODICT` will not load an LTO generated auto dictionary
    that is compiled into the target.

  - Setting `AFL_NO_COLOR` or `AFL_NO_COLOUR` will omit control sequences for
    coloring console output when configured with USE_COLOR and not
    ALWAYS_COLORED.

  - 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 for very high CPU usages, you can
    set `AFL_NO_CPU_RED`.

  - 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_NO_SNAPSHOT` will advise afl-fuzz not to use the snapshot feature if
    the snapshot lkm is loaded.

  - 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.

  - Setting `AFL_NO_STARTUP_CALIBRATION` will skip the initial calibration
    of all starting seeds, and start fuzzing at once. Use with care, this
    degrades the fuzzing performance!

  - Setting `AFL_NO_WARN_INSTABILITY` will suppress instability warnings.

  - In QEMU mode (-Q) and FRIDA mode (-O), `AFL_PATH` will be searched for
    afl-qemu-trace and afl-frida-trace.so.

  - If you are using persistent mode (you should, see
    [instrumentation/README.persistent_mode.md](../instrumentation/README.persistent_mode.md)),
    some targets keep inherent state due which a detected crash test case does
    not crash the target again when the test case is given. To be able to still
    re-trigger these crashes, you can use the `AFL_PERSISTENT_RECORD` variable
    with a value of how many previous fuzz cases to keep prior a crash. If set to
    e.g., 10, then the 9 previous inputs are written to out/default/crashes as
    RECORD:000000,cnt:000000 to RECORD:000000,cnt:000008 and
    RECORD:000000,cnt:000009 being the crash case. NOTE: This option needs to be
    enabled in config.h first!

  - Note that `AFL_POST_LIBRARY` is deprecated, use `AFL_CUSTOM_MUTATOR_LIBRARY`
    instead.

  - 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 libdislocator.so.

  - In QEMU mode (-Q), setting `AFL_QEMU_CUSTOM_BIN` will cause afl-fuzz to skip
    prepending `afl-qemu-trace` to your command line. Use this if you wish to
    use a custom afl-qemu-trace or if you need to modify the afl-qemu-trace
    arguments.

  - `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. Note that this also turns off
    auto map size detection.

  - 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_STATSD` enables StatsD metrics collection. By default, AFL++
    will send these metrics over UDP to 127.0.0.1:8125. The host and port are
    configurable with `AFL_STATSD_HOST` and `AFL_STATSD_PORT` respectively. To
    enable tags (banner and afl_version), you should provide
    `AFL_STATSD_TAGS_FLAVOR` that matches your StatsD server (see
    `AFL_STATSD_TAGS_FLAVOR`).

  - Setting `AFL_STATSD_TAGS_FLAVOR` to one of `dogstatsd`, `influxdb`,
    `librato`, or `signalfx` allows you to add tags to your fuzzing instances.
    This is especially useful when running multiple instances (`-M/-S` for
    example). Applied tags are `banner` and `afl_version`. `banner` corresponds
    to the name of the fuzzer provided through `-M/-S`. `afl_version`
    corresponds to the currently running AFL++ version (e.g., `++3.0c`). Default
    (empty/non present) will add no tags to the metrics. For more information,
    see [rpc_statsd.md](rpc_statsd.md).

  - `AFL_SYNC_TIME` allows you to specify a different minimal time (in minutes)
    between fuzzing instances synchronization. Default sync time is 30 minutes,
    note that time is halved for -M main nodes.

  - Setting `AFL_TARGET_ENV` causes AFL++ to set extra environment variables for
    the target binary. Example: `AFL_TARGET_ENV="VAR1=1 VAR2='a b c'" afl-fuzz
    ... `. This exists mostly for things like `LD_LIBRARY_PATH` but it would
    theoretically allow fuzzing of AFL++ itself (with 'target' AFL++ using some
    AFL_ vars that would disrupt work of 'fuzzer' AFL++). Note that when using
    QEMU mode, the `AFL_TARGET_ENV` environment variables will apply to QEMU, as
    well as the target binary. Therefore, in this case, you might want to use
    QEMU's `QEMU_SET_ENV` environment variable (see QEMU's documentation because
    the format is different from `AFL_TARGET_ENV`) to apply the environment
    variables to the target and not QEMU.

  - `AFL_TESTCACHE_SIZE` allows you to override the size of `#define
    TESTCASE_CACHE` in config.h. Recommended values are 50-250MB - or more if
    your fuzzing finds a huge amount of paths for large inputs.

  - `AFL_TMPDIR` is used to write the `.cur_input` file to if it exists, and in
    the normal output directory otherwise. You would use this to point to a
    ramdisk/tmpfs. This increases the speed by a small value but also reduces
    the stress on SSDs.

  - Setting `AFL_TRY_AFFINITY` tries to attempt binding to a specific CPU core
    on Linux systems, but will not terminate if that fails.

  - The following environment variables are only needed if you implemented
    your own forkserver or persistent mode, or if __AFL_LOOP or __AFL_INIT
    are in a shared library and not the main binary:
    - `AFL_DEFER_FORKSRV` enforces a deferred forkserver even if none was
      detected in the target binary
    - `AFL_PERSISTENT` enforces persistent mode even if none was detected
      in the target binary

  - If you need an early forkserver in your target because of early
    constructors in your target, you can set `AFL_EARLY_FORKSERVER`.
    Note that this is not a compile time option but a runtime option :-)

  - Set `AFL_PIZZA_MODE` to 1 to enable the April 1st stats menu, set to -1
    to disable although it is 1st of April. 0 is the default and means enable
    on the 1st of April automatically.

  - If you need a specific interval to update fuzzer_stats file, you can
    set `AFL_FUZZER_STATS_UPDATE_INTERVAL` to the interval in seconds you'd
    the file to be updated.
    Note that will not be exact and with slow targets it can take seconds
    until there is a slice for the time test.

## 5) Settings for afl-qemu-trace

The QEMU wrapper used to instrument binary-only code supports several settings:

  - Setting `AFL_COMPCOV_LEVEL` enables the CompareCoverage tracing of all cmp
    and sub in x86 and x86_64 and memory comparison functions (e.g., strcmp,
    memcmp, ...) when libcompcov is preloaded using `AFL_PRELOAD`. More info at
    [qemu_mode/libcompcov/README.md](../qemu_mode/libcompcov/README.md).

    There are two levels at the moment, `AFL_COMPCOV_LEVEL=1` that instruments
    only comparisons with immediate values / read-only memory and
    `AFL_COMPCOV_LEVEL=2` that instruments all the comparisons. Level 2 is more
    accurate but may need a larger shared memory.

  - `AFL_DEBUG` will print the found entry point for the binary to stderr. Use
    this if you are unsure if the entry point might be wrong - but use it
    directly, e.g., `afl-qemu-trace ./program`.

  - `AFL_ENTRYPOINT` allows you to specify a specific entry point into the
    binary (this can be very good for the performance!). The entry point is
    specified as hex address, e.g., `0x4004110`. Note that the address must be
    the address of a basic block.

  - Setting `AFL_INST_LIBS` causes the translator to also instrument the code
    inside any dynamically linked libraries (notably including glibc).

  - You can use `AFL_QEMU_INST_RANGES=0xaaaa-0xbbbb,0xcccc-0xdddd` to just
    instrument specific memory locations, e.g. a specific library.
    Excluding ranges takes priority over any included ranges or `AFL_INST_LIBS`.

  - You can use `AFL_QEMU_EXCLUDE_RANGES=0xaaaa-0xbbbb,0xcccc-0xdddd` to **NOT**
    instrument specific memory locations, e.g. a specific library.
    Excluding ranges takes priority over any included ranges or `AFL_INST_LIBS`.

  - 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
    binaries.

  - Setting `AFL_QEMU_COMPCOV` enables the CompareCoverage tracing of all cmp
    and sub in x86 and x86_64. This is an alias of `AFL_COMPCOV_LEVEL=1` when
    `AFL_COMPCOV_LEVEL` is not specified.

  - With `AFL_QEMU_FORCE_DFL`, you force QEMU to ignore the registered signal
    handlers of the target.

  - When the target is i386/x86_64, you can specify the address of the function
    that has to be the body of the persistent loop using
    `AFL_QEMU_PERSISTENT_ADDR=start addr`.

  - With `AFL_QEMU_PERSISTENT_GPR=1`, QEMU will save the original value of
    general purpose registers and restore them in each persistent cycle.

  - Another modality to execute the persistent loop is to specify also the
    `AFL_QEMU_PERSISTENT_RET=end addr` environment variable. With this variable
    assigned, instead of patching the return address, the specified instruction
    is transformed to a jump towards `start addr`.

  - With `AFL_QEMU_PERSISTENT_RETADDR_OFFSET`, you can specify the offset from
    the stack pointer in which QEMU can find the return address when `start
    addr` is hit.

  - With `AFL_USE_QASAN`, you can enable QEMU AddressSanitizer for dynamically
    linked binaries.

  - 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.

  - Normally a `README.txt` is written to the `crashes/` directory when a first
    crash is found. Setting `AFL_NO_CRASH_README` will prevent this. Useful when
    counting crashes based on a file count in that directory.

## 7) Settings for afl-frida-trace

The FRIDA wrapper used to instrument binary-only code supports many of the same
options as `afl-qemu-trace`, but also has a number of additional advanced
options. These are listed in brief below (see
[frida_mode/README.md](../frida_mode/README.md) for more details). These
settings are provided for compatibility with QEMU mode, the preferred way to
configure FRIDA mode is through its [scripting](../frida_mode/Scripting.md)
support.

* `AFL_FRIDA_DEBUG_MAPS` - See `AFL_QEMU_DEBUG_MAPS`
* `AFL_FRIDA_DRIVER_NO_HOOK` - See `AFL_QEMU_DRIVER_NO_HOOK`. When using the
  QEMU driver to provide a `main` loop for a user provided
  `LLVMFuzzerTestOneInput`, this option configures the driver to read input from
  `stdin` rather than using in-memory test cases.
* `AFL_FRIDA_EXCLUDE_RANGES` - See `AFL_QEMU_EXCLUDE_RANGES`
* `AFL_FRIDA_INST_COVERAGE_FILE` - File to write DynamoRio format coverage
  information (e.g., to be loaded within IDA lighthouse).
* `AFL_FRIDA_INST_DEBUG_FILE` - File to write raw assembly of original blocks
  and their instrumented counterparts during block compilation.
* `AFL_FRIDA_INST_JIT` - Enable the instrumentation of Just-In-Time compiled
  code. Code is considered to be JIT if the executable segment is not backed by
  a file.
* `AFL_FRIDA_INST_NO_DYNAMIC_LOAD` - Don't instrument the code loaded late at
  runtime. Strictly limits instrumentation to what has been included.
* `AFL_FRIDA_INST_NO_OPTIMIZE` - Don't use optimized inline assembly coverage
  instrumentation (the default where available). Required to use
  `AFL_FRIDA_INST_TRACE`.
* `AFL_FRIDA_INST_NO_BACKPATCH` - Disable backpatching. At the end of executing
  each block, control will return to FRIDA to identify the next block to
  execute.
* `AFL_FRIDA_INST_NO_PREFETCH` - Disable prefetching. By default, the child will
  report instrumented blocks back to the parent so that it can also instrument
  them and they be inherited by the next child on fork, implies
  `AFL_FRIDA_INST_NO_PREFETCH_BACKPATCH`.
* `AFL_FRIDA_INST_NO_PREFETCH_BACKPATCH` - Disable prefetching of stalker
  backpatching information. By default, the child will report applied
  backpatches to the parent so that they can be applied and then be inherited by
  the next child on fork.
* `AFL_FRIDA_INST_RANGES` - See `AFL_QEMU_INST_RANGES`
* `AFL_FRIDA_INST_SEED` - Sets the initial seed for the hash function used to
  generate block (and hence edge) IDs. Setting this to a constant value may be
  useful for debugging purposes, e.g., investigating unstable edges.
* `AFL_FRIDA_INST_TRACE` - Log to stdout the address of executed blocks, implies
  `AFL_FRIDA_INST_NO_OPTIMIZE`.
* `AFL_FRIDA_INST_TRACE_UNIQUE` - As per `AFL_FRIDA_INST_TRACE`, but each edge
  is logged only once, requires `AFL_FRIDA_INST_NO_OPTIMIZE`.
* `AFL_FRIDA_INST_UNSTABLE_COVERAGE_FILE` - File to write DynamoRio format
  coverage information for unstable edges (e.g., to be loaded within IDA
  lighthouse).
* `AFL_FRIDA_JS_SCRIPT` - Set the script to be loaded by the FRIDA scripting
  engine. See [frida_mode/Scripting.md](../frida_mode/Scripting.md) for details.
* `AFL_FRIDA_OUTPUT_STDOUT` - Redirect the standard output of the target
  application to the named file (supersedes the setting of `AFL_DEBUG_CHILD`)
* `AFL_FRIDA_OUTPUT_STDERR` - Redirect the standard error of the target
  application to the named file (supersedes the setting of `AFL_DEBUG_CHILD`)
* `AFL_FRIDA_PERSISTENT_ADDR` - See `AFL_QEMU_PERSISTENT_ADDR`
* `AFL_FRIDA_PERSISTENT_CNT` - See `AFL_QEMU_PERSISTENT_CNT`
* `AFL_FRIDA_PERSISTENT_DEBUG` - Insert a Breakpoint into the instrumented code
  at `AFL_FRIDA_PERSISTENT_HOOK` and `AFL_FRIDA_PERSISTENT_RET` to allow the
  user to detect issues in the persistent loop using a debugger.
* `AFL_FRIDA_PERSISTENT_HOOK` - See `AFL_QEMU_PERSISTENT_HOOK`
* `AFL_FRIDA_PERSISTENT_RET` - See `AFL_QEMU_PERSISTENT_RET`
* `AFL_FRIDA_SECCOMP_FILE` - Write a log of any syscalls made by the target to
  the specified file.
* `AFL_FRIDA_STALKER_ADJACENT_BLOCKS` - Configure the number of adjacent blocks
  to fetch when generating instrumented code. By fetching blocks in the same
  order they appear in the original program, rather than the order of execution
  should help reduce locality and adjacency. This includes allowing us to
  vector between adjacent blocks using a NOP slide rather than an immediate
  branch.
* `AFL_FRIDA_STALKER_IC_ENTRIES` - Configure the number of inline cache entries
  stored along-side branch instructions which provide a cache to avoid having to
  call back into FRIDA to find the next block. Default is 32.
* `AFL_FRIDA_STATS_FILE` - Write statistics information about the code being
  instrumented to the given file name. The statistics are written only for the
  child process when new block is instrumented (when the
  `AFL_FRIDA_STATS_INTERVAL` has expired). Note that just because a new path is
  found does not mean a new block needs to be compiled. It could be that the
  existing blocks instrumented have been executed in a different order.
* `AFL_FRIDA_STATS_INTERVAL` - The maximum frequency to output statistics
  information. Stats will be written whenever they are updated if the given
  interval has elapsed since last time they were written.
* `AFL_FRIDA_TRACEABLE` - Set the child process to be traceable by any process
  to aid debugging and overcome the restrictions imposed by YAMA. Supported on
  Linux only. Permits a non-root user to use `gcore` or similar to collect a
  core dump of the instrumented target. Note that in order to capture the core
  dump you must set a sufficient timeout (using `-t`) to avoid `afl-fuzz`
  killing the process whilst it is being dumped.

## 8) Settings for afl-cmin

The corpus minimization script offers very little customization:

  - `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.

  - `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
    `<out_dir>/.traces/` directory.

  - Setting `AFL_PATH` offers a way to specify the location of afl-showmap and
    afl-qemu-trace (the latter only in `-Q` mode).

  - `AFL_PRINT_FILENAMES` prints each filename to stdout, as it gets processed.
    This can help when embedding `afl-cmin` or `afl-showmap` in other scripts.

## 9) 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.

## 10) Settings for afl-analyze

You can set `AFL_ANALYZE_HEX` to get file offsets printed as hexadecimal instead
of decimal.

## 11) Settings for libdislocator

The library honors these environment variables:

  - `AFL_ALIGNED_ALLOC=1` will force the alignment of the allocation size to
    `max_align_t` to be compliant with the C standard.

  - `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_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_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.

  - `AFL_LD_VERBOSE` causes the library to output some diagnostic messages that
    may be useful for pinpointing the cause of any observed issues.

## 11) Settings for libtokencap

This library accepts `AFL_TOKEN_FILE` to indicate the location to which the
discovered tokens should be written.

## 12) 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, `ASAN_OPTIONS` are set to (among others):

    ```
    abort_on_error=1
    detect_leaks=0
    malloc_context_size=0
    symbolize=0
    allocator_may_return_null=1
    ```

    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.

  - Similarly, the default `LSAN_OPTIONS` are set to:

    ```
    exit_code=23
    fast_unwind_on_malloc=0
    symbolize=0
    print_suppressions=0
    ```

    Be sure to include the first ones for LSAN and MSAN when customizing
    anything, since some MSAN and LSAN versions don't call `abort()` on error,
    and we need a way to detect faults.

  - In the same vein, by default, `MSAN_OPTIONS` are set to:

    ```
    exit_code=86 (required for legacy reasons)
    abort_on_error=1
    symbolize=0
    msan_track_origins=0
    allocator_may_return_null=1
    ```

  - 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.