# 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.md](README.md) for the general instruction manual. ## 1) Settings for all compilers Starting with afl++ 3.0 there is only one compiler: afl-cc To select the different instrumentation modes this can be done by 1. passing the --afl-MODE command line option to the compiler 2. or using a symlink to afl-cc: afl-gcc, afl-g++, afl-clang, afl-clang++, afl-clang-fast, afl-clang-fast++, afl-clang-lto, afl-clang-lto++, afl-gcc-fast, afl-g++-fast 3. or using the environment variable `AFL_CC_COMPILER` with `MODE` `MODE` can be one of `LTO` (afl-clang-lto*), `LLVM` (afl-clang-fast*), `GCC_PLUGIN` (afl-g*-fast) or `GCC` (afl-gcc/afl-g++). Because (with the exception of the --afl-MODE command line option) the compile-time tools do not accept afl specific command-line options, they make fairly broad use of environmental variables instead: - 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. - 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`. However if `-O...` and/or `-fno-unroll-loops` are set, these are not overridden. - 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.md](notes_for_asan.md). (You can also enable MSAN via `AFL_USE_MSAN`; ASAN and MSAN come with the same gotchas; the modes are mutually exclusive. UBSAN can be enabled similarly by setting the environment variable `AFL_USE_UBSAN=1`. Finally there is the Control Flow Integrity sanitizer that can be activated by `AFL_USE_CFISAN=1`) - 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 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_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. 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). - `TMPDIR` is used by afl-as for temporary files; if this variable is not set, the tool defaults to /tmp. - 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` - Setting `AFL_QUIET` will prevent afl-cc and afl-as banners from being displayed during compilation, in case you find them distracting. - Setting `AFL_CAL_FAST` will speed up the initial calibration, if the application is very slow. ## 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: - 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. - `AFL_AS`, since this toolchain does not directly invoke GNU as. - `TMPDIR` and `AFL_KEEP_ASSEMBLY`, since no temporary assembly files are created. - `AFL_INST_RATIO`, as we by default use collision free instrumentation. Not all passes support this option though as it is an outdated feature. 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: PCGUARD - our own pcgard based instrumentation (default) NATIVE - clang's original pcguard based instrumentation CLASSIC - classic AFL (map[cur_loc ^ prev_loc >> 1]++) (default) CFG - InsTrim instrumentation (see below) LTO - LTO instrumentation (see below) CTX - context sensitive instrumentation (see below) NGRAM-x - deeper previous location coverage (from NGRAM-2 up to NGRAM-16) GCC - outdated gcc instrumentation CLANG - outdated clang instrumentation In CLASSIC (default) and CFG/INSTRIM you can also specify CTX and/or NGRAM, seperate the options with a comma "," then, e.g.: `AFL_LLVM_INSTRUMENT=CFG,CTX,NGRAM-4` Not that this is a good idea to use both CTX and NGRAM :) ### LTO This is a different kind 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_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_STARTID` sets the starting location ID for the instrumentation. This defaults to 1 - `AFL_LLVM_LTO_DONTWRITEID` prevents that the highest location ID written into the instrumentation is set in a global variable See [instrumentation/README.lto.md](../instrumentation/README.lto.md) for more information. ### INSTRIM This feature increases the speed by ~15% without any disadvantages to the classic instrumentation. Note that there is also an LTO version (if you have llvm 11 or higher) - that is the best instrumentation we have. Use `afl-clang-lto` to activate. The InsTrim LTO version additionally has all the options and features of LTO (see above). - Setting `AFL_LLVM_INSTRIM` or `AFL_LLVM_INSTRUMENT=CFG` activates this mode - Setting `AFL_LLVM_INSTRIM_LOOPHEAD=1` expands on INSTRIM to optimize loops. afl-fuzz will only be able to see the path the loop took, but not how many times it was called (unless it is a complex loop). See [instrumentation/README.instrim.md](../instrumentation/README.instrim.md) ### NGRAM - Setting `AFL_LLVM_NGRAM_SIZE` or `AFL_LLVM_INSTRUMENT=NGRAM-{value}` 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. See [instrumentation/README.ngram.md](../instrumentation/README.ngram.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. See [instrumentation/README.ctx.md](../instrumentation/README.ctx.md) ### 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_SWITCHES` will split all `switch` constructs - 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_ALL` sets all of the above See [instrumentation/README.laf-intel.md](../instrumentation/README.laf-intel.md) for more information. ### 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 filenames and/or function will only instrument (or 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. ### 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 few loops then this will give a small performance boost. See [instrumentation/README.neverzero.md](../instrumentation/README.neverzero.md) ### CMPLOG - Setting `AFL_LLVM_CMPLOG=1` during compilation will tell afl-clang-fast to produce a CmpLog binary. See [instrumentation/README.cmplog.md](../instrumentation/README.cmplog.md) ## 3) Settings for GCC / GCC_PLUGIN modes Then there are a few specific features that are only available in GCC and GCC_PLUGIN mode. - Setting `AFL_KEEP_ASSEMBLY` prevents afl-as from deleting instrumented assembly files. Useful for troubleshooting problems or understanding how the tool works. (GCC mode only) 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_GCC_INSTRUMENT_FILE` with a filename will only instrument those files that match the names listed in this file (one filename per line). See [instrumentation/README.instrument_list.md](../instrumentation/README.instrument_list.md) for more information. (GCC_PLUGIN mode only) ## 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_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. - `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. - `AFL_MAP_SIZE` sets the size of the shared map that afl-fuzz, afl-showmap, afl-tmin and afl-analyze create to gather instrumentation data from the target. This must be equal or larger than the size the target was compiled with. - `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. - Setting `AFL_DISABLE_TRIM` tells afl-fuzz not to trim test cases. This is usually a bad idea! - 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). - Setting `AFL_NO_AUTODICT` will not load an LTO generated auto dictionary that is compiled into the target. - `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 conditions. - 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. - Setting `AFL_FORKSRV_INIT_TMOUT` allows you to specify a different timeout to wait for the forkserver to spin up. 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. 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. - `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_NO_SNAPSHOT` will advice afl-fuzz not to use the snapshot feature if the snapshot lkm is loaded - `AFL_SHUFFLE_QUEUE` randomly reorders the input queue on startup. Requested by some users for unorthodox parallelized fuzzing setups, but not advisable otherwise. - `AFL_TMPDIR` is used to write the `.cur_input` file to if 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. - 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. - Note that `AFL_POST_LIBRARY` is deprecated, use `AFL_CUSTOM_MUTATOR_LIBRARY` instead (see below). - `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. - 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 builing 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. Please see [custom_mutators.md](custom_mutators.md). - `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_CYCLE_SCHEDULES` will switch to a different schedule everytime a cycle is finished. - 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. - 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. - 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_COLOR` or `AFL_NO_COLOUR` will omit control sequences for coloring console output when configured with USE_COLOR and not ALWAYS_COLORED. - Setting `AFL_FORCE_UI` will force painting the UI on the screen even if no valid terminal was detected (for virtual consoles) - 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. - 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_NO_CPU_RED` will not display very high cpu usages in red color. - 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. - Setting `AFL_MAX_DET_EXRAS` 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 entires will be used all of the times 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_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. - 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`, `librato`, `signalfx` or `influxdb` 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. - 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 an `-1` return code (i.e. `exit(-1)` got called), will be treated as if a crash had ocurred. This may be beneficial if you look for higher-level faulty conditions in which your target still exits gracefully. - Outdated environment variables that are not supported anymore: `AFL_DEFER_FORKSRV` `AFL_PERSISTENT` ## 5) 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 binaries. - Setting `AFL_INST_LIBS` causes the translator to also instrument the code inside any dynamically linked libraries (notably including glibc). - Setting `AFL_COMPCOV_LEVEL` enables the CompareCoverage tracing of all cmp and sub in x86 and x86_64 and memory comparions functions (e.g. strcmp, memcmp, ...) when libcompcov is preloaded using `AFL_PRELOAD`. More info at 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 comparions. Level 2 is more accurate but may need a larger shared memory. - 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. - 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. - `AFL_DEBUG` will print the found entrypoint for the binary to stderr. Use this if you are unsure if the entrypoint might be wrong - but use it directly, e.g. `afl-qemu-trace ./program` - `AFL_ENTRYPOINT` allows you to specify a specific entrypoint into the binary (this can be very good for the performance!). The entrypoint is specified as hex address, e.g. `0x4004110` Note that the address must be the address of a basic block. - 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`. - Another modality to execute the persistent loop is to specify also the `AFL_QEMU_PERSISTENT_RET=end addr` env variable. With this variable assigned, instead of patching the return address, the specified instruction is transformed to a jump towards `start addr`. - `AFL_QEMU_PERSISTENT_GPR=1` QEMU will save the original value of general purpose registers and restore them in each persistent cycle. - 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. ## 6) 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 `/.traces/` directory. - `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. ## 7) 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. ## 8) Settings for afl-analyze You can set `AFL_ANALYZE_HEX` to get file offsets printed as hexadecimal instead of decimal. ## 9) Settings for libdislocator The library honors these 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. - `AFL_ALIGNED_ALLOC=1` will force the alignment of the allocation size to `max_align_t` to be compliant with the C standard. ## 10) Settings for libtokencap This library accepts `AFL_TOKEN_FILE` to indicate the location to which the discovered tokens should be written. ## 11) 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: ``` 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. - 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 ``` 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 faults.