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features.md

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Important features of AFL++

AFL++ supports llvm from 3.8 up to version 12, very fast binary fuzzing with QEMU 5.1 with laf-intel and Redqueen, FRIDA mode, unicorn mode, gcc plugin, full *BSD, Mac OS, Solaris and Android support and much, much, much more.

Features and instrumentation

Feature/Instrumentation afl-gcc llvm gcc_plugin FRIDA mode(9) QEMU mode(10) unicorn_mode(10) nyx_mode(12) coresight_mode(11)
Threadsafe counters [A] x(3) x
NeverZero [B] x86[_64] x(1) x x x x
Persistent Mode [C] x x x86[_64]/arm64 x86[_64]/arm[64] x
LAF-Intel / CompCov [D] x x86[_64]/arm[64] x86[_64]/arm[64] x86[_64]
CmpLog [E] x x x86[_64]/arm64 x86[_64]/arm[64]
Selective Instrumentation [F] x x x x
Non-Colliding Coverage [G] x(4) (x)(5)
Ngram prev_loc Coverage [H] x(6)
Context Coverage [I] x(6)
Auto Dictionary [J] x(7)
Snapshot Support [K] (x)(8) (x)(8) (x)(5) x
Shared Memory Test cases [L] x x x86[_64]/arm64 x x x

More information about features

A. Default is not thread-safe coverage counter updates for better performance, see instrumentation/README.llvm.md

B. On wrapping coverage counters (255 + 1), skip the 0 value and jump to 1 instead. This has shown to give better coverage data and is the default; see instrumentation/README.llvm.md.

C. Instead of forking, reiterate the fuzz target function in a loop (like LLVMFuzzerTestOneInput. Great speed increase but only works with target functions that do not keep state, leak memory, or exit; see instrumentation/README.persistent_mode.md

D. Split any non-8-bit comparison to 8-bit comparison; see instrumentation/README.laf-intel.md

E. CmpLog is our enhanced Redqueen implementation, see instrumentation/README.cmplog.md

F. Similar and compatible to clang 13+ sancov sanitize-coverage-allow/deny but for all llvm versions and all our compile modes, only instrument what should be instrumented, for more speed, directed fuzzing and less instability; see instrumentation/README.instrument_list.md

G. Vanilla AFL uses coverage where edges could collide to the same coverage bytes the larger the target is. Our default instrumentation in LTO and afl-clang-fast (PCGUARD) uses non-colliding coverage that also makes it faster. Vanilla AFL style is available with AFL_LLVM_INSTRUMENT=AFL; see instrumentation/README.llvm.md.

H.+I. Alternative coverage based on previous edges (NGRAM) or depending on the caller (CTX), based on https://www.usenix.org/system/files/raid2019-wang-jinghan.pdf; see instrumentation/README.llvm.md.

J. An LTO feature that creates a fuzzing dictionary based on comparisons found during compilation/instrumentation. Automatic feature :) See instrumentation/README.lto.md

K. The snapshot feature requires a kernel module that was a lot of work to get right and maintained so it is no longer supported. We have nyx_mode instead.

L. Faster fuzzing and less kernel syscall overhead by in-memory fuzz testcase delivery, see instrumentation/README.persistent_mode.md

More information about instrumentation

  1. Default for LLVM >= 9.0, environment variable for older version due an efficiency bug in previous llvm versions
  2. GCC creates non-performant code, hence it is disabled in gcc_plugin
  3. With AFL_LLVM_THREADSAFE_INST, disables NeverZero
  4. With pcguard mode and LTO mode for LLVM 11 and newer
  5. Upcoming, development in the branch
  6. Not compatible with LTO instrumentation and needs at least LLVM v4.1
  7. Automatic in LTO mode with LLVM 11 and newer, an extra pass for all LLVM versions that write to a file to use with afl-fuzz' -x
  8. The snapshot LKM is currently unmaintained due to too many kernel changes coming too fast :-(
  9. FRIDA mode is supported on Linux and MacOS for Intel and ARM
  10. QEMU/Unicorn is only supported on Linux
  11. Coresight mode is only available on AARCH64 Linux with a CPU with Coresight extension
  12. Nyx mode is only supported on Linux and currently restricted to x86_x64

Integrated features and patches

Among others, the following features and patches have been integrated:

  • NeverZero patch for afl-gcc, instrumentation, QEMU mode and unicorn_mode which prevents a wrapping map value to zero, increases coverage
  • Persistent mode, deferred forkserver and in-memory fuzzing for QEMU mode
  • Unicorn mode which allows fuzzing of binaries from completely different platforms (integration provided by domenukk)
  • The new CmpLog instrumentation for LLVM and QEMU inspired by Redqueen
  • Win32 PE binary-only fuzzing with QEMU and Wine
  • AFLfast's power schedules by Marcel Böhme: https://github.com/mboehme/aflfast
  • The MOpt mutator: https://github.com/puppet-meteor/MOpt-AFL
  • LLVM mode Ngram coverage by Adrian Herrera https://github.com/adrianherrera/afl-ngram-pass
  • LAF-Intel/CompCov support for instrumentation, QEMU mode and unicorn_mode (with enhanced capabilities)
  • Radamsa and honggfuzz mutators (as custom mutators).
  • QBDI mode to fuzz android native libraries via Quarkslab's QBDI framework
  • Frida and ptrace mode to fuzz binary-only libraries, etc.

So all in all this is the best-of AFL that is out there :-)