The purpose of FRIDA mode is to provide an alternative binary only fuzzer for AFL just like that provided by QEMU mode. The intention is to provide a very similar user experience, right down to the options provided through environment variables.
Whilst AFLplusplus already has some support for running on FRIDA here this requires the code to be fuzzed to be provided as a shared library, it cannot be used to fuzz executables. Additionally, it requires the user to write a small harness around their target code of interest, FRIDA mode instead takes a different approach to avoid these limitations.
As FRIDA mode is new, it is missing a lot of features. Most importantly, persistent mode. The design is such that it should be possible to add these features in a similar manner to QEMU mode and perhaps leverage some of its design and implementation.
| Feature/Instrumentation | frida-mode |
|---|---|
| NeverZero | |
| Persistent Mode | |
| LAF-Intel / CompCov | |
| CmpLog | |
| Selective Instrumentation | x |
| Non-Colliding Coverage | |
| Ngram prev_loc Coverage | |
| Context Coverage | |
| Auto Dictionary | |
| Snapshot LKM Support |
Currently FRIDA mode supports Linux and macOS targets on both x86/x64 architecture and aarch64. Later releases may add support for aarch32 and Windows targets as well as embedded linux environments.
FRIDA has been used on various embedded targets using both uClibc and musl C runtime libraries, so porting should be possible. However, the current build system does not support cross compilation.
To build everything run make.
To run the benchmark sample with qemu run make png_qemu. To run the benchmark sample with frida run make png_frida.
FRIDA mode requires some small modifications to afl-fuzz and similar tools in AFLplusplus. The intention is that it behaves identically to QEMU, but uses the ‘O’ switch rather than ‘Q’. Whilst the options ‘f’, ‘F’, ‘s’ or ‘S’ may have made more sense for a mode powered by FRIDA Stalker, they were all taken, so instead we use ‘O’ in hommage to the author of FRIDA.
Similarly, the intention is to mimic the use of environment variables used by QEMU where possible (although replacing s/QEMU/FRIDA/g). Accodingly, the following options are currently supported.
AFL_FRIDA_DEBUG_MAPS - See AFL_QEMU_DEBUG_MAPSAFL_FRIDA_EXCLUDE_RANGES - See AFL_QEMU_EXCLUDE_RANGESAFL_FRIDA_INST_RANGES - See AFL_QEMU_INST_RANGESAdditionally, the intention is to be able to make a direct performance comparison between the two approaches. Accordingly, FRIDA mode includes a test target based on the libpng benchmark used by fuzzbench and integrated with the StandaloneFuzzTargetMain from the llvm project. This is built and linked without any special modifications to suit FRIDA or QEMU. We use the test data provided with libpng as our corpus.
Whilst not much performance tuning has been completed to date, performance is around 30-50% of that of QEMU mode, however, this gap may reduce with the introduction of persistent mode. Performance can be tested by running make compare, albeit a longer time measurement may be required for more accurate results.
Whilst afl_frida claims a 5-10x performance increase over QEMU, it has not been possible to reproduce these claims. However, the number of executions per second can vary dramatically as a result of the randomization of the fuzzer input. Some inputs may traverse relatively few paths before being rejected as invalid whilst others may be valid inputs or be subject to much more processing before rejection. Accordingly, it is recommended that testing be carried out over prolongued periods to gather timings which are more than indicative.
FRIDA mode is supported by using LD_PRELOAD (DYLD_INSERT_LIBRARIES on macOS) to inject a shared library (afl-frida-trace.so) into the target. This shared library is built using the frida-gum devkit from the FRIDA project. One of the components of frida-gum is Stalker, this allows the dynamic instrumentation of running code for AARCH32, AARCH64, x86 and x64 architectures. Implementation details can be found here.
Dynamic instrumentation is used to augment the target application with similar coverage information to that inserted by afl-gcc or afl-clang. The shared library is also linked to the compiler-rt component of AFLplusplus to feedback this coverage information to AFL++ and also provide a fork server. It also makes use of the FRIDA prefetch support to feedback instrumented blocks from the child to the parent using a shared memory region to avoid the need to regenerate instrumented blocks on each fork.
Whilst FRIDA allows for a normal C function to be used to augment instrumented code, to minimize the costs of storing and restoring all of the registers, FRIDA mode instead makes use of optimized assembly instead on AARCH64 and x86/64 targets.
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_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.AFL_FRIDA_INST_STRICT - Under certain conditions, Stalker may encroach into excluded regions and generate both instrumented blocks and coverage data (e.g. indirect calls on x86). The excluded block is generally honoured as soon as another function is called within the excluded region and so such encroachment is usually of little consequence. This detail may however, hinder you when checking that the correct number of paths are found for testing purposes or similar. There is a performance penatly for this option during block compilation where we check the block isn't in a list of excluded ranges.AFL_FRIDA_INST_TRACE - Generate some logging when running instrumented code. Requires AFL_FRIDA_INST_NO_OPTIMIZE.As can be seen from the progress section above, there are a number of features which are missing in its currently form. Chief amongst which is persistent mode. The intention is to achieve feature parity with QEMU mode in due course. Contributions are welcome, but please get in touch to ensure that efforts are deconflicted.