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diff --git a/gcc_plugin/README.gcc b/gcc_plugin/README.gcc deleted file mode 100644 index a002c741..00000000 --- a/gcc_plugin/README.gcc +++ /dev/null @@ -1,163 +0,0 @@ -=========================================== -Fast GCC-based instrumentation for afl-fuzz -=========================================== - - (See ../docs/README for the general instruction manual.) - (See ../llvm_mode/README.llvm for the LLVM-based instrumentation.) - - -!!! -!!! gcc_plugin is not stable yet and can crash when compiling -!!! - - -1) Introduction ---------------- - -The code in this directory allows you to instrument programs for AFL using -true compiler-level instrumentation, instead of the more crude -assembly-level rewriting approach taken by afl-gcc and afl-clang. This has -several interesting properties: - - - The compiler can make many optimizations that are hard to pull off when - manually inserting assembly. As a result, some slow, CPU-bound programs will - run up to around 2x faster. - - The gains are less pronounced for fast binaries, where the speed is limited - chiefly by the cost of creating new processes. In such cases, the gain will - probably stay within 10%. - - - The instrumentation is CPU-independent. At least in principle, you should - be able to rely on it to fuzz programs on non-x86 architectures (after - building afl-fuzz with AFL_NOX86=1). - - - Because the feature relies on the internals of GCC, it is gcc-specific - and will *not* work with LLVM (see ../llvm_plugin for an alternative). - -Once this implementation is shown to be sufficiently robust and portable, it -will probably replace afl-gcc. For now, it can be built separately and -co-exists with the original code. - -The idea and much of the implementation comes from Laszlo Szekeres. - -2) How to use -------------- - -In order to leverage this mechanism, you need to have GCC and the plugin headers -installed on your system. That should be all you need. On Debian machines, these -headers can be acquired by installing the `gcc-<VERSION>-plugin-dev` packages. - -To build the instrumentation itself, type 'make'. This will generate binaries -called afl-gcc-fast and afl-g++-fast in the parent directory. Once this -is done, you can instrument third-party code in a way similar to the standard -operating mode of AFL, e.g.: - - CC=/path/to/afl/afl-gcc-fast ./configure [...options...] - make - -Be sure to also include CXX set to afl-g++-fast for C++ code. - -The tool honors roughly the same environmental variables as afl-gcc (see -../docs/env_variables.txt). This includes AFL_INST_RATIO, AFL_USE_ASAN, -AFL_HARDEN, and AFL_DONT_OPTIMIZE. - -Note: if you want the GCC plugin to be installed on your system for all -users, you need to build it before issuing 'make install' in the parent -directory. - -3) Gotchas, feedback, bugs --------------------------- - -This is an early-stage mechanism, so field reports are welcome. You can send bug -reports to <aseipp@pobox.com>. - -4) Bonus feature #1: deferred initialization --------------------------------------------- - -AFL tries to optimize performance by executing the targeted binary just once, -stopping it just before main(), and then cloning this "master" process to get -a steady supply of targets to fuzz. - -Although this approach eliminates much of the OS-, linker- and libc-level -costs of executing the program, it does not always help with binaries that -perform other time-consuming initialization steps - say, parsing a large config -file before getting to the fuzzed data. - -In such cases, it's beneficial to initialize the forkserver a bit later, once -most of the initialization work is already done, but before the binary attempts -to read the fuzzed input and parse it; in some cases, this can offer a 10x+ -performance gain. You can implement delayed initialization in LLVM mode in a -fairly simple way. - -First, locate a suitable location in the code where the delayed cloning can -take place. This needs to be done with *extreme* care to avoid breaking the -binary. In particular, the program will probably malfunction if you select -a location after: - - - The creation of any vital threads or child processes - since the forkserver - can't clone them easily. - - - The initialization of timers via setitimer() or equivalent calls. - - - The creation of temporary files, network sockets, offset-sensitive file - descriptors, and similar shared-state resources - but only provided that - their state meaningfully influences the behavior of the program later on. - - - Any access to the fuzzed input, including reading the metadata about its - size. - -With the location selected, add this code in the appropriate spot: - -#ifdef __AFL_HAVE_MANUAL_CONTROL - __AFL_INIT(); -#endif - -You don't need the #ifdef guards, but they will make the program still work as -usual when compiled with a tool other than afl-gcc-fast/afl-clang-fast. - -Finally, recompile the program with afl-gcc-fast (afl-gcc or afl-clang will -*not* generate a deferred-initialization binary) - and you should be all set! - -5) Bonus feature #2: persistent mode ------------------------------------- - -Some libraries provide APIs that are stateless, or whose state can be reset in -between processing different input files. When such a reset is performed, a -single long-lived process can be reused to try out multiple test cases, -eliminating the need for repeated fork() calls and the associated OS overhead. - -The basic structure of the program that does this would be: - - while (__AFL_LOOP(1000)) { - - /* Read input data. */ - /* Call library code to be fuzzed. */ - /* Reset state. */ - - } - - /* Exit normally */ - -The numerical value specified within the loop controls the maximum number -of iterations before AFL will restart the process from scratch. This minimizes -the impact of memory leaks and similar glitches; 1000 is a good starting point. - -A more detailed template is shown in ../experimental/persistent_demo/. -Similarly to the previous mode, the feature works only with afl-gcc-fast or -afl-clang-fast; #ifdef guards can be used to suppress it when using other -compilers. - -Note that as with the previous mode, the feature is easy to misuse; if you -do not reset the critical state fully, you may end up with false positives or -waste a whole lot of CPU power doing nothing useful at all. Be particularly -wary of memory leaks and the state of file descriptors. - -When running in this mode, the execution paths will inherently vary a bit -depending on whether the input loop is being entered for the first time or -executed again. To avoid spurious warnings, the feature implies -AFL_NO_VAR_CHECK and hides the "variable path" warnings in the UI. - -PS. Because there are task switches still involved, the mode isn't as fast as -"pure" in-process fuzzing offered, say, by LLVM's LibFuzzer; but it is a lot -faster than the normal fork() model, and compared to in-process fuzzing, -should be a lot more robust. |