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Diffstat (limited to 'llvm_mode/README.llvm')
| -rw-r--r-- | llvm_mode/README.llvm | 53 |
1 files changed, 39 insertions, 14 deletions
diff --git a/llvm_mode/README.llvm b/llvm_mode/README.llvm index dc860e97..a0c40211 100644 --- a/llvm_mode/README.llvm +++ b/llvm_mode/README.llvm @@ -3,6 +3,7 @@ Fast LLVM-based instrumentation for afl-fuzz ============================================ (See ../docs/README for the general instruction manual.) + (See ../gcc_plugin/README.gcc for the GCC-based instrumentation.) 1) Introduction --------------- @@ -30,7 +31,7 @@ several interesting properties: - The instrumentation can cope a bit better with multi-threaded targets. - Because the feature relies on the internals of LLVM, it is clang-specific - and will *not* work with GCC. + and will *not* work with GCC (see ../gcc_plugin/ for an alternative). Once this implementation is shown to be sufficiently robust and portable, it will probably replace afl-clang. For now, it can be built separately and @@ -38,8 +39,8 @@ co-exists with the original code. The idea and much of the implementation comes from Laszlo Szekeres. -2) How to use -------------- +2) How to use this +------------------ In order to leverage this mechanism, you need to have clang installed on your system. You should also make sure that the llvm-config tool is in your path @@ -69,21 +70,47 @@ operating mode of AFL, e.g.: Be sure to also include CXX set to afl-clang-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. +../docs/env_variables.txt). This includes AFL_USE_ASAN, +AFL_HARDEN, and AFL_DONT_OPTIMIZE. However AFL_INST_RATIO is not honored +as it does not serve a good purpose with the more effective instrim CFG +analysis. Note: if you want the LLVM helper 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 +3) Options + +Several options are present to make llvm_mode faster or help it rearrange +the code to make afl-fuzz path discovery easier. + +If you need just to instrument specific parts of the code, you can whitelist +which C/C++ files to actually intrument. See README.whitelist + +For splitting memcmp, strncmp, etc. please see README.laf-intel + +Then there is an optimized instrumentation strategy that uses CFGs and +markers to just instrument what is needed. This increases speed by 20-25% +however has a lower path discovery. +If you want to use this, set AFL_LLVM_INSTRIM=1 +See README.instrim + +Finally if your llvm version is 8 or lower, you can activate a mode that +prevents that a counter overflow result in a 0 value. This is good for +path discovery, but the llvm implementation for intel for this functionality +is not optimal and was only fixed in llvm 9. +You can set this with AFL_LLVM_NOT_ZERO=1 +See README.neverzero + + +4) Gotchas, feedback, bugs -------------------------- This is an early-stage mechanism, so field reports are welcome. You can send bug reports to <afl-users@googlegroups.com>. -4) Bonus feature #1: deferred instrumentation ---------------------------------------------- +5) 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 @@ -129,7 +156,7 @@ will keep working normally when compiled with a tool other than afl-clang-fast. Finally, recompile the program with afl-clang-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 +6) Bonus feature #2: persistent mode ------------------------------------ Some libraries provide APIs that are stateless, or whose state can be reset in @@ -169,7 +196,7 @@ PS. Because there are task switches still involved, the mode isn't as fast as faster than the normal fork() model, and compared to in-process fuzzing, should be a lot more robust. -6) Bonus feature #3: new 'trace-pc-guard' mode +8) Bonus feature #3: new 'trace-pc-guard' mode ---------------------------------------------- Recent versions of LLVM are shipping with a built-in execution tracing feature @@ -178,10 +205,8 @@ post-process the assembly or install any compiler plugins. See: http://clang.llvm.org/docs/SanitizerCoverage.html#tracing-pcs-with-guards -As of this writing, the feature is only available on SVN trunk, and is yet to -make it to an official release of LLVM. Nevertheless, if you have a -sufficiently recent compiler and want to give it a try, build afl-clang-fast -this way: +If you have a sufficiently recent compiler and want to give it a try, build +afl-clang-fast this way: AFL_TRACE_PC=1 make clean all |