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Diffstat (limited to 'instrumentation/README.llvm.md')
| -rw-r--r-- | instrumentation/README.llvm.md | 24 |
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diff --git a/instrumentation/README.llvm.md b/instrumentation/README.llvm.md index c0677474..126cf1a2 100644 --- a/instrumentation/README.llvm.md +++ b/instrumentation/README.llvm.md @@ -280,3 +280,27 @@ Please note that the default counter implementations are not thread safe! Support for thread safe counters in mode LLVM CLASSIC can be activated with setting `AFL_LLVM_THREADSAFE_INST=1`. + +## 8) Source code coverage through instrumentation + +Measuring source code coverage is a common task in fuzzing, but it is very +difficut to do in some situations (e.g. when using snapshot fuzzing). + +When using the `AFL_LLVM_INSTRUMENT=llvm-codecov` option, afl-cc will use +native trace-pc-guard instrumentation but additionally select options that +are required to utilize the instrumentation for source code coverage. + +In particular, it will switch the instrumentation to be per basic block +instead of instrumenting edges, disable all guard pruning and enable the +experimental pc-table support that allows the runtime to gather 100% of +instrumented basic blocks at start, including their locations. + +Note: You must compile AFL with the `CODE_COVERAGE=1` option to enable the +respective parts in the AFL compiler runtime. Support is currently only +implemented for Nyx, but can in theory also work without Nyx. + +Note: You might have to adjust `MAP_SIZE_POW2` in include/config.h to ensure +that your coverage map is large enough to hold all basic blocks of your +target program without any collisions. + +More documentation on how to utilize this with Nyx will follow. |