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diff --git a/instrumentation/README.lto.md b/instrumentation/README.lto.md index 626bc9cb..38252308 100644 --- a/instrumentation/README.lto.md +++ b/instrumentation/README.lto.md @@ -138,6 +138,34 @@ make NOTE: some targets also need to set the linker, try both `afl-clang-lto` and `afl-ld-lto` for `LD=` before `configure`. +## Instrumenting shared libraries + +Note: this is highly discouraged! Try to compile to static libraries with +afl-clang-lto instead of shared libraries! + +To make instrumented shared libraries work with afl-clang-lto you have to do +quite some extra steps. + +Every shared library you want to instrument has to be individually compiled- +The environment variable `AFL_LLVM_LTO_DONTWRITEID=1` has to be set during +compilation. +Additionally the environment variable `AFL_LLVM_LTO_STARTID` has to be set to +the combined edge values of all previous compiled instrumented shared +libraries for that target. +E.g. for the first shared library this would be `AFL_LLVM_LTO_STARTID=0` and +afl-clang-lto will then report how many edges have been instrumented (let's say +it reported 1000 instrumented edges). +The second shared library then has to be set to that value +(`AFL_LLVM_LTO_STARTID=1000` in our example), the third to all previous +combined, etc. + +The final program compilation step then may *not* have `AFL_LLVM_LTO_DONTWRITEID` +set, and `AFL_LLVM_LTO_STARTID` must be set to all combined edges of all shared +libaries it will be linked to. + +This is quite some hands-on work, so better stay away from instrumenting +shared libraries :-) + ## AUTODICTIONARY feature While compiling, a dictionary based on string comparisons is automatically |