diff options
| -rw-r--r-- | docs/FAQ.md | 9 | ||||
| -rw-r--r-- | llvm_mode/afl-llvm-rt.o.c | 21 |
2 files changed, 26 insertions, 4 deletions
diff --git a/docs/FAQ.md b/docs/FAQ.md index ee221d02..c15cd484 100644 --- a/docs/FAQ.md +++ b/docs/FAQ.md @@ -103,10 +103,11 @@ afl-clang-fast PCGUARD and afl-clang-lto LTO instrumentation! b) For PCGUARD instrumented binaries it is much more difficult. Here you can either modify the __sanitizer_cov_trace_pc_guard function in llvm_mode/afl-llvm-rt.o.c to write a backtrace to a file if the ID in - __afl_area_ptr[*guard] is one of the unstable edge IDs. Then recompile - and reinstall llvm_mode and rebuild your target. Run the recompiled - target with afl-fuzz for a while and then check the file that you - wrote with the backtrace information. + __afl_area_ptr[*guard] is one of the unstable edge IDs. + (Example code is already there). + Then recompile and reinstall llvm_mode and rebuild your target. + Run the recompiled target with afl-fuzz for a while and then check the + file that you wrote with the backtrace information. Alternatively you can use `gdb` to hook __sanitizer_cov_trace_pc_guard_init on start, check to which memory address the edge ID value is written and set a write breakpoint to that address (`watch 0x.....`). diff --git a/llvm_mode/afl-llvm-rt.o.c b/llvm_mode/afl-llvm-rt.o.c index c0ed1bcf..c2859d9c 100644 --- a/llvm_mode/afl-llvm-rt.o.c +++ b/llvm_mode/afl-llvm-rt.o.c @@ -859,6 +859,27 @@ __attribute__((constructor(CONST_PRIO))) void __afl_auto_init(void) { void __sanitizer_cov_trace_pc_guard(uint32_t *guard) { + // For stability analysis, if you want to know to which function unstable + // edge IDs belong to - uncomment, recompile+install llvm_mode, recompile + // the target. libunwind and libbacktrace are better solutions. + // Set AFL_DEBUG_CHILD_OUTPUT=1 and run afl-fuzz with 2>file to capture + // the backtrace output + /* + uint32_t unstable[] = { ... unstable edge IDs }; + uint32_t idx; + char bt[1024]; + for (idx = 0; i < sizeof(unstable)/sizeof(uint32_t); i++) { + if (unstable[idx] == __afl_area_ptr[*guard]) { + int bt_size = backtrace(bt, 256); + if (bt_size > 0) { + char **bt_syms = backtrace_symbols(bt, bt_size); + if (bt_syms) + fprintf(stderr, "DEBUG: edge=%u caller=%s\n", unstable[idx], bt_syms[0]); + } + } + } + */ + __afl_area_ptr[*guard]++; } |