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edge id documentation example for sancov

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This commit is contained in:
van Hauser 2020-08-03 10:03:45 +02:00
parent 1cddd51662
commit e0d1529061
2 changed files with 26 additions and 4 deletions
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9
docs/FAQ.md
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@ -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.....`).

21
llvm_mode/afl-llvm-rt.o.c
View File

@ -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]++;
}