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[libfuzzer] Fix UB when calculating Log(0) in StackDepthStepFunction(). 

Summary:
__builtin_clz used for Log calculation returns an undefined result
when argument is 0. I noticed that issue when was testing some fuzzers:

```
/src/libfuzzer/FuzzerTracePC.h:282:33: runtime error: shift exponent 450349 is too large for 32-bit type 'uint32_t' (aka 'unsigned int')
  #0 0x43d83f in operator() /src/libfuzzer/FuzzerTracePC.h:283:33
  #1 0x43d83f in void fuzzer::TracePC::CollectFeatures<fuzzer::Fuzzer::RunOne(unsigned char const*, unsigned long, bool, fuzzer::InputInfo*, bool*)::$_1>(fuzzer::Fuzzer::RunOne(unsigned char const*, unsigned long, bool, fuzzer::InputInfo*, bool*)::$_1) const /src/libfuzzer/FuzzerTracePC.h:290
  #2 0x43cbd4 in fuzzer::Fuzzer::RunOne(unsigned char const*, unsigned long, bool, fuzzer::InputInfo*, bool*) /src/libfuzzer/FuzzerLoop.cpp:445:7
  #3 0x43e5f1 in fuzzer::Fuzzer::ReadAndExecuteSeedCorpora(std::__1::vector<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >, fuzzer::fuzzer_allocator<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > > > const&) /src/libfuzzer/FuzzerLoop.cpp:706:5
  #4 0x43e9e1 in fuzzer::Fuzzer::Loop(std::__1::vector<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >, fuzzer::fuzzer_allocator<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > > > const&) /src/libfuzzer/FuzzerLoop.cpp:739:3
  #5 0x432f8c in fuzzer::FuzzerDriver(int*, char***, int (*)(unsigned char const*, unsigned long)) /src/libfuzzer/FuzzerDriver.cpp:754:6
  #6 0x42ee18 in main /src/libfuzzer/FuzzerMain.cpp:20:10
  #7 0x7f17ffeb182f in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x2082f)
  #8 0x407838 in _start (/out/rotate_fuzzer+0x407838)

Reviewers: kcc

Reviewed By: kcc

Subscribers: llvm-commits, #sanitizers

Differential Revision: https://reviews.llvm.org/D41457

llvm-svn: 321211
This commit is contained in:
Max Moroz 2017-12-20 19:31:51 +00:00
parent cb35c5d5c2
commit a40ce7bc36
1 changed files with 1 additions and 0 deletions
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1
compiler-rt/lib/fuzzer/FuzzerTracePC.h
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@ -276,6 +276,7 @@ void TracePC::CollectFeatures(Callback HandleFeature) const {
// Step function, grows similar to 8 * Log_2(A). // Step function, grows similar to 8 * Log_2(A).
auto StackDepthStepFunction = [](uint32_t A) -> uint32_t { auto StackDepthStepFunction = [](uint32_t A) -> uint32_t {
if (!A) return A;
uint32_t Log2 = Log(A); uint32_t Log2 = Log(A);
if (Log2 < 3) return A; if (Log2 < 3) return A;
Log2 -= 3; Log2 -= 3;