[fuzzer] Avoid buffer overflow in CrossOverTest.cpp (and fix-forward build breakage from llvm#155943) (llvm#156103)

thurstond · mdenson · commit 403c6ac8e836 · 2025-08-29T16:46:46.000-05:00
ASan now detects dereferences of zero-sized allocations (llvm#155943; the corresponding MSan change is llvm#155944). This appears to have detected a bug in CrossOverTest.cpp, causing a buildbot breakage. This patch fixes the test. Buildbot report: https://lab.llvm.org/buildbot/#/builders/4/builds/8732 ``` 7: ==949882==ERROR: AddressSanitizer: heap-buffer-overflow on address 0xf169cfbe0010 at pc 0xb5f45efc6d1c bp 0xffffd933e460 sp 0xffffd933e458 check:20'0 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 8: READ of size 1 at 0xf169cfbe0010 thread T0 check:20'0 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 9: #0 0xb5f45efc6d18 in LLVMFuzzerTestOneInput /home/tcwg-buildbot/worker/clang-aarch64-sve-vls-2stage/llvm/compiler-rt/test/fuzzer/CrossOverTest.cpp:48:7 check:20'0 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ check:20'1 ? possible intended match 10: llvm#1 0xb5f45eec7288 in fuzzer::Fuzzer::ExecuteCallback(unsigned char const*, unsigned long) /home/tcwg-buildbot/worker/clang-aarch64-sve-vls-2stage/llvm/compiler-rt/lib/fuzzer/FuzzerLoop.cpp:619:13 check:20'0 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 11: llvm#2 0xb5f45eec85d4 in fuzzer::Fuzzer::ReadAndExecuteSeedCorpora(std::vector<fuzzer::SizedFile, std::allocator<fuzzer::SizedFile>>&) /home/tcwg-buildbot/worker/clang-aarch64-sve-vls-2stage/llvm/compiler-rt/lib/fuzzer/FuzzerLoop.cpp:812:3 check:20'0 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 12: llvm#3 0xb5f45eec8c60 in fuzzer::Fuzzer::Loop(std::vector<fuzzer::SizedFile, std::allocator<fuzzer::SizedFile>>&) /home/tcwg-buildbot/worker/clang-aarch64-sve-vls-2stage/llvm/compiler-rt/lib/fuzzer/FuzzerLoop.cpp:872:3 check:20'0 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 13: llvm#4 0xb5f45eeb5c64 in fuzzer::FuzzerDriver(int*, char***, int (*)(unsigned char const*, unsigned long)) /home/tcwg-buildbot/worker/clang-aarch64-sve-vls-2stage/llvm/compiler-rt/lib/fuzzer/FuzzerDriver.cpp:923:6 check:20'0 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 14: llvm#5 0xb5f45eee09d0 in main /home/tcwg-buildbot/worker/clang-aarch64-sve-vls-2stage/llvm/compiler-rt/lib/fuzzer/FuzzerMain.cpp:20:10 check:20'0 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ``` For context, FuzzerLoop.cpp:812 tries empty input: ``` 810 // Test the callback with empty input and never try it again. 811 uint8_t dummy = 0; 812 ExecuteCallback(&dummy, 0); ```
diff --git a/compiler-rt/test/fuzzer/CrossOverTest.cpp b/compiler-rt/test/fuzzer/CrossOverTest.cpp
@@ -45,6 +45,9 @@ extern "C" int LLVMFuzzerTestOneInput(const uint8_t *Data, size_t Size) {
   // fprintf(stderr, "ExpectedHash: %x\n", ExpectedHash);
   if (Size == 10 && ExpectedHash == simple_hash(Data, Size))
     *NullPtr = 0;
+  // It's UB to read *Data when Size == 0
+  if (Size == 0)
+    return;
   if (*Data == 'A')
     Sink++;
   if (*Data == 'Z')
