[libFuzzer] Disable MSan interceptors in SIGINT handler.

[morehouse]

Summary

Avoids an MSan false positive if the SIGINT comes while the user
callback is running. The false positive happens when the interrupt
handler calls opendir() to remove some temporary files, which is
intercepted by MSan.

Fixes https://github.com/google/oss-fuzz/issues/2332.

compiler-rt/lib/fuzzer/FuzzerLoop.cpp

void Fuzzer::MaybeExitGracefully() {
   RmDirRecursive(TempPath(".dir"));
   F->PrintFinalStats();
   _Exit(0);
 }
 
 void Fuzzer::InterruptCallback() {
   Printf("==%lu== libFuzzer: run interrupted; exiting\n", GetPid());
   PrintFinalStats();
+  ScopedDisableMsanInterceptorChecks S; // RmDirRecursive may call opendir().
   RmDirRecursive(TempPath(".dir"));
   // Stop right now, don't perform any at-exit actions.
   _Exit(Options.InterruptExitCode);
 }
 
 NO_SANITIZE_MEMORY
 void Fuzzer::AlarmCallback() {
   assert(Options.UnitTimeoutSec > 0);
for (int i = 0; i < Options.MutateDepth; i++) {
     if (TotalNumberOfRuns >= Options.MaxNumberOfRuns)
       break;
     MaybeExitGracefully();
     size_t NewSize = 0;
     if (II.HasFocusFunction && !II.DataFlowTraceForFocusFunction.empty() &&
         Size <= CurrentMaxMutationLen)
       NewSize = MD.MutateWithMask(CurrentUnitData, Size, Size,
                                   II.DataFlowTraceForFocusFunction);
-    
     // If MutateWithMask either failed or wasn't called, call default Mutate.
     if (!NewSize)
       NewSize = MD.Mutate(CurrentUnitData, Size, CurrentMaxMutationLen);
     assert(NewSize > 0 && "Mutator returned empty unit");
     assert(NewSize <= CurrentMaxMutationLen && "Mutator return oversized unit");
     Size = NewSize;
     II.NumExecutedMutations++;
 

compiler-rt/test/fuzzer/sigint.test

+# FIXME: Disabled on Windows for now because of reliance on posix only features
+# (eg: export, "&", pkill).
+UNSUPPORTED: darwin, windows
+
+# Check that libFuzzer exits gracefully under SIGINT with MSan.
+RUN: rm -rf %t
+RUN: mkdir -p %t
+RUN: %msan_compiler %S/SleepOneSecondTest.cpp -o %t/LFSIGINT
+
+RUN: %run %t/LFSIGINT 2> %t/log & export PID=$!
+RUN: sleep 2
+RUN: kill -SIGINT $PID
+RUN: sleep 3
+RUN: cat %t/log | FileCheck %s
+
+CHECK: libFuzzer: run interrupted; exiting
+CHECK-NOT: WARNING: MemorySanitizer