[libFuzzer] Don't replace custom signal handlers.

[vitalybuka]

Summary

This allows to keep handlers installed by sanitizers.
In other cases third-party code can replace handlers after libFuzzer
initialization anyway.

[kcc]

LGTM with a nit.

lib/Fuzzer/FuzzerUtilPosix.cpp

60 ↗

(On Diff #100167)

no {}

[vitalybuka]

{}

llvm/trunk/lib/Fuzzer/FuzzerUtilPosix.cpp

 }
 
 static void FileSizeExceedHandler(int, siginfo_t *, void *) {
   Fuzzer::StaticFileSizeExceedCallback();
 }
 
 static void SetSigaction(int signum,
                          void (*callback)(int, siginfo_t *, void *)) {
-  struct sigaction sigact;
-  memset(&sigact, 0, sizeof(sigact));
+  struct sigaction sigact = {};
+  if (sigaction(signum, nullptr, &sigact)) {
+    Printf("libFuzzer: sigaction failed with %d\n", errno);
+    exit(1);
+  }
+  if (sigact.sa_flags & SA_SIGINFO) {
+    if (sigact.sa_sigaction)
+      return;
+  } else {
+    if (sigact.sa_handler != SIG_DFL && sigact.sa_handler != SIG_IGN &&
+        sigact.sa_handler != SIG_ERR)
+      return;
+  }
+
+  sigact = {};
   sigact.sa_sigaction = callback;
   if (sigaction(signum, &sigact, 0)) {
     Printf("libFuzzer: sigaction failed with %d\n", errno);
     exit(1);
   }
 }
 
 void SetTimer(int Seconds) {

llvm/trunk/lib/Fuzzer/test/fuzzer-segv.test

 RUN: env ASAN_OPTIONS=handle_segv=0 not LLVMFuzzer-NullDerefTest 2>&1 | FileCheck %s --check-prefix=LIBFUZZER_OWN_SEGV_HANDLER
 LIBFUZZER_OWN_SEGV_HANDLER: == ERROR: libFuzzer: deadly signal
 LIBFUZZER_OWN_SEGV_HANDLER: SUMMARY: libFuzzer: deadly signal
 LIBFUZZER_OWN_SEGV_HANDLER: Test unit written to ./crash-
 
+RUN: env ASAN_OPTIONS=handle_segv=1 not LLVMFuzzer-NullDerefTest 2>&1 | FileCheck %s --check-prefix=LIBFUZZER_ASAN_SEGV_HANDLER
+LIBFUZZER_ASAN_SEGV_HANDLER: ERROR: AddressSanitizer: {{SEGV|access-violation}} on unknown address