[sanitizer_common] Block asynchronous signals only (llvm#98200)

This changes the behavior of `BlockSignals` and `ScopedBlockSignals` to
block only asynchronous signals.

This extension is intended to be used in a future fix for MSan (block
async signals during `MsanThread::Destroy`).

Author: thurstond

compiler-rt/lib/sanitizer_common/sanitizer_linux.cpp

@@ -155,6 +155,7 @@ void SetSigProcMask(__sanitizer_sigset_t *set, __sanitizer_sigset_t *oldset) {
   CHECK_EQ(0, internal_sigprocmask(SIG_SETMASK, set, oldset));
 }
 
+// Block asynchronous signals
 void BlockSignals(__sanitizer_sigset_t *oldset) {
   __sanitizer_sigset_t set;
   internal_sigfillset(&set);
@@ -170,6 +171,16 @@ void BlockSignals(__sanitizer_sigset_t *oldset) {
   // hang.
   internal_sigdelset(&set, 31);
 #  endif
+
+  // Don't block synchronous signals
+  internal_sigdelset(&set, SIGSEGV);
+  internal_sigdelset(&set, SIGBUS);
+  internal_sigdelset(&set, SIGILL);
+  internal_sigdelset(&set, SIGTRAP);
+  internal_sigdelset(&set, SIGABRT);
+  internal_sigdelset(&set, SIGFPE);
+  internal_sigdelset(&set, SIGPIPE);
+
   SetSigProcMask(&set, oldset);
 }