[libFuzzer] always install signal handler with SA_ONSTACK

[randall77]

Summary

SA_ONSTACK is required for certain runtimes that use small stacks, for instance the Go runtime.
See https://github.com/golang/go/issues/49075
SA_ONSTACK is a no-op unless someone also calls sigaltstack.

[sebastianpoeplau]

Looks good to me in principle. I'm wondering whether there could be situations where a signal stack has been established with sigaltstack but it's intended for some specific signal(s) only - if we were to set SA_ONSTACK uncontitionally, we would run all of the fuzzer's signal handlers on the signal stack. That said, none of the handlers seem to require a lot of stack space, so it's probably fine.

[randall77]

I agree there is a possibility of a problem if signal handlers need more stack than provided by sigaltstack. That seems like a pretty hypothetical problem, however. In contrast, there is a current real problem with executing signals on user's stacks.

[randall77]

Ping. Can we get this in?

[sebastianpoeplau]

Since nobody has objected thus far let's get it in. I don't know if approval from me is enough to be able to merge, but let's try...

[randall77]

Is there something I need to do to actually get this landed?

compiler-rt/lib/fuzzer/FuzzerUtilPosix.cpp

   }
 
   struct sigaction new_sigact = {};
-  // Address sanitizer needs SA_ONSTACK (causing the signal handler to run on a
-  // dedicated stack) in order to be able to detect stack overflows; keep the
-  // flag if it's set.
-  new_sigact.sa_flags = SA_SIGINFO | (sigact.sa_flags & SA_ONSTACK);
+  // SA_ONSTACK is required for certain runtimes that use small stacks, for
+  // instance the Go runtime.
+  // See https://github.com/golang/go/issues/49075
+  // Address sanitizer also wants SA_ONSTACK, and the fuzzer and sanitizer
+  // often run together.
+  // SA_ONSTACK is a no-op unless someone also calls sigaltstack. That is left
+  // up to code that needs it.
+  new_sigact.sa_flags = SA_SIGINFO | SA_ONSTACK;
   new_sigact.sa_sigaction = callback;
   if (sigaction(signum, &new_sigact, nullptr)) {
     Printf("libFuzzer: sigaction failed with %d\n", errno);