[libFuzzer] always install signal handler with SA_ONSTACK

[randall77]

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

[llvmbot]

@llvm/pr-subscribers-compiler-rt-sanitizer

Author: Keith Randall (randall77)

Changes

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

---

Full diff: https://github.com/llvm/llvm-project/pull/147422.diff

1 Files Affected:

• (modified) compiler-rt/lib/fuzzer/FuzzerUtilPosix.cpp (+8-4)

diff --git a/compiler-rt/lib/fuzzer/FuzzerUtilPosix.cpp b/compiler-rt/lib/fuzzer/FuzzerUtilPosix.cpp
index 392c1e5be4eea..b6cc3bcf8a3f3 100644
--- a/compiler-rt/lib/fuzzer/FuzzerUtilPosix.cpp
+++ b/compiler-rt/lib/fuzzer/FuzzerUtilPosix.cpp
@@ -78,10 +78,14 @@ static void SetSigaction(int signum,
   }
 
   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);

[randall77]

Previous patch and discussion on Phabricator. It's been a few years, trying to get this landed.

https://reviews.llvm.org/D150231

Why this matters: it causes random stack corruption in Go when running with libFuzzer. See golang/go#49075 (comment) and subsequent comments.

@dvyukov @sebastianpoeplau @vitalybuka @ramosian-glider

[sebastianpoeplau]

Like I said on the old Phabricator discussion, I think this is good to go in. However, I don't have rights to formally approve or merge 🤷‍♂️