[compiler-rt] [Fuzzer] Use user signal to coordinate handler shutdown (PR #82067)

[James Robinson via llvm-commits]

https://github.com/jamesr created https://github.com/llvm/llvm-project/pull/82067

This updates the signal handle thread coordinating to use a user signal bit on the SignalHandlerEvent to coordinate shutdown instead of closing the event handle. Closing the event handle is racy as the handle may be closed before the signal handler thread resolves the handle value in _zx_object_wait_many() and we would like to make this an explicit error. Using the user signal bit 1 instead and then closing the event object after the signal handler thread is joined cannot race as the wait will terminate whether the signal is raised before or after the wait begins.

>From 41b9130ddd27f1d7513db2aea52706352766070a Mon Sep 17 00:00:00 2001
From: James Robinson <jamesr at google.com>
Date: Fri, 16 Feb 2024 14:55:28 -0800
Subject: [PATCH] [Fuzzer] Use user signal to coordinate handler shutdown

This updates the signal handle thread coordinating to use a user signal
bit on the SignalHandlerEvent to coordinate shutdown instead of closing
the event handle. Closing the event handle is racy as the handle may be
closed before the signal handler thread resolves the handle value in
_zx_object_wait_many() and we would like to make this an explicit error.
Using the user signal bit 1 instead and then closing the event object
after the signal handler thread is joined cannot race as the wait will
terminate whether the signal is raised before or after the wait begins.
---
 compiler-rt/lib/fuzzer/FuzzerUtilFuchsia.cpp | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/compiler-rt/lib/fuzzer/FuzzerUtilFuchsia.cpp b/compiler-rt/lib/fuzzer/FuzzerUtilFuchsia.cpp
index cfb81cd3f780bb..fe79e1908d6029 100644
--- a/compiler-rt/lib/fuzzer/FuzzerUtilFuchsia.cpp
+++ b/compiler-rt/lib/fuzzer/FuzzerUtilFuchsia.cpp
@@ -292,7 +292,7 @@ void CrashHandler() {
     zx_wait_item_t WaitItems[] = {
         {
             .handle = SignalHandlerEvent,
-            .waitfor = ZX_SIGNAL_HANDLE_CLOSED,
+            .waitfor = ZX_USER_SIGNAL_1,
             .pending = 0,
         },
         {
@@ -378,10 +378,11 @@ void CrashHandler() {
 }
 
 void StopSignalHandler() {
-  _zx_handle_close(SignalHandlerEvent);
+  _zx_object_signal(SignalHandlerEvent, 0, ZX_USER_SIGNAL_1);
   if (SignalHandler.joinable()) {
     SignalHandler.join();
   }
+  _zx_handle_close(SignalHandlerEvent);
 }
 
 } // namespace