1 files changed, 18 insertions, 4 deletions
diff --git a/sigchainlib/sigchain.cc b/sigchainlib/sigchain.cc
index c5015e8..601e321 100644
--- a/sigchainlib/sigchain.cc
+++ b/sigchainlib/sigchain.cc
@@ -35,6 +35,8 @@
 
 namespace art {
 
+typedef int (*SigActionFnPtr)(int, const struct sigaction*, struct sigaction*);
+
 class SignalAction {
  public:
   SignalAction() : claimed_(false), uses_old_style_(false) {
@@ -147,7 +149,20 @@ extern "C" void InvokeUserSignalHandler(int sig, siginfo_t* info, void* context)
   }
 }
 
-// These functions are C linkage since they replace the functions in libc.
+extern "C" void EnsureFrontOfChain(int signal, struct sigaction* expected_action) {
+  CheckSignalValid(signal);
+  // Read the current action without looking at the chain, it should be the expected action.
+  SigActionFnPtr linked_sigaction = reinterpret_cast<SigActionFnPtr>(linked_sigaction_sym);
+  struct sigaction current_action;
+  linked_sigaction(signal, nullptr, &current_action);
+  // If the sigactions don't match then we put the current action on the chain and make ourself as
+  // the main action.
+  if (current_action.sa_sigaction != expected_action->sa_sigaction) {
+    log("Warning: Unexpected sigaction action found %p\n", current_action.sa_sigaction);
+    user_sigactions[signal].Claim(current_action);
+    linked_sigaction(signal, expected_action, nullptr);
+  }
+}
 
 extern "C" int sigaction(int signal, const struct sigaction* new_action, struct sigaction* old_action) {
   // If this signal has been claimed as a signal chain, record the user's
@@ -179,9 +194,7 @@ extern "C" int sigaction(int signal, const struct sigaction* new_action, struct
     log("Unable to find next sigaction in signal chain");
     abort();
   }
-
-  typedef int (*SigAction)(int, const struct sigaction*, struct sigaction*);
-  SigAction linked_sigaction = reinterpret_cast<SigAction>(linked_sigaction_sym);
+  SigActionFnPtr linked_sigaction = reinterpret_cast<SigActionFnPtr>(linked_sigaction_sym);
   return linked_sigaction(signal, new_action, old_action);
 }
 
@@ -287,5 +300,6 @@ extern "C" void InitializeSignalChain() {
   }
   initialized = true;
 }
+
 }   // namespace art