Add way to ensure we are at the front of the sigaction chain

Calling this after jni_on_load fixes the unity apps. This is
not exactly correct since we may already have the following chain.

Start up:
Us -> debuggerd

After app goes in front:
App -> us -> debuggerd

After we put ourself back at the front:
Us -> app -> us -> app -> .... stack overflow.

Bug: 17620677
Change-Id: I9183997e3d5ebd51c320b5d51425be5142e938f3

(cherry picked from commit 1f24296c7c8a6501ee2388c0d20b48f471b48660)

4 files changed, 30 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
 
diff --git a/sigchainlib/sigchain.h b/sigchainlib/sigchain.h
index 0de0d08..79b76a7 100644
--- a/sigchainlib/sigchain.h
+++ b/sigchainlib/sigchain.h
@@ -19,6 +19,8 @@
 
 #include <signal.h>
 
+namespace art {
+
 extern "C" void InitializeSignalChain();
 
 extern "C" void ClaimSignalChain(int signal, struct sigaction* oldaction);
@@ -27,4 +29,8 @@ extern "C" void UnclaimSignalChain(int signal);
 
 extern "C" void InvokeUserSignalHandler(int sig, siginfo_t* info, void* context);
 
+extern "C" void EnsureFrontOfChain(int signal, struct sigaction* expected_action);
+
+}  // namespace art
+
 #endif  // ART_SIGCHAINLIB_SIGCHAIN_H_
diff --git a/sigchainlib/sigchain_dummy.cc b/sigchainlib/sigchain_dummy.cc
index 7176f05..fbc8c3f 100644
--- a/sigchainlib/sigchain_dummy.cc
+++ b/sigchainlib/sigchain_dummy.cc
@@ -57,3 +57,8 @@ extern "C" void InitializeSignalChain() {
   log("InitializeSignalChain is not exported by the main executable.");
   abort();
 }
+
+extern "C" void EnsureFrontOfChain(int signal, struct sigaction* expected_action) {
+  log("EnsureFrontOfChain is not exported by the main executable.");
+  abort();
+}
diff --git a/sigchainlib/version-script.txt b/sigchainlib/version-script.txt
index 8030da4..ce15054 100644
--- a/sigchainlib/version-script.txt
+++ b/sigchainlib/version-script.txt
@@ -4,6 +4,7 @@ global:
   UnclaimSignalChain;
   InvokeUserSignalHandler;
   InitializeSignalChain;
+  EnsureFrontOfChain;
   sigaction;
   signal;
   sigprocmask;