Work around a kernel bug on Android.

runtime/vm/thread_interrupter_android.cc

Index: runtime/vm/thread_interrupter_android.cc
diff --git a/runtime/vm/thread_interrupter_android.cc b/runtime/vm/thread_interrupter_android.cc
index 82be148cbd07f3b85c7f594eaf585ef9b832ca6b..0cc1b2a71e4285f16b479aadb25c93051e5a7cc4 100644
--- a/runtime/vm/thread_interrupter_android.cc
+++ b/runtime/vm/thread_interrupter_android.cc
@@ -21,29 +21,59 @@ namespace dart {
 DECLARE_FLAG(bool, thread_interrupter);
 DECLARE_FLAG(bool, trace_thread_interrupter);
 
-class ThreadInterrupterAndroid : public AllStatic {
- public:
-  static void ThreadInterruptSignalHandler(int signal, siginfo_t* info,
-                                           void* context_) {
-    if (signal != SIGPROF) {
-      return;
-    }
-    Thread* thread = Thread::Current();
-    if (thread == NULL) {
-      return;
-    }
-    // Extract thread state.
-    ucontext_t* context = reinterpret_cast<ucontext_t*>(context_);
-    mcontext_t mcontext = context->uc_mcontext;
-    InterruptedThreadState its;
-    its.pc = SignalHandler::GetProgramCounter(mcontext);
-    its.fp = SignalHandler::GetFramePointer(mcontext);
-    its.csp = SignalHandler::GetCStackPointer(mcontext);
-    its.dsp = SignalHandler::GetDartStackPointer(mcontext);
-    its.lr = SignalHandler::GetLinkRegister(mcontext);
-    Profiler::SampleThread(thread, its);
+static void ThreadInterruptSignalHandler(int signal, siginfo_t* info,
+                                         void* context_) {
+  if (signal != SIGPROF) {
+    return;
   }
-};
+  Thread* thread = Thread::Current();
+  if (thread == NULL) {
+    return;
+  }
+  // Extract thread state.
+  ucontext_t* context = reinterpret_cast<ucontext_t*>(context_);
+  mcontext_t mcontext = context->uc_mcontext;
+  InterruptedThreadState its;
+  its.pc = SignalHandler::GetProgramCounter(mcontext);
+  its.fp = SignalHandler::GetFramePointer(mcontext);
+  its.csp = SignalHandler::GetCStackPointer(mcontext);
+  its.dsp = SignalHandler::GetDartStackPointer(mcontext);
+  its.lr = SignalHandler::GetLinkRegister(mcontext);
+  Profiler::SampleThread(thread, its);
+}
+
+
+// Work around for a bug in old kernels (only fixed in 3.18 Android kernel):
+// kernel does not clear If-Then execution state bits when entering ARM signal
+// handler which violates requirements imposed by ARM architecture reference.
+// Some CPUs look at these bits even while in ARM mode which causes them
+// to skip some instructions in the prologue of the signal handler.
+// To work around the issue we insert enough NOPs in the prologue to ensure that
+// no actual instructions are skipped and then branch to the actual signal
+// handler.
+// For the kernel patch that fixes the issue see: http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=6ecf830e5029598732e04067e325d946097519cb
+// Note: this function is marked "naked" because we must guarantee that
+// our NOPs occur before any compiler generated prologue.
+static __attribute__((naked)) void SignalHandlerTrampoline(int signal,
+                                                           siginfo_t* info,
+                                                           void* context_) {
+  // IT (If-Then) instruction makes up to four instructions that follow it
+  // conditional.
+  asm volatile("nop; nop; nop; nop" : : : "memory");
+
+  // Tail-call into the actual signal handler.
+  // Note: this code is split into a separate inline assembly block because
+  // any code that compiler generates to satisfy register constraints must
+  // be generated after four NOPs.
+  register int arg0 asm("r0") = signal;
+  register siginfo_t* arg1 asm("r1") = info;
+  register void* arg2 asm("r2") = context_;
+  asm volatile("bx %3"

[Florian Schneider, 2016/05/02 13:44:45]

Need this only on ARM, not on Android ia32 for example.

+                :
+                : "r"(arg0), "r"(arg1), "r"(arg2),
+                  "r"(&ThreadInterruptSignalHandler)
+                : "memory");
+}
 
 
 void ThreadInterrupter::InterruptThread(OSThread* thread) {
@@ -57,8 +87,7 @@ void ThreadInterrupter::InterruptThread(OSThread* thread) {
 
 
 void ThreadInterrupter::InstallSignalHandler() {
-  SignalHandler::Install(
-      ThreadInterrupterAndroid::ThreadInterruptSignalHandler);
+  SignalHandler::Install(&SignalHandlerTrampoline);
 }