[libunwind][ARM] Handle end of stack during unwind

[danielkiss]

Summary

When unwind step reaches the end of the stack that means the force unwind should notify the stop function.
This is not an error, it could mean just the thread is cleaned up completely.

[danielkiss]

Rerun clang-format.

[danielkiss]

Make the CI happy.

[danielkiss]

Ping

[mstorsjo]

Looks reasonable I think. Is this a deficiency in the EHABI implementation only, i.e. this aspect works as it should in the regular dwarf implementation?

[danielkiss]

In D109856#3154763, @mstorsjo wrote:

Looks reasonable I think. Is this a deficiency in the EHABI implementation only, i.e. this aspect works as it should in the regular dwarf implementation?

Yes, it works on dwarf already ( also the test pass on X86/Aarch64 ).

[mstorsjo]

In D109856#3155235, @danielkiss wrote:

In D109856#3154763, @mstorsjo wrote:

Looks reasonable I think. Is this a deficiency in the EHABI implementation only, i.e. this aspect works as it should in the regular dwarf implementation?

Yes, it works on dwarf already ( also the test pass on X86/Aarch64 ).

Ok, thanks for the clarification!

libcxxabi/src/cxa_personality.cpp

 
 // Helper function to unwind one frame.
 // ARM EHABI 7.3 and 7.4: If the personality function returns _URC_CONTINUE_UNWIND, the
 // personality routine should update the virtual register set (VRS) according to the
 // corresponding frame unwinding instructions (ARM EHABI 9.3.)
 static _Unwind_Reason_Code continue_unwind(_Unwind_Exception* unwind_exception,
                                            _Unwind_Context* context)
 {
-    if (__gnu_unwind_frame(unwind_exception, context) != _URC_OK)
-        return _URC_FAILURE;
+  switch (__gnu_unwind_frame(unwind_exception, context)) {
+  case _URC_OK:
     return _URC_CONTINUE_UNWIND;
+  case _URC_END_OF_STACK:
+    return _URC_END_OF_STACK;
+  default:
+    return _URC_FAILURE;
+  }
 }
 
 // ARM register names
 #if !defined(LIBCXXABI_USE_LLVM_UNWINDER)
 static const uint32_t REG_UCB = 12;  // Register to save _Unwind_Control_Block
 #endif
 static const uint32_t REG_SP = 13;
 

libcxxabi/test/forced_unwind3.pass.cpp

+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// Let's run ForcedUnwind until it reaches end of the stack, this test simulates
+// what pthread_cancel does.
+
+// UNSUPPORTED: c++03
+// UNSUPPORTED: libcxxabi-no-threads
+// UNSUPPORTED: no-exceptions
+
+#include <assert.h>
+#include <exception>
+#include <stdlib.h>
+#include <string.h>
+#include <unwind.h>
+#include <thread>
+#include <tuple>
+#include <__cxxabi_config.h>
+
+// TODO: dump version back to 14 once clang is updated on the CI.
+#if defined(_LIBCXXABI_ARM_EHABI) && defined(__clang__) && __clang_major__ < 15
+// _Unwind_ForcedUnwind is not available or broken before version 14.
+int main(int, char**) { return 0; }
+
+#else
+static bool destructorCalled = false;
+
+struct myClass {
+  myClass() {}
+  ~myClass() {
+    assert(destructorCalled == false);
+    destructorCalled = true;
+  };
+};
+
+template <typename T>
+struct Stop;
+
+template <typename R, typename... Args>
+struct Stop<R (*)(Args...)> {
+  // The third argument of _Unwind_Stop_Fn is uint64_t in Itanium C++ ABI/LLVM
+  // libunwind while _Unwind_Exception_Class in libgcc.
+  typedef typename std::tuple_element<2, std::tuple<Args...>>::type type;
+
+  static _Unwind_Reason_Code stop(int, _Unwind_Action actions, type,
+                                  struct _Unwind_Exception*,
+                                  struct _Unwind_Context*, void*) {
+    if (actions & _UA_END_OF_STACK) {
+      assert(destructorCalled == true);
+      exit(0);
+    }
+    return _URC_NO_REASON;
+  }
+};
+
+static void forced_unwind() {
+  _Unwind_Exception* exc = new _Unwind_Exception;
+  memset(&exc->exception_class, 0, sizeof(exc->exception_class));
+  exc->exception_cleanup = 0;
+  _Unwind_ForcedUnwind(exc, Stop<_Unwind_Stop_Fn>::stop, 0);
+  abort();
+}
+
+__attribute__((__noinline__)) static void test() {
+  myClass c{};
+  forced_unwind();
+  abort();
+}
+
+int main(int, char**) {
+  std::thread t{test};
+  t.join();
+  return -1;
+}
+#endif

libunwind/src/Unwind-EHABI.cpp

static _Unwind_Reason_Code unwindOneFrame(_Unwind_State state,
   // of the correct stack frame.
   _Unwind_Reason_Code result =
       ProcessDescriptors(state, ucbp, context, format, lsda,
                          ucbp->pr_cache.additional);
 
   if (result != _URC_CONTINUE_UNWIND)
     return result;
 
-  if (__unw_step(reinterpret_cast<unw_cursor_t *>(context)) != UNW_STEP_SUCCESS)
-    return _URC_FAILURE;
+  switch (__unw_step(reinterpret_cast<unw_cursor_t *>(context))) {
+  case UNW_STEP_SUCCESS:
     return _URC_CONTINUE_UNWIND;
+  case UNW_STEP_END:
+    return _URC_END_OF_STACK;
+  default:
+    return _URC_FAILURE;
+  }
 }
 
 // Generates mask discriminator for _Unwind_VRS_Pop, e.g. for _UVRSC_CORE /
 // _UVRSD_UINT32.
 uint32_t RegisterMask(uint8_t start, uint8_t count_minus_one) {
   return ((1U << (count_minus_one + 1)) - 1) << start;
 }
 
#endif
   // said it would...
   return _URC_FATAL_PHASE2_ERROR;
 }
 
 static _Unwind_Reason_Code
 unwind_phase2_forced(unw_context_t *uc, unw_cursor_t *cursor,
                      _Unwind_Exception *exception_object, _Unwind_Stop_Fn stop,
                      void *stop_parameter) {
+  bool endOfStack = false;
   // See comment at the start of unwind_phase1 regarding VRS integrity.
   __unw_init_local(cursor, uc);
   _LIBUNWIND_TRACE_UNWINDING("unwind_phase2_force(ex_ojb=%p)",
                              static_cast<void *>(exception_object));
   // Walk each frame until we reach where search phase said to stop
-  while (true) {
+  while (!endOfStack) {
     // Update info about this frame.
     unw_proc_info_t frameInfo;
     if (__unw_get_proc_info(cursor, &frameInfo) != UNW_ESUCCESS) {
       _LIBUNWIND_TRACE_UNWINDING("unwind_phase2_forced(ex_ojb=%p): __unw_step "
                                  "failed => _URC_END_OF_STACK",
                                  (void *)exception_object);
       return _URC_FATAL_PHASE2_ERROR;
     }
if (frameInfo.handler != 0) {
       case _URC_INSTALL_CONTEXT:
         _LIBUNWIND_TRACE_UNWINDING("unwind_phase2_forced(ex_ojb=%p): "
                                    "personality returned "
                                    "_URC_INSTALL_CONTEXT",
                                    (void *)exception_object);
         // We may get control back if landing pad calls _Unwind_Resume().
         __unw_resume(cursor);
         break;
+      case _URC_END_OF_STACK:
+        _LIBUNWIND_TRACE_UNWINDING("unwind_phase2_forced(ex_ojb=%p): "
+                                   "personality returned "
+                                   "_URC_END_OF_STACK",
+                                   (void *)exception_object);
+        // Personalty routine did the step and it can't step forward.
+        endOfStack = true;
+        break;
       default:
         // Personality routine returned an unknown result code.
         _LIBUNWIND_TRACE_UNWINDING("unwind_phase2_forced(ex_ojb=%p): "
                                    "personality returned %d, "
                                    "_URC_FATAL_PHASE2_ERROR",
                                    (void *)exception_object, personalityResult);
         return _URC_FATAL_PHASE2_ERROR;
       }
if (exception_object->exception_cleanup != NULL)
     (*exception_object->exception_cleanup)(_URC_FOREIGN_EXCEPTION_CAUGHT,
                                            exception_object);
 }
 
 extern "C" _LIBUNWIND_EXPORT _Unwind_Reason_Code
 __gnu_unwind_frame(_Unwind_Exception *exception_object,
                    struct _Unwind_Context *context) {
   unw_cursor_t *cursor = (unw_cursor_t *)context;
-  if (__unw_step(cursor) != UNW_STEP_SUCCESS)
-    return _URC_FAILURE;
+  switch (__unw_step(cursor)) {
+  case UNW_STEP_SUCCESS:
     return _URC_OK;
+  case UNW_STEP_END:
+    return _URC_END_OF_STACK;
+  default:
+    return _URC_FAILURE;
+  }
 }
 
 #endif  // defined(_LIBUNWIND_ARM_EHABI)