Support dereferencing a DWARF scalar stack value

[aprantl]

Summary

Swift async functions receive function arguments inside a heap-allocated data structure, similar to how ObjC block captures or C++ coroutine arguments are implement. In DWARF they are described relative to an entry value that produces a pointer into that heap object. At typical location looks like

DW_OP_entry_value [ DW_OP_reg14 ] DW_OP_deref DW_OP_plus_uconst 32 DW_OP_deref

This allows the unwinder (which has special ABI knowledge to restore the contents of r14) to push the base address onto the stack thus allowing the deref/offset operations to continue. The result of the entry value is a scalar, because DW_OP_reg14 is a register location — as it should be since we want to restore the pointer value contained in r14 at the beginning of the function and not the historical memory contents it was pointing to. The entry value should restore the address, which is still valid, not the contents at function entry.

To make this work, we need to allow LLDB to dereference Scalar stack results like load addresses, which is what this patch does. Unfortunately it is difficult to test this in isolation, since the DWARFExpression unit test doesn't have a process. Are there any situations where we shouldn't allow Scalars to be dereferences? Could they ever hold a File- or Hostaddress? I don't think so.

[vsk]

The logic change seems reasonable to me. Any chance we can provide a mock memory in the DwarfExpression unit test, or check in assembly for a program that uses such an entry value expression?

[JDevlieghere]

I think this makes sense. From a DWARF perspective this is sound, all the standard says is that the popped value must have an integral type.

[aprantl]

That took me a little to get right, but here you go! Mock process.

[aprantl]

There's actually no reason to fill a second memory buffer. Replaced all of DoReadMemory with a single for loop.

[vsk]

Thanks, lgtm.

lldb/unittests/Expression/DWARFExpressionTest.cpp

347

Neat, so lit4 is the address and the read pulls out [4, 5, 6, 7].

[aprantl]

lldb/unittests/Expression/DWARFExpressionTest.cpp

347

Yeah that's the lowest sensible address without dereferencing a nullptr :-)

lldb/source/Expression/DWARFExpression.cpp

case DW_OP_deref: {
           if (error_ptr)
             error_ptr->SetErrorString("failed to resolve load address");
           return false;
         }
         stack.back().GetScalar() = load_Addr;
         stack.back().SetValueType(Value::ValueType::LoadAddress);
         // Fall through to load address code below...
       } LLVM_FALLTHROUGH;
+      case Value::ValueType::Scalar:
       case Value::ValueType::LoadAddress:
         if (exe_ctx) {
           if (process) {
             lldb::addr_t pointer_addr =
                 stack.back().GetScalar().ULongLong(LLDB_INVALID_ADDRESS);
             Status error;
             lldb::addr_t pointer_value =
                 process->ReadPointerFromMemory(pointer_addr, error);
case DW_OP_deref: {
         } else {
           if (error_ptr)
             error_ptr->SetErrorString(
                 "NULL execution context for DW_OP_deref.\n");
           return false;
         }
         break;
 
-      case Value::ValueType::Scalar:
       case Value::ValueType::Invalid:
         if (error_ptr)
           error_ptr->SetErrorString("Invalid value type for DW_OP_deref.\n");
         return false;
       }
 
     } break;
 

lldb/unittests/Expression/CMakeLists.txt

 add_lldb_unittest(ExpressionTests
   ClangParserTest.cpp
   ClangExpressionDeclMapTest.cpp
   DiagnosticManagerTest.cpp
   DWARFExpressionTest.cpp
   CppModuleConfigurationTest.cpp
 
   LINK_LIBS
     lldbCore
+    lldbPluginObjectFileELF
+    lldbPluginPlatformLinux
     lldbPluginExpressionParserClang
     lldbPluginTypeSystemClang
     lldbUtility
     lldbUtilityHelpers
     lldbSymbolHelpers
     LLVMTestingSupport
   )

lldb/unittests/Expression/DWARFExpressionTest.cpp

 //===-- DWARFExpressionTest.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
 //
 //===----------------------------------------------------------------------===//
 
 #include "lldb/Expression/DWARFExpression.h"
+#include "Plugins/Platform/Linux/PlatformLinux.h"
 #include "Plugins/TypeSystem/Clang/TypeSystemClang.h"
 #include "TestingSupport/Symbol/YAMLModuleTester.h"
 #include "lldb/Core/Value.h"
+#include "lldb/Core/Debugger.h"
 #include "lldb/Core/dwarf.h"
+#include "lldb/Host/HostInfo.h"
 #include "lldb/Symbol/ObjectFile.h"
+#include "lldb/Utility/Reproducer.h"
 #include "lldb/Utility/StreamString.h"
 #include "llvm/ADT/StringExtras.h"
 #include "llvm/Testing/Support/Error.h"
 #include "gtest/gtest.h"
 
 using namespace lldb_private;
 
 static llvm::Expected<Scalar> Evaluate(llvm::ArrayRef<uint8_t> expr,
                                        lldb::ModuleSP module_sp = {},
-                                       DWARFUnit *unit = nullptr) {
+                                       DWARFUnit *unit = nullptr,
+                                       ExecutionContext *exe_ctx = nullptr) {
   DataExtractor extractor(expr.data(), expr.size(), lldb::eByteOrderLittle,
                           /*addr_size*/ 4);
   Value result;
   Status status;
-  if (!DWARFExpression::Evaluate(
-          /*exe_ctx*/ nullptr, /*reg_ctx*/ nullptr, module_sp, extractor, unit,
-          lldb::eRegisterKindLLDB,
+  if (!DWARFExpression::Evaluate(exe_ctx, /*reg_ctx*/ nullptr, module_sp,
+                                 extractor, unit, lldb::eRegisterKindLLDB,
                                  /*initial_value_ptr*/ nullptr,
-          /*object_address_ptr*/ nullptr, result, &status))
+                                 /*object_address_ptr*/ nullptr, result,
+                                 &status))
     return status.ToError();
 
   switch (result.GetValueType()) {
   case Value::ValueType::Scalar:
     return result.GetScalar();
   case Value::ValueType::HostAddress: {
     // Convert small buffers to scalars to simplify the tests.
     DataBufferHeap &buf = result.GetBuffer();

 
 /// Unfortunately Scalar's operator==() is really picky.
 static Scalar GetScalar(unsigned bits, uint64_t value, bool sign) {
   Scalar scalar(value);
   scalar.TruncOrExtendTo(bits, sign);
   return scalar;
 }
 
+/// This is needed for the tests that use a mock process.
+class DWARFExpressionMockProcessTest : public ::testing::Test {
+public:
+  void SetUp() override {
+    llvm::cantFail(repro::Reproducer::Initialize(repro::ReproducerMode::Off, {}));
+    FileSystem::Initialize();
+    HostInfo::Initialize();
+    platform_linux::PlatformLinux::Initialize();
+  }
+  void TearDown() override {
+    platform_linux::PlatformLinux::Terminate();
+    HostInfo::Terminate();
+    FileSystem::Terminate();
+    repro::Reproducer::Terminate();
+  }
+};
+
 TEST(DWARFExpression, DW_OP_pick) {
   EXPECT_THAT_EXPECTED(Evaluate({DW_OP_lit1, DW_OP_lit0, DW_OP_pick, 0}),
                        llvm::HasValue(0));
   EXPECT_THAT_EXPECTED(Evaluate({DW_OP_lit1, DW_OP_lit0, DW_OP_pick, 1}),
                        llvm::HasValue(1));
   EXPECT_THAT_EXPECTED(Evaluate({DW_OP_lit1, DW_OP_lit0, DW_OP_pick, 2}),
                        llvm::Failed());
 }

 
 TEST(DWARFExpression, DW_OP_unknown) {
   EXPECT_THAT_EXPECTED(
       Evaluate({0xff}),
       llvm::FailedWithMessage(
           "Unhandled opcode DW_OP_unknown_ff in DWARFExpression"));
 }
 
-TEST(DWARFExpression, DW_OP_deref) {
+TEST_F(DWARFExpressionMockProcessTest, DW_OP_deref) {
   EXPECT_THAT_EXPECTED(Evaluate({DW_OP_lit0, DW_OP_deref}), llvm::Failed());
+
+  struct MockProcess : Process {
+    using Process::Process;
+    ConstString GetPluginName() override { return ConstString("mock process"); }
+    uint32_t GetPluginVersion() override { return 0; }
+    bool CanDebug(lldb::TargetSP target,
+                  bool plugin_specified_by_name) override {
+      return false;
+    };
+    Status DoDestroy() override { return {}; }
+    void RefreshStateAfterStop() override {}
+    bool DoUpdateThreadList(ThreadList &old_thread_list,
+                            ThreadList &new_thread_list) override {
+      return false;
+    };
+    size_t DoReadMemory(lldb::addr_t vm_addr, void *buf, size_t size,
+                        Status &error) override {
+      for (size_t i = 0; i < size; ++i)
+        ((char *)buf)[i] = (vm_addr + i) & 0xff;
+      error.Clear();
+      return size;
+    }
+  };
+
+  // Set up a mock process.
+  ArchSpec arch("i386-pc-linux");
+  Platform::SetHostPlatform(
+      platform_linux::PlatformLinux::CreateInstance(true, &arch));
+  lldb::DebuggerSP debugger_sp = Debugger::CreateInstance();
+  ASSERT_TRUE(debugger_sp);
+  lldb::TargetSP target_sp;
+  lldb::PlatformSP platform_sp;
+  debugger_sp->GetTargetList().CreateTarget(
+      *debugger_sp, "", arch, eLoadDependentsNo, platform_sp, target_sp);
+  ASSERT_TRUE(target_sp);
+  ASSERT_TRUE(target_sp->GetArchitecture().IsValid());
+  ASSERT_TRUE(platform_sp);
+  lldb::ListenerSP listener_sp(Listener::MakeListener("dummy"));
+  lldb::ProcessSP process_sp =
+      std::make_shared<MockProcess>(target_sp, listener_sp);
+  ASSERT_TRUE(process_sp);
+
+  ExecutionContext exe_ctx(process_sp);
+  EXPECT_THAT_EXPECTED(Evaluate({DW_OP_lit4, DW_OP_deref}, {}, {}, &exe_ctx),
+                       llvm::HasValue(GetScalar(32, 0x07060504, false)));
 }