[lldb] Fix dynamic type resolutions for core files (#138698)

We're reading from the object's vtable to determine the pointer to the
full object. The vtable is normally in the "rodata" section of the
executable, which is often not included in the core file because it's
not supposed to change and the debugger can extrapolate its contents
from the executable file. We weren't doing that.

This patch changes the read operation to use the target class (which
falls back onto the executable module as expected) and adds the missing
ReadSignedIntegerFromMemory API. The fix is tested by creating a core
(minidump) file which deliberately omits the vtable pointer.

Author: labath

lldb/include/lldb/Target/Target.h

@@ -1158,6 +1158,11 @@ class Target : public std::enable_shared_from_this<Target>,
                                      Status &error,
                                      bool force_live_memory = false);
 
+  int64_t ReadSignedIntegerFromMemory(const Address &addr,
+                                      size_t integer_byte_size,
+                                      int64_t fail_value, Status &error,
+                                      bool force_live_memory = false);
+
   uint64_t ReadUnsignedIntegerFromMemory(const Address &addr,
                                          size_t integer_byte_size,
                                          uint64_t fail_value, Status &error,

lldb/source/Plugins/LanguageRuntime/CPlusPlus/ItaniumABI/ItaniumABILanguageRuntime.cpp

@@ -350,7 +350,7 @@ bool ItaniumABILanguageRuntime::GetDynamicTypeAndAddress(
   if (offset_to_top_location >= vtable_load_addr)
     return false;
   Status error;
-  const int64_t offset_to_top = m_process->ReadSignedIntegerFromMemory(
+  const int64_t offset_to_top = target.ReadSignedIntegerFromMemory(
       offset_to_top_location, addr_byte_size, INT64_MIN, error);
 
   if (offset_to_top == INT64_MIN)

lldb/source/Target/Target.cpp

@@ -2270,6 +2270,17 @@ size_t Target::ReadScalarIntegerFromMemory(const Address &addr, uint32_t byte_si
   return 0;
 }
 
+int64_t Target::ReadSignedIntegerFromMemory(const Address &addr,
+                                            size_t integer_byte_size,
+                                            int64_t fail_value, Status &error,
+                                            bool force_live_memory) {
+  Scalar scalar;
+  if (ReadScalarIntegerFromMemory(addr, integer_byte_size, false, scalar, error,
+                                  force_live_memory))
+    return scalar.SLongLong(fail_value);
+  return fail_value;
+}
+
 uint64_t Target::ReadUnsignedIntegerFromMemory(const Address &addr,
                                                size_t integer_byte_size,
                                                uint64_t fail_value, Status &error,

lldb/test/API/lang/cpp/dynamic-value/TestDynamicValue.py

@@ -279,3 +279,54 @@ def test_from_forward_decl(self):
             "frame var -d run-target --ptr-depth=1 --show-types a",
             substrs=["(B *) a", "m_b_value = 10"],
         )
+
+    @no_debug_info_test
+    @expectedFailureAll(oslist=["windows"], bugnumber="llvm.org/pr24663")
+    @expectedFailureDarwin  # dynamic loader unloads modules
+    def test_from_core_file(self):
+        """Test fetching C++ dynamic values from core files. Specifically, test
+        that we can determine the dynamic type of the value if the core file
+        does not contain the type vtable."""
+        self.build()
+        lldbutil.run_to_name_breakpoint(self, "take_A")
+
+        # Get the address of our object and its vtable
+        a = self.frame().FindVariable("a")
+        self.assertSuccess(a.GetError())
+        vtable = a.GetVTable()
+        self.assertSuccess(vtable.GetError())
+        a = a.GetValueAsAddress()
+        vtable = vtable.GetValueAsAddress()
+
+        # Create a core file which will only contain the memory region
+        # containing `a`. The object is on the stack, so this will automatically
+        # include the stack of the main thread.
+        core = self.getBuildArtifact("a.dmp")
+        options = lldb.SBSaveCoreOptions()
+        options.SetPluginName("minidump")
+        options.SetStyle(lldb.eSaveCoreCustomOnly)
+        options.SetOutputFile(lldb.SBFileSpec(core))
+        region = lldb.SBMemoryRegionInfo()
+        self.assertSuccess(self.process().GetMemoryRegionInfo(a, region))
+        self.assertSuccess(options.AddMemoryRegionToSave(region))
+
+        # Save the core file and load it.
+        self.assertSuccess(self.process().SaveCore(options))
+        self.process().Kill()
+        error = lldb.SBError()
+        self.target().LoadCore(core, error)
+        self.assertSuccess(error)
+
+        # Sanity check -- the process should be able to read the object but not
+        # its vtable..
+        self.process().ReadPointerFromMemory(a, error)
+        self.assertSuccess(error)
+        self.process().ReadPointerFromMemory(vtable, error)
+        self.assertTrue(error.Fail())
+
+        # .. but we should still be able to see the dynamic type by reading the
+        # vtable from the executable file.
+        self.expect(
+            "frame var -d run-target --ptr-depth=1 --show-types a",
+            substrs=["(B *) a", "m_b_value = 10"],
+        )