I have Delphi 6 DLL which internally uses a third party library to do the automatic resource string translation.
In the library source code 2 RTL functions (ShortCutToText and LoadResString) are replaced with the multilingual counterparts (trShortCutToText and trLoadResString). Code is shown below:
VirtualProtect(@ShortCutToText, SIZE_C, PAGE_READWRITE, @protectType);
Move((@ShortCutToText)^, shortCutBuffer, SIZE_C);
Move((@trShortCutToText)^, (@ShortCutToText)^, SIZE_C);
VirtualProtect(@ShortCutToText, SIZE_C, protectType, @protectType);
FlushInstructionCache(GetCurrentProcess, @ShortCutToText, SIZE_C);
and
VirtualProtect(@LoadResString, SIZE_C, PAGE_READWRITE, @protectType);
Move((@LoadResString)^, loadResStringBuffer, SIZE_C);
Move((@trLoadResString)^, (@LoadResString)^, SIZE_C);
VirtualProtect(@LoadResString, SIZE_C, protectType, @protectType); //<--Exception thrown at this line
FlushInstructionCache(GetCurrentProcess, @LoadResString, SIZE_C);
Now when I call functions of this DLL from within another Delphi 6 application everything works fine.
However when I call the DLL from a C# application (compiled as 32bit app on 64bit machine), my c# application simply crashes.
On debugging the DLL, I found that the first block of the above 3rd party code executes without any issue however the second block fails at second last line.
The error message is shown below:
System.AccessViolationException was unhandled
Message="Attempted to read or write protected memory. This is often an indication that other memory is corrupt."
Source="TestWinform"
StackTrace:
at TestWinform.TestSwitching()
at TestWinform.Form1.button1_Click(Object sender, EventArgs e)
at System.Windows.Forms.Control.OnClick(EventArgs e)
at System.Windows.Forms.Button.OnClick(EventArgs e)
at System.Windows.Forms.Button.OnMouseUp(MouseEventArgs mevent)
at System.Windows.Forms.Control.WmMouseUp(Message& m, MouseButtons button, Int32 clicks)
at System.Windows.Forms.Control.WndProc(Message& m)
at System.Windows.Forms.ButtonBase.WndProc(Message& m)
at System.Windows.Forms.Button.WndProc(Message& m)
at System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m)
at System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m)
at System.Windows.Forms.NativeWindow.DebuggableCallback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)
at System.Windows.Forms.UnsafeNativeMethods.DispatchMessageW(MSG& msg)
at System.Windows.Forms.Application.ComponentManager.System.Windows.Forms.UnsafeNativeMethods.IMsoComponentManager.FPushMessageLoop(Int32 dwComponentID, Int32 reason, Int32 pvLoopData)
at System.Windows.Forms.Application.ThreadContext.RunMessageLoopInner(Int32 reason, ApplicationContext context)
at System.Windows.Forms.Application.ThreadContext.RunMessageLoop(Int32 reason, ApplicationContext context)
at System.Windows.Forms.Application.Run(Form mainForm)
at TestWinform.Program.Main()
at System.AppDomain._nExecuteAssembly(Assembly assembly, String[] args)
at System.AppDomain.ExecuteAssembly(String assemblyFile, Evidence assemblySecurity, String[] args)
at Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly()
at System.Threading.ThreadHelper.ThreadStart_Context(Object state)
at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
at System.Threading.ThreadHelper.ThreadStart()
InnerException:
EDIT:
I have created a sample DLL project in Delphi 6 to replicate the problem.
Source code from DelphiDllTest.dpr file is given below:
library DelphiDllTest;
uses
SysUtils,
Classes,
MyUnit in 'MyUnit.pas';
{$R *.res}
procedure TestSwitching; export;
var
myDict : TMyDictionary;
begin
myDict := TMyDictionary.Create(nil);
try
myDict.Open;
finally
myDict.Close;
FreeAndNil(myDict);
end;
end;
exports
TestSwitching;
begin
end.
Source of MyUnit.pas is given below:
unit MyUnit;
interface
uses
Classes, Windows;
type
TMyDictionary = class(TComponent)
public
constructor Create(owner: TComponent); override;
destructor Destroy; override;
procedure Open; virtual;
procedure Close; virtual;
end;
function TranslateLoadResString(resStringRec: PResStringRec): String;
function trLoadResString(resStringRec: PResStringRec): String;
const
SIZE_C = 32;
var
loadResStringBuffer: array[0..SIZE_C] of Byte;
implementation
constructor TMyDictionary.Create(owner: TComponent);
begin
inherited Create(owner);
end;
destructor TMyDictionary.Destroy;
begin
inherited Destroy;
end;
procedure TMyDictionary.Open;
var
protect: Integer;
begin
VirtualProtect(@LoadResString, SIZE_C, PAGE_READWRITE, @protect);
Move((@LoadResString)^, loadResStringBuffer, SIZE_C);
Move((@trLoadResString)^, (@LoadResString)^, SIZE_C);
VirtualProtect(@LoadResString, SIZE_C, protect, @protect);
FlushInstructionCache(GetCurrentProcess, @LoadResString, SIZE_C);
end;
procedure TMyDictionary.Close;
var
protect: Integer;
begin
VirtualProtect(@LoadResString, SIZE_C, PAGE_READWRITE, @protect);
Move(loadResStringBuffer, (@LoadResString)^, SIZE_C);
VirtualProtect(@LoadResString, SIZE_C, protect, @protect);
end;
function TranslateLoadResString(resStringRec: PResStringRec): String;
var
buffer: array[0..1023] of Char;
begin
try
if resStringRec <> nil then
begin
if resStringRec.Identifier < 64*1024 then
begin
SetString(
Result,
buffer,
LoadString(
FindResourceHInstance(resStringRec.Module^),
resStringRec.Identifier,
buffer,
SizeOf(buffer)));
end
else
Result := PChar(resStringRec.Identifier);
end
else
Result := '';
except
Result := '';
end;
end;
function trLoadResString(resStringRec: PResStringRec): String;
asm
PUSH EBP
MOV EBP, ESP
ADD ESP, $-8
MOV [EBP-$8], EDX
MOV [EBP-$4], EAX
MOV EDX, [EBP-$8]
MOV EAX, [EBP-$4]
MOV ECX, OFFSET Addr(TranslateLoadResString)-$1
CALL ECX
MOV ESP,EBP
POP EBP
end;
end.
Now finally the C# code which uses that DLL is given below:
using System;
using System.Windows.Forms;
using System.Runtime.InteropServices;
namespace TestWinform
{
public partial class Form1 : Form
{
[DllImport("DelphiDllTest.dll")]
public static extern void TestSwitching();
public Form1()
{
InitializeComponent();
}
private void button1_Click(object sender, EventArgs e)
{
TestSwitching();
}
}
}
Hope this is enough to generate the AV.
I believe that DEP is responsible for the exceptions. The problem is with the protection option that you are using. Instead of PAGE_READWRITE you need to use PAGE_EXECUTE_READWRITE. In Open, you need to protect @trLoadResString as well as @LoadResString, you are missing that call to VirtualProtect.
Perhaps like this:
VirtualProtect(@LoadResString, SIZE_C, PAGE_EXECUTE_READWRITE, @protect);
Move((@LoadResString)^, loadResStringBuffer, SIZE_C);
VirtualProtect(@LoadResString, SIZE_C, protect, @protect);
FlushInstructionCache(GetCurrentProcess, @LoadResString, SIZE_C);
VirtualProtect(@trLoadResString, SIZE_C, PAGE_EXECUTE_READWRITE, @protect);
Move((@trLoadResString)^, (@LoadResString)^, SIZE_C);
VirtualProtect(@trLoadResString, SIZE_C, protect, @protect);
FlushInstructionCache(GetCurrentProcess, @trLoadResString, SIZE_C);
And you need to do something similar in the Close method. Perhaps you need to restore trLoadResString.
Your exported function is using the register calling convention, but it should export as stdcall. The export directive is ignored. It's only exports that matters.