Exception Handling in 64bit(Windows) Delphi assembly

~ Davis Miller

Exception handling in 64-bit Windows, particularly when working with Delphi assembly, involves understanding the Windows Structured Exception Handling (SEH) mechanism and how Delphi integrates with it. Below is a comprehensive guide to handling exceptions in 64-bit Delphi assembly on Windows.

Overview of 64-bit Windows Exception Handling

Windows 64-bit uses a table-based Structured Exception Handling (SEH) mechanism, which differs significantly from the 32-bit frame-based SEH. Key aspects include:

  • Unwind Tables: Exception handling information is stored in data structures called unwind tables, which the system uses to unwind the stack during exceptions.
  • Registration: Functions register their exception handling information in these tables rather than using linked lists as in 32-bit SEH.
  • Function Prologues: Each function has associated unwind information that describes how to clean up the stack and which handlers to invoke during an exception.

Delphi's Integration with 64-bit SEH

Delphi's compiler is designed to generate the necessary unwind information and integrate seamlessly with the Windows 64-bit SEH. When writing assembly within Delphi, especially inline assembly, it's crucial to adhere to the conventions expected by both Delphi and the Windows SEH system.

Writing Exception-Safe Assembly in Delphi

1. Adhering to Calling Conventions

Ensure that your assembly code follows the 64-bit Windows calling conventions:

  • Register Usage:
    • Volatile Registers: RCX, RDX, R8, R9, R10, R11 are used for passing arguments and can be modified by the callee.
    • Non-Volatile Registers: RBX, RBP, RDI, RSI, R12–R15 must be preserved by the callee.
  • Shadow Space: Allocate 32 bytes of shadow space on the stack before calling other functions.
  • Stack Alignment: Ensure the stack is 16-byte aligned at the point of a call instruction.

2. Maintaining Proper Stack Frames

Proper stack frame setup is essential for the unwinding process during exceptions:

  • Frame Pointer: While not strictly required in 64-bit, maintaining a consistent frame can aid debugging and exception handling.
  • Local Variables and Saved Registers: Allocate space for local variables and save non-volatile registers if your assembly code modifies them.

3. Integrating with Delphi's Exception Handling

When writing inline assembly within Delphi's high-level constructs (like try...except or try...finally blocks), the compiler manages the exception handling. However, for standalone assembly routines, you must ensure that your code does not disrupt the exception handling mechanism.

Example: Inline Assembly within a Delphi Function

procedure ExampleProcedure;
begin
  try
    asm
      // Your assembly code here
      // Ensure adherence to calling conventions and stack alignment
      // For example, preserving non-volatile registers if modified
      push    rbx
      mov     rbx, 1234h
      // ... more assembly instructions ...
      pop     rbx
    end;
  except
    on E: Exception do
      // Handle exception
  end;
end;

In this example:

  • Preserving Registers: If you modify any non-volatile registers (like RBX), you must save and restore them.
  • Stack Alignment: Ensure that your assembly code does not disrupt the 16-byte stack alignment.

4. Writing External Assembly Routines

For external assembly routines (e.g., written in .asm files and linked with Delphi), you need to:

  • Provide Unwind Information: Use the Microsoft Assembler (MASM) or compatible tools to generate unwind metadata.
  • Use Proper Directives: Include directives that inform the assembler about function boundaries and exception handling.

Example: MASM-Compatible Assembly Function with Unwind Info

.686
.MODEL flat, C
PUBLIC MyAsmFunction

.data
; Data declarations

.code
MyAsmFunction PROC
    ; Function prologue
    push    rbp
    mov     rbp, rsp
    ; Function body
    ; ... assembly instructions ...
    ; Function epilogue
    pop     rbp
    ret
MyAsmFunction ENDP
END

Notes:

  • Prologue/Epilogue: Maintain standard function prologue and epilogue to allow the unwind mechanism to recognize stack frames.
  • Exception Safety: Ensure that all code paths properly clean up the stack and restore registers to prevent corruption during exceptions.

5. Handling Exceptions Within Assembly

Directly handling exceptions within assembly is complex and generally not recommended unless absolutely necessary. Instead, allow Delphi's high-level exception handling constructs to manage exceptions. If you must handle specific exceptions:

  • Use Structured Exception Handling (SEH): Implement SEH within your assembly code, adhering to the 64-bit Windows SEH conventions.
  • Register Exception Handlers: Manually register your exception handlers using the appropriate Windows APIs, ensuring that the unwind tables are correctly updated.

Caution: Improper implementation can lead to undefined behavior, application crashes, or security vulnerabilities.

Best Practices

  1. Minimize Assembly Usage: Use high-level Delphi code where possible to leverage the compiler's exception handling and optimizations.
  2. Follow Conventions Strictly: Adhere to calling conventions, stack alignment, and register preservation to maintain compatibility with Delphi's exception handling.
  3. Testing: Rigorously test assembly routines, especially under exception conditions, to ensure stability and correctness.
  4. Documentation: Clearly document assembly code to indicate how it interacts with Delphi's exception handling mechanisms.

Conclusion

Exception handling in 64-bit Delphi assembly on Windows requires careful adherence to the Windows SEH model and Delphi's calling conventions. By following best practices and ensuring that your assembly code integrates seamlessly with Delphi's exception handling mechanisms, you can create robust and exception-safe assembly routines within your Delphi applications.

If you have specific scenarios or code snippets you'd like to discuss further, feel free to provide more details!

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