Fix two issues that undefined behavior sanitizer caught

[henrylhtsang]

Fixes #413

Changes

1. string.h: Guard std::memcpy call with size check to avoid passing null source pointer with size 0
2. object.h: Replace null pointer member access with standard offsetof macro for computing object header offsets

I tested manually for clang and gcc. Didn't test locally for msvc since I don't have access to windows. CI should test that.

script:

#include <tvm/ffi/string.h>
#include <tvm/ffi/object.h>
#include <iostream>

class TestObj : public tvm::ffi::Object {
  public:
  static constexpr const char* _type_key = "test.TestObj";
};

int main() {
  std::cout << "Test 1: Creating string from null TVMFFIByteArray...\n";
  TVMFFIByteArray null_bytes{nullptr, 0};
  tvm::ffi::String str_from_null(null_bytes);
  std::cout << "String size: " << str_from_null.size() << "\n";

  std::cout << "Test 2: Computing object offset...\n";
  int64_t offset = tvm::ffi::details::ObjectUnsafe::GetObjectOffsetToSubclass<TestObj>();
  std::cout << "Offset: " << offset << "\n";

  std::cout << "Done - no UBSan errors!\n";
  return 0;
}

commands:

cd ~/tvm-ffi
git submodule update --init --recursive

clang++ -std=c++17 -fsanitize=undefined -I./include -I./3rdparty/dlpack/include -x c++  repro.cpp  -o repro

g++ -std=c++17 -fsanitize=undefined -I./include -I./3rdparty/dlpack/include repro.cpp -o repro

[gemini-code-assist]

Summary of Changes

Hello @henrylhtsang, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses two distinct instances of undefined behavior identified by the Undefined Behavior Sanitizer (UBSan). The changes enhance the robustness and correctness of the codebase by ensuring safer memory operations and standard-compliant offset calculations, thereby preventing potential crashes or unexpected program behavior.

Highlights

• string.h fix: Guarded std::memcpy call with a size check to prevent passing a null source pointer when the size is zero, addressing potential undefined behavior.
• object.h fix: Replaced a non-standard null pointer member access pattern with the offsetof macro for computing object header offsets, improving standard compliance and safety.

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[henrylhtsang]

cc @junrushao

[gemini-code-assist]

Code Review

This pull request correctly addresses two sources of undefined behavior. The change in object.h to use the standard offsetof macro instead of manual pointer arithmetic is a great improvement for correctness and portability. Similarly, adding a size check before std::memcpy in string.h correctly prevents undefined behavior. The changes are approved.

[gemini-code-assist]

This is a great fix to prevent undefined behavior when memcpy is called with a size of 0 and a null source pointer. I noticed that the Bytes::InitData method in this same file has a similar implementation without this check. It would be beneficial to apply the same fix there to ensure consistency and prevent the same undefined behavior in the Bytes class.