A year ago, I wrote a short post about us banning memcpy in the SDL for new code. Well, I’m happy to announce that in VC++ 2010, we have made it much easier to remove potentially insecure calls to memcpy and replace them with more secure calls to memcpy_s; it’s automagic, just like we do did for other banned functions!

As I said in a previous post, I am a huge fan of adding defenses to code automatically, and making such changes as easy as possible for software engineers, and this auto-migration is a great example.

In short, if your code has a call to memcpy, and the compiler can determine the destination buffer size at compile time, the compiler will replace the call to memcpy with a call to memcpy_s.

For example, if you compile the code below with:

cl /D_CRT_SECURE_CPP_OVERLOAD_STANDARD_NAMES_MEMORY=1 foo.cpp

#include <memory.h>

int main() {

    int src[100];
    const size_t src_size =  _countof(src) * sizeof(int);

    memset(src, 12, src_size);

    const size_t dst_size_int = _countof(src);
    int dst[dst_size_int];

    memcpy(dst, src, src_size);

    return 0;

}

You’ll see that the calls to memcpy are replaced with memcpy_s courtesy of this code in memory.h:

#if defined(__cplusplus) && _CRT_SECURE_CPP_OVERLOAD_STANDARD_NAMES_MEMORY

extern "C++" {

#ifndef _CRT_ENABLE_IF_DEFINED

  #define _CRT_ENABLE_IF_DEFINED

    template<bool _Enable, typename _Ty>

    struct _CrtEnableIf;

    template<typename _Ty>

    struct _CrtEnableIf<true, _Ty>

    {

        typedef _Ty _Type;

    };

#endif

    template <size_t _Size, typename _DstType>

    inline

    typename _CrtEnableIf<(_Size > 1), void *>::_Type __cdecl memcpy(_DstType (&_Dst)[_Size], _In_opt_bytecount_(_SrcSize) const void *_Src, _In_ size_t _SrcSize) _CRT_SECURE_CPP_NOTHROW

    {

        return memcpy_s(_Dst, _Size * sizeof(_DstType), _Src, _SrcSize) == 0 ? _Dst : 0;

    }

}

#endif

Note that for this to work, you must define a preprocessor variable:

_CRT_SECURE_CPP_OVERLOAD_STANDARD_NAMES_MEMORY = 1

This is another great example of why migrating your C++ code to Visual C++ 2010 can help improve the security of the code with virtually no engineering effort.

(Big thanks to the C++ folks over in the Developer Division for getting this work done in time for VC++ 2010)

- Michael