c++ - Most efficient replacement for IsBadReadPtr?

Question

I have some Visual C++ code that receives a pointer to a buffer with data that needs to be processed by my code and the length of that buffer. Due to a bug outside my control, sometimes this pointer comes into my code uninitialized or otherwise unsuitable for reading (i.e. it causes a crash when I try to access the data in the buffer.)

So, I need to verify this pointer before I use it. I don't want to use IsBadReadPtr or IsBadWritePtr because everyone agrees that they're buggy. (Google them for examples.) They're also not thread-safe -- that's probably not a concern in this case, though a thread-safe solution would be nice.

I've seen suggestions on the net of accomplishing this by using VirtualQuery, or by just doing a memcpy inside an exception handler. However, the code where this check needs to be done is time sensitive so I need the most efficient check possible that is also 100% effective. Any ideas would be appreciated.

Just to be clear: I know that the best practice would be to just read the bad pointer, let it cause an exception, then trace that back to the source and fix the actual problem. However, in this case the bad pointers are coming from Microsoft code that I don't have control over so I have to verify them.

Note also that I don't care if the data pointed at is valid. My code is looking for specific data patterns and will ignore the data if it doesn't find them. I'm just trying to prevent the crash that occurs when running memcpy on this data, and handling the exception at the point memcpy is attempted would require changing a dozen places in legacy code (but if I had something like IsBadReadPtr to call I would only have to change code in one place).

Answer 1

bool IsBadReadPtr(void* p)
{
    MEMORY_BASIC_INFORMATION mbi = {0};
    if (::VirtualQuery(p, &mbi, sizeof(mbi)))
    {
        DWORD mask = (PAGE_READONLY|PAGE_READWRITE|PAGE_WRITECOPY|PAGE_EXECUTE_READ|PAGE_EXECUTE_READWRITE|PAGE_EXECUTE_WRITECOPY);
        bool b = !(mbi.Protect & mask);
        // check the page is not a guard page
        if (mbi.Protect & (PAGE_GUARD|PAGE_NOACCESS)) b = true;

        return b;
    }
    return true;
}