C++ interop issue due to non-null pointers

[Florian Weimer <fweimer at redhat dot com>]

In GCC 4.9, we have optimizations that make use of non-null annotations, 
at least for removing null pointer checks.  Some libc functions are 
annotated with it, such as qsort, memcpy, memset, memcmp.

On the other hand, it is unspecified if the data() member of std::vector 
returns null pointer if empty() returns true.

As a result, code like this is invalid if the functions are ever called 
with empty vectors:

  void clear(std::vector<char> &vec)
  {
    memset(vec.data(), '\0', vec.size());
  }

  int comparefn(void *, void *);

  void sort(std::vector<T> &vec)
  {
    qsort(vec.data(), vec.size(), sizeof(T), comparefn);
  }

I think this is quite surprising.  What can we do about it?

--
Florian Weimer / Red Hat Product Security Team