C++ interop issue due to non-null pointers

[Florian Weimer]

On 06/17/2014 04:39 PM, Jakub Jelinek wrote:
> On Tue, Jun 17, 2014 at 04:34:16PM +0200, Florian Weimer wrote:
>>> I'm not that familiar with the exact requirements of std::vector, could
>>> we use the same trick as
>>> http://mxr.mozilla.org/mozilla-central/source/xpcom/glue/nsTArray.h#275
>>> that is instead of pointing at null point at a global header used for
>>> all empty vectors?
>>
>> For std::vector, we could use reinterpret_cast<T*>(alignof(T)) or
>> reinterpret_cast<T*>(sizeof(T)) if those are sufficiently well-defined as
>> far as GCC is concerned.  The smaller constant is easier to load, and no
>> relocation is required.
>
> If there is no object at those addresses, that is still undefined behavior.

GCC is the implementation, so it could be made defined there.

> GCC will likely not optimize it away at this point, but having code with
> undefined behavior is just asking for future trouble.  Just use "" instead?

It's always const and may lack sufficient alignment.  The former isn't a 
problem in C++ (I think), but the alignment is an issue.

-- 
Florian Weimer / Red Hat Product Security Team