Re: GCC5 library transition for xml-security-c -- needed or not?

[Jakub Wilk <jwilk@debian.org>]

* Ferenc Wagner <wferi@niif.hu>, 2015-11-29, 22:26:
> > If you grep for "std::map<std::string", you'll find this in 
> > OpenSSLCryptoProvider.hpp:
> >
> > class DSIG_EXPORT OpenSSLCryptoProvider : public XSECCryptoProvider {
> > #ifdef XSEC_OPENSSL_HAVE_EC
> >   std::map<std::string,int> m_namedCurveMap;
> > #endif

[...]

> Now I still fail to follow the last step of your reasoning: how does a 
> change in the internal representaion of curve names (std::string) break 
> the ABI?

In general, changing type of a non-static member of a (public) class 
constitutes ABI breakage, even if it's a private member.

You could get away from it sometimes, and this might be the case here, 
so perhaps I was overly pessimistic. :)

I can think of two reasons why changing private member can break ABI in 
practice:

1) Changing the member type could change its size, leading to changing 
memory layout of the class. (However, in this case 
std::map<std::string,int> size didn't change AFAICS.)

2) User of the class could be accessing private members via inlined 
code. If the user of the class has a different idea about the member 
type than the library, this is clearly not going to fly.

Now, looking at the OpenSSLCryptoProvider header, it doesn't have any 
explicitly declared inline methods, constructors, etc. But it does have 
implicit inline copy constructor and implicit inline assignment 
operator. So I was able to craft a small test program, which crashes if 
it was compiled with GCC 4.X (and the library was compiled with GCC 5):

$ g++-5 test.cc -l xml-security-c -o test && ./test
moo!
$ g++-4.9 test.cc -l xml-security-c -o test && ./test
moo!
Segmentation fault

That said, this example is a bit contrived, because 
OpenSSLCryptoProvider doesn't look like something that is safe to 
instantiate multiple times anyway.

(Classes that are not supposed to be copied should declare private copy 
constructors and assignment operators. You might want to ask upstream to 
add them.)

--
Jakub Wilk

#include <iostream>

#include <xsec/enc/OpenSSL/OpenSSLCryptoProvider.hpp>

int main(int argc, char **argv)
{
	OpenSSLCryptoProvider op;
	OpenSSLCryptoProvider op2(op);
	std::cout << "moo!\n";
}