Symbol versioning for secondary libraries is not effective

[Florian Weimer <fweimer at redhat dot com>]

I looked at underlinking issues in the upcoming Fedora 28 distribution. 
It is fairly common that functions from <dlfcn.h> are called from DSOs 
which are not linked with -ldl.  The net result is that the references 
to dlsym etc. are not properly versioned.  <pthread.h> and <math.h> are 
also affected.

In the past, we have seen actual bugs related to missing symbol versions 
for functions from librt.

In short, I think this affects all secondary libraries besides 
libc.so.6.  This means that bumping symbol versions for those libraries 
is not an effective way to maintain backwards compatibility.  This is 
something to keep in mind for future changes.  It is also a mild 
argument in favor of moving symbols into libc.so.6, I think.

(Technically, those DSOs are of course incorrectly linked and not 
ABI-compliant, but that doesn't help users who are stuck with them.)

Thanks,
Florian