Diagnostics for missing libraries in DSOs

[Florian Weimer]

Currently, it is extremely easy to produce a DSO which links against 
compat symbols at run time: Just omit a required DSO when invoking the 
static linker.  As a result, the symbol reference in the created DSO 
will lack a version, and will be bound to a compat symbol at run time:

   https://sourceware.org/bugzilla/show_bug.cgi?id=20489

This is very difficult to diagnose properly.  It can also lead to 
current binaries reintroducing historic bugs which we did not want to 
push on applications for backwards compatibility reasons.

What can we do here?  Can we provide a diagnostic at static link time? 
Should ld.so print a run-time diagnostic?

Non-versioned to versioned symbol bindings happen quite frequently 
outside glibc (right after a library has introduced symbol versioning). 
But perhaps we can produce a run-time warning if such a binding occurs 
when the referring object lacks a DT_NEEDED entry for the referent.

Florian