Is there a way to detect a null reference function pointer during assembly or linking ?

[William Tambe]

On Sat, Nov 30, 2019 at 9:31 PM Alan Modra <amodra@gmail.com> wrote:
>
> On Sat, Nov 30, 2019 at 08:59:29AM -0500, William Tambe wrote:
> > To supplement my previous question, a null reference function pointer
> > can occur when using a weak function for which there is no definition;
> > ie:
> >
> > extern void func (void) __attribute__((weak));
> >
> > int main () {
> >         if (func)
> >                 func();
> > }
> >
> > In the above example, func will be null if there is no default implementation.
> >
> > The issue I am running into is that  the value of a null reference
> > function pointer is not null when the executable is not loaded at the
> > address specified in the ELF header, because that value is computed by
> > offsetting a constant value from the
> > start of the section .text.
>
> "if (func)" when -fPIC is generally implemented as a load of an
> address from the GOT or similar, and a comparison of that address with
> zero.  You have a compiler problem if -fPIC code for "if (func)" uses
> a relative offset for the address of func.

Could I please have pointers to examples of code that implement "if
(func)" when -fPIC ?

>
> You have a linker problem if ld is emitting dynamic relocs, or
> resolving symbols and/or editing code so that what wasn't a relative
> offset in the relocatable object file becomes one in the final
> executable.
>
> --
> Alan Modra
> Australia Development Lab, IBM