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

[Alan Modra]

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.

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