Re: Preventing preemption of 'protected' symbols in GNU ld 2.26 [aka should we revert the fix for 65248]

[Michael Matz <matz@suse.de>]

Hi,

On Mon, 18 Apr 2016, H.J. Lu wrote:

> > reason is DSO code (also handcoded assembly) may reasonably expect to 
> > be able to load the address with a PC-relative load-address type 
> > instruction (ADDIUPC, LEA, MOVAB, etc.) and the target may not even 
> > have suitable dynamic relocations available to apply any load-time 
> > fixup if the symbol referred turns up outside of the DSO.  The 
> > instruction used may have a PC-relative range limit too.
> 
> That is why protected visibility is such a mess.

Not mess, but it comes with certain limitations.  And that's okay.  It's 
intended as an optimization, and it should do that optimization if 
requested, and error out if it can't be done for whatever reason.

E.g. one limitation might very well be that function pointer comparison 
for protected functions doesn't work (gives different outcomes if the 
pointer is built from inside the exe or from a shared lib).  (No matter 
how it's built, it will still _work_ when called).  Alternatively we can 
make comparison work (by using the exe PLT slot), in which case Alans 
testcase will need more complications to show that protected visibility 
currently is broken.  Alans testcase will work right now (as in showing 
protected being broken) on data symbols.


Ciao,
Michael.