linux-kernel - Re: [REGRESSION] "efi: efistub: Convert into static library" and preparation patches

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Date:	Thu, 04 Sep 2014 13:54:09 +0100
From:	Michael Brown <mbrown@...systems.co.uk>
To:	Maarten Lankhorst <maarten.lankhorst@...onical.com>,
	Ard Biesheuvel <ard.biesheuvel@...aro.org>,
	Matt Fleming <matt@...sole-pimps.org>
CC:	Ulf Winkelvos <ulf@...kelvos.de>,
	Matt Fleming <matt.fleming@...el.com>,
	LKML <linux-kernel@...r.kernel.org>,
	"x86@...nel.org" <x86@...nel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	"linux-efi@...r.kernel.org" <linux-efi@...r.kernel.org>,
	Seth Forshee <seth.forshee@...onical.com>,
	Matthew Garrett <mjg59@...f.ucam.org>
Subject: Re: [REGRESSION] "efi: efistub: Convert into static library" and
 preparation patches

On 04/09/14 11:48, Maarten Lankhorst wrote:
>>> If not, Ard please go ahead with option #2 above. Overkill yes, but I've
>>> done the single __attribute__() hacks in other projects and someone
>>> (usually me) always eventually forgets to tag some instance.
>>>
>> It appears we just got lucky on arm64, since we don't have any global
>> variables, but the issue does exist there as well.
>>
> FWIW, visibility pushing doesn't seem to work for functions declared with extern.
> Following seems to get rid of all GOTPCREL:

Are you sure?  The iPXE build process relies on this:

   /* Force visibility of all symbols to "hidden", i.e. inform gcc that
    * all symbol references resolve strictly within our final binary.
    * This avoids unnecessary PLT/GOT entries on x86_64.
    *
    * This is a stronger claim than specifying "-fvisibility=hidden",
    * since it also affects symbols marked with "extern".
    */
   #ifndef ASSEMBLY
   #if __GNUC__ >= 4
   #pragma GCC visibility push(hidden)
   #endif
   #endif /* ASSEMBLY */

and https://gcc.gnu.org/wiki/Visibility states that

   "#pragma GCC visibility is stronger than -fvisibility; it affects 
extern declarations as well. -fvisibility only affects definitions, so 
that existing code can be recompiled with minimal changes. This is more 
true for C than C++; C++ interfaces tend use classes, which are affected 
by -fvisibility."

Michael

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