linux-kernel - Re: linux-next: build warnings after merge of the kbuild tree

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Date:   Fri, 19 Aug 2016 20:44:55 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     Michal Marek <mmarek@...e.cz>
Cc:     Stephen Rothwell <sfr@...b.auug.org.au>,
        linux-kernel@...r.kernel.org, linux-next@...r.kernel.org,
        Al Viro <viro@...iv.linux.org.uk>,
        PowerPC <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: linux-next: build warnings after merge of the kbuild tree

On Fri, 19 Aug 2016 10:37:00 +0200
Michal Marek <mmarek@...e.cz> wrote:

> On 2016-08-19 07:09, Stephen Rothwell wrote:
> > Hi Nick,
> > 
> > On Fri, 19 Aug 2016 13:38:54 +1000 Stephen Rothwell <sfr@...b.auug.org.au> wrote:  
> >>
> >> On Thu, 18 Aug 2016 11:09:48 +1000 Nicholas Piggin <npiggin@...il.com> wrote:  
> >>>
> >>> On Wed, 17 Aug 2016 14:59:59 +0200
> >>> Michal Marek <mmarek@...e.cz> wrote:
> >>>     
> >>>> On 2016-08-17 03:44, Stephen Rothwell wrote:      
> >>>>>
> >>>>> After merging the kbuild tree, today's linux-next build (powerpc
> >>>>> ppc64_defconfig) produced these warnings:
> >>>>>
> >>>>> WARNING: 25 bad relocations
> >>>>> c000000000cf2570 R_PPC64_ADDR64    __crc___arch_hweight16        
> >>>> [...]      
> >>>>> Introduced by commit
> >>>>>
> >>>>>   9445aa1a3062 ("ppc: move exports to definitions")
> >>>>>
> >>>>> I have reverted that commit for today.
> >>>>>
> >>>>> [cc-ing the ppc guys for clues - also involved is commit
> >>>>>
> >>>>>   22823ab419d8 ("EXPORT_SYMBOL() for asm")
> >>>>> ]        
> >>>>
> >>>> FWIW, I see these warnings as well. Any help from ppc developers is
> >>>> appreciated - should the R_PPC64_ADDR64 be whitelisted for exported asm
> >>>> symbols (their CRCs actually)?      
> >>>
> >>> The dangling relocation is a side effect of linker unable to resolve the
> >>> reference to the undefined weak symbols. So the real question is, why has
> >>> genksyms not overridden these symbols with their CRC values?
> >>>
> >>> This may not even be powerpc specific, but  I'll poke at it a bit more
> >>> when I get a chance.    
> >>
> >> Not sure if this is relevant, but with the commit reverted, the
> >> __crc___... symbols are absolute.
> >>
> >> 00000000f55b3b3d A __crc___arch_hweight16  
> > 
> > Ignore that :-)
> > 
> > I just had a look at a x86_64 allmodconfig result and it looks like the
> > weak symbols are not resolved their either ...
> > 
> > I may be missing something, but genksyms generates the crc's off the
> > preprocessed C source code and we don't have any for the asm files ...  
> 
> Of course you are right. Which means that we are losing type information
> for these exports for CONFIG_MODVERSIONS purposes. I guess it's
> acceptable, since the asm functions are pretty basic and their
> signatures do not change.

I don't completely agree. It would be nice to have the functionality
still there.

What happens if you just run cmd_modversions on the as rule? It relies on
!defined(__ASSEMBLY__), but we're feeding the result to genksyms, not as.
It would require the header be included in the .S file and be protected for
asm builds.

Stephen wasn't a fan of suck a hack and I can't say I blame him. Another
possibility I suppose is an EXPORT_SYMBOL_ASM() variant that takes string
containing C function declaration and just inserts it as an assembler
comment somewhere that genksysms can find.


Thanks,
Nick