linux-kernel - Re: [PATCH] x86/kbuild: enable modversions for symbols exported from asm

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Message-ID: <20161124210101.76fc21ff@roar.ozlabs.ibm.com>
Date:   Thu, 24 Nov 2016 21:01:01 +1100
From:   Nicholas Piggin <npiggin@...il.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Ingo Molnar <mingo@...nel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Adam Borowski <kilobyte@...band.pl>,
        Michal Marek <mmarek@...e.com>,
        Philip Muller <philm@...jaro.org>,
        linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        linux-arch <linux-arch@...r.kernel.org>,
        linux-kbuild <linux-kbuild@...r.kernel.org>,
        Russell King <rmk+lkml@....linux.org.uk>
Subject: Re: [PATCH] x86/kbuild: enable modversions for symbols exported
 from asm

On Thu, 24 Nov 2016 10:38:04 +0100
Arnd Bergmann <arnd@...db.de> wrote:

> On Thursday, November 24, 2016 6:53:22 PM CET Nicholas Piggin wrote:
> > >   
> > > > Google tells me
> > > > Linus is not a neutral bystander on the topic of symbol versioning, so I'm
> > > > bracing for a robust response  (actually I don't much care either way, I'm
> > > > happy to put a couple of bandaids on it and keep it going)    
> > > 
> > > There are tools that people are working on to make it more obvious where
> > > API breaks happen by looking at the .o debug data instead of our crazy
> > > current system (which is really better than nothing), perhaps we should
> > > start using them instead?
> > > 
> > > See here for more details about this:
> > >       https://kernel-recipes.org/en/2016/talks/would-an-abi-changes-visualization-tool-be-useful-to-linux-kernel-maintenance/  
> > 
> > Hmm. I guess it's basically similar to modversions, so has downsides of not
> > detecting a semantic change unless it changes the type. But still, if we could
> > replace our custom code with a tool like this for modversions functionality,
> > that alone would be a massive improvement. But requiring debug info might be
> > a bit of a show stopper. I also don't know if that would handle asm functions.  
> 
> It's certainly not an option for v4.9 at this point. There is also no

Yeah I wasn't suggesting that for 4.9, or 4.10 even.

> realistic way we can get a correct asm/asm-prototypes.h for all the
> other architectures in place. At the moment, powerpc is the only
> one that actually works with modversions.
> 
> We can either make CONFIG_MODVERSIONS a per-architecture opt-in
> and let only the ones that have the header file select that, or
> revert all of Al's original patches that moved the exports.

alpha, m68k, s390, sparc, ia64 are affected and have no patch (or none
I've been cc'ed on) for asm-prototypes.h. Doesn't seem infeasible to add
them before 4.9, considering the fairly low risk of patch. Opt-in would
be pointless IMO, might as well just revert the arch part of Al's patch
instead.

Thanks,
Nick