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

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Date:   Thu, 24 Nov 2016 10:38:04 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Nicholas Piggin <npiggin@...il.com>
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 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
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.

	Arnd