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

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Date:   Thu, 24 Nov 2016 18:53:22 +1100
From:   Nicholas Piggin <npiggin@...il.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     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>
Subject: Re: [PATCH] x86/kbuild: enable modversions for symbols exported
 from asm

On Thu, 24 Nov 2016 08:36:39 +0100
Greg Kroah-Hartman <gregkh@...uxfoundation.org> wrote:

> On Thu, Nov 24, 2016 at 06:20:26PM +1100, Nicholas Piggin wrote:
> > But still, modversions is pretty complicated for what it gives us. It sends
> > preprocessed C into a C parser that makes CRCs using type definitions of
> > exported symbols, then turns those CRCs into a linker script which which is
> > used to link the .o file with. What we get in return is a quite limited and
> > symbol "versioning" system.
> > 
> > What if we ripped all that out and just attached an explicit version to
> > each export, and incompatible changes require an increment?  
> 
> How would that work for structures?  Would that be required for every
> EXPORT_SYMBOL* somehow?

Yeah just have EXPORT_SYMBOL take another parameter which attaches a version
number and use that as the value for the __crc_ symbol versions rather than
a calculated CRC.

Yes it would require some level of care from developers and may be a small
annoyance when changing exports. But making people think a tiny bit more
before chnaging exported ABI shouldn't be the end of the world.

> 
> > 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.

Thanks,
Nick