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

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Date:   Thu, 24 Nov 2016 21:03:55 +1100
From:   Nicholas Piggin <npiggin@...il.com>
To:     Michal Marek <mmarek@...e.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>,
        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 10:32:12 +0100
Michal Marek <mmarek@...e.com> wrote:

> On 2016-11-24 08:53, Nicholas Piggin wrote:
> > 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.  
> 
> The problem is that with every kernel release, the structures change in
> a way that you would have to bump the version of virtually every export.
>
> At which point, there would be little difference between
> CONFIG_MODVERSION on and off (without CONFIG_MODVERSION, we compare the
> kernel version strings when loading modules).


I'm not sure about that. If they are truly incompatible changes and
MODVERSIONS does not pick up a different CRC, then it's even worse if
incompatible modules are missed so often.

Thanks,
Nick