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

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Message-ID: <20161215120615.34fd0f22@roar.ozlabs.ibm.com>
Date:   Thu, 15 Dec 2016 12:06:15 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     Hannes Frederic Sowa <hannes@...hat.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Stanislav Kozina <skozina@...hat.com>,
        Don Zickus <dzickus@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Ben Hutchings <ben@...adent.org.uk>,
        Michal Marek <mmarek@...e.com>,
        Adam Borowski <kilobyte@...band.pl>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Debian kernel maintainers <debian-kernel@...ts.debian.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>, Ingo Molnar <mingo@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86/kbuild: enable modversions for symbols exported
 from asm

On Wed, 14 Dec 2016 15:04:36 +0100
Hannes Frederic Sowa <hannes@...hat.com> wrote:

> On 09.12.2016 17:03, Greg Kroah-Hartman wrote:
> > On Sat, Dec 10, 2016 at 01:56:53AM +1000, Nicholas Piggin wrote:  
> >> On Fri, 9 Dec 2016 15:36:04 +0100
> >> Stanislav Kozina <skozina@...hat.com> wrote:
> >>  
> >>>>>>> The question is how to provide a similar guarantee if a different way?    
> >>>>>> As a tool to aid distro reviewers, modversions has some value, but the
> >>>>>> debug info parsing tools that have been mentioned in this thread seem
> >>>>>> superior (not that I've tested them).    
> >>>>> On the other hand the big advantage of modversions is that it also
> >>>>> verifies the checksum during runtime (module loading). In other words, I
> >>>>> believe that any other solution should still generate some form of
> >>>>> checksum/watermark which can be easily checked for compatibility on
> >>>>> module load.
> >>>>> It should not be hard to add to the DWARF based tools though. We'd just
> >>>>> parse DWARF data instead of the C code.    
> >>>> A runtime check is still done, with per-module vermagic which distros
> >>>> can change when they bump the ABI version. Is it really necessary to
> >>>> have more than that (i.e., per-symbol versioning)?    
> >>>
> >>>  From my point of view, it is. We need to allow changing ABI for some 
> >>> modules while maintaining it for others.
> >>> In fact I think that there should be version not only for every exported 
> >>> symbol (in the EXPORT_SYMBOL() sense), but also for every public type 
> >>> (in the sense of eg. structure defined in the public header file).  
> >>
> >> Well the distro can just append _v2, _v3 to the name of the function
> >> or type if it has to break compat for some reason. Would that be enough?  
> > 
> > There are other ways that distros can work around when upstream "breaks"
> > the ABI, sometimes they can rename functions, and others they can
> > "preload" structures with padding in anticipation for when/if fields get
> > added to them.  But that's all up to the distros, no need for us to
> > worry about that at all :)  
> 
> The _v2 and _v3 functions are probably the ones that also get used by
> future backports in the distro kernel itself and are probably the reason
> for the ABI change in the first place. Thus going down this route will
> basically require distros to touch every future backport patch and will
> in general generate a big mess internally.

What kind of big mess? You have to check the logic of each backport even
if it does apply cleanly, so the added overhead of the name change should
be relatively tiny, no?

> 
> I think it is important to keep versioning information outside of the
> source code. Some kind of modversions will still be required, but
> distros should be able to decide if they put in some kind of checksum or
> a string, what suites them most.

The module crc symbols are just an integer that requires a match, so it
could easily be populated by a list that the distro keeps, rather than
by genksyms. Most of the complexity is on the build side, so that would
still be an improvement for the kernel. So we *could* do this if the
distros need it.

Thanks,
Nick