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

Nicholas Piggin
2016-12-15T14:15:31Z

On Thu, 15 Dec 2016 14:15:31 +0100
Hannes Frederic Sowa <hannes@redhat.com> wrote:

> On 15.12.2016 13:03, Nicholas Piggin wrote:
> > On Thu, 15 Dec 2016 12:19:02 +0100
> > Hannes Frederic Sowa <hannes@redhat.com> wrote:
> >
> >> On 15.12.2016 03:06, Nicholas Piggin wrote:
> >>> On Wed, 14 Dec 2016 15:04:36 +0100
> >>> Hannes Frederic Sowa <hannes@redhat.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@redhat.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?
> >>
> >> Basically single patches are backported in huge series. Reviewing each
> >> single patch also definitely makes sense, a review of the series as a
> >> whole is much more worthwhile because it focuses more on logic.
> >>
> >> The patches themselves are checked by individual robots or humans
> >> against merge conflict introduced mistakes which ring alarm bells for
> >> people to look more closely during review.
> >>
> >> Merge conflicts introduced mistakes definitely can happen because
> >> developers/backporters lose the focus from the actual logic but deal
> >> with shifting lines around or just fixing up postfixes to function names.
> >>
> >> We still try to align the kernel as much as possible with upstream,
> >> because most developers can't really hold the differences between
> >> upstream and the internal functions in their heads (is this function RMW
> >> safe in this version but not that kernel version...).
> >
> > I agree with all this, but in the case of a function rename, you can
> > automate it all with scripts if that's what you want.
> >
> > When you have your list of exported symbols with non-zero version number,
> > then you can script that __abivXXX into the changeset applying process,
> > or alternatively apply the rename after your patches are applied, or
> > use the c preprocessor to define names to something else.
>
> Yes, probably one could come up with coccinelle patches to do this,
> preprocessing/string matching could have false positives. But as I wrote
> above, we need one stable ABI and not multiple for our particular
> kernels, so it seems like a lot of overhead to rename particular
> functions internally all the time to make them inaccessible for external
> modules.

I can't be sure until it's implemented in a workflow that distros are
happy with of course, but I just don't see why it would be a lot of
overhead. Particularly if you just scripted everything.

How frequently do symbols become incompatible within an ostensibly ABI-
stable release?

> >> Like Don also already said, genksyms already did a pretty good job so
> >> far. We are right now working with Dodji to come up with a way to
> >> replace genksyms, in case people really want to have very specific
> >> control about what causes the symbol version to be changed.
> >
> > Yeah it's great work, so is Stanislav's checker. I wouldn't mind having
> > a kernel-centric checker tool merged in the kernel if it is small,
> > maintained, and does a sufficient job for distros.
>
> Yes, I think this needs more experimentation and thought right now
> before we can make a decision.

Sure, I wanted to mention it in case people had a concern about out
of tree tools. It will depend on what distros end up settling with.

> >> Also I wonder what Ben's opinion on this is.. As I understood that he
> >> wants to maintain a super-long-term stable kernel with kabi guarantees.
> >>
> >> Note, what we want is to weaken the check for kabi, by excluding parts
> >> of the struct from genksyms with libabigail. For Red Hat genksyms is too
> >> strict in the checks.
> >
> > Sure, that makes sense.
> >
> > So if I understand where we are, moving the ABI compatibility checking
> > to one of these tools looks possible. What to do when we have an ABI change
> > is not settled, but feeding version numbers explicitly into modversions
> > is an option that would be close to what distros do today.
>
> Agreed!

Thanks,
Nick