linux.kernel

Re: a bug in genksysms/CONFIG_MODVERSIONS w/ __attribute__((foo))?


On Tue, Aug 27, 2019 at 7:26 PM Nicholas Piggin <npiggin_at_gmail.com> wrote:
>
> Ben Hutchings's on August 28, 2019 1:34 am:
> > On Tue, 2019-08-27 at 22:42 +1000, Nicholas Piggin wrote:
> >> Masahiro Yamada's on August 27, 2019 8:49 pm:
> >> > Hi.
> >> >
> >> > On Tue, Aug 27, 2019 at 6:59 PM Nicholas Piggin <npiggin_at_gmail.com> wrote:
> >> > > Nick Desaulniers's on August 27, 2019 8:57 am:
> >> > > > On Mon, Aug 26, 2019 at 2:22 PM Nick Desaulniers
> >> > > > <ndesaulniers_at_google.com> wrote:
> >> > > > > I'm looking into a linkage failure for one of our device kernels, and
> >> > > > > it seems that genksyms isn't producing a hash value correctly for
> >> > > > > aggregate definitions that contain __attribute__s like
> >> > > > > __attribute__((packed)).
> >> > > > >
> >> > > > > Example:
> >> > > > > $ echo 'struct foo { int bar; };' | ./scripts/genksyms/genksyms -d
> >> > > > > Defn for struct foo == <struct foo { int bar ; } >
> >> > > > > Hash table occupancy 1/4096 = 0.000244141
> >> > > > > $ echo 'struct __attribute__((packed)) foo { int bar; };' |
> >> > > > > ./scripts/genksyms/genksyms -d
> >> > > > > Hash table occupancy 0/4096 = 0
> >> > > > >
> >> > > > > I assume the __attribute__ part isn't being parsed correctly (looks
> >> > > > > like genksyms is a lex/yacc based C parser).
> >> > > > >
> >> > > > > The issue we have in our out of tree driver (*sadface*) is basically a
> >> > > > > EXPORT_SYMBOL'd function whose signature contains a packed struct.
> >> > > > >
> >> > > > > Theoretically, there should be nothing wrong with exporting a function
> >> > > > > that requires packed structs, and this is just a bug in the lex/yacc
> >> > > > > based parser, right? I assume that not having CONFIG_MODVERSIONS
> >> > > > > coverage of packed structs in particular could lead to potentially
> >> > > > > not-fun bugs? Or is using packed structs in exported function symbols
> >> > > > > with CONFIG_MODVERSIONS forbidden in some documentation somewhere I
> >> > > > > missed?
> >> > > >
> >> > > > Ah, looks like I'm late to the party:
> >> > > > https://lwn.net/Articles/707520/
> >> > >
> >> > > Yeah, would be nice to do something about this.
> >> >
> >> > modversions is ugly, so it would be great if we could dump it.
> >> >
> >> > > IIRC (without re-reading it all), in theory distros would be okay
> >> > > without modversions if they could just provide their own explicit
> >> > > versioning. They take care about ABIs, so they can version things
> >> > > carefully if they had to change.
> >
> > Debian doesn't currently have any other way of detecting ABI changes
> > (other than eyeballing diffs).
> >
> > I know there have been proposals of using libabigail for this instead,
> > but I'm not sure how far those progressed.
> >
> >> > We have not provided any alternative solution for this, haven't we?
> >> >
> >> > In your patch (https://lwn.net/Articles/707729/),
> >> > you proposed CONFIG_MODULE_ABI_EXPLICIT.
> >>
> >> Right, that was just my first proposal, but I am not confident that I
> >> understood everybody's requirements. I don't think the distro people
> >> had much time to to test things out.
> >>
> >> One possible shortcoming with that patch is no per-symbol version. The
> >> distro may break an ABI for a security fix, but they don't want to break
> >> all out of tree modules if it's an obscure ABI.
> >
> > Right, for example the KVM kABI is only meant for in-tree modules (like
> > kvm_intel) and in Debian we do not change the "ABI version" and require
> > rebuilding out-of-tree modules just because that ABI changes.
> > Currently we maintain explicit lists of exported symbols and exporting
> > modules for which we ignore ABI changes at build time.
> >
> >> The counter argument to
> >> that is they should just rename the symbol in their kernel for such
> >> cases, so I didn't implement it without somebody describing a good
> >> requirement.
> > `...`
> >
> > Sometimes it is just a single function that changes, but often a
> > structure change can affect large numbers of functions. For example,
> > if KVM adds a member to an operations struct that can indirectly change
> > the ABI for most of its exported functions. We wouldn't want to change
> > the ABI version but would still want to prevent loading mismatched kvm
> > and kvm_intel versions. It would be a lot more work to change all of
> > the affected function names.
>
> You could change just a single symbol name though :)
>
> > An alternative to symbol version matching that I think would work for
> > us is: if a module's exports or imports match the "changes ignored"
> > list then the module can only be loaded on the exact version of the
> > kernel, otherwise it only needs to match the ABI version. I think that
> > would avoid the need for carrying symbol versions, but we would still
> > need a build-time ABI check and a way of flagging which symbols need
> > the tighter version match.
>
> Just trying to think how best to express that.
>
> ` Aside, the whole symbol name resolution linking stuff does matching on
> on any number of ~arbitrary strings that you can generate as you like,
> and symbol tables are something that all existing tools and libs
> understand.
>
> So I strongly favour using that as the back end for our "version"
> resolution system _if at all possible_ rather than adding these extra
> bits of crud that really just do the same thing. At least for a first
> pass, I don't want to over-engineer things.
>
> Then it hopefully becomes a matter of adding some helper macros and
> build facilities on top of that which can contain everyone's
> requirements mostly within .config and perhaps a very small patch.
> A bit more work with preprocessor macros etc is far preferable to
> linking and loading "features" IMO`
>
> Back to your case, is it sufficient to have just an internal and an
> external module version where the kernel provides both and your in-tree
> modules match on the internal, others match on external?
>
> Thanks,
> Nick
>
+ some Android folks who are looking into libabigail
(root thread for context:
https://lore.kernel.org/l



Written by Nick Desaulniers 28/08/2019 19:40:01
Check some pics on this site!
06/12/2019 03:51:05