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 <npig...@gmail.com> wrote: >> > > Nick Desaulniers's on August 27, 2019 8:57 am: >> > > > On Mon, Aug 26, 2019 at 2:22 PM Nick Desaulniers >> > > > <ndesaulni...@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