linux-kernel - Re: a bug in genksysms/CONFIG_MODVERSIONS w/ __attribute_

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Date:   Tue, 27 Aug 2019 19:49:51 +0900
From:   Masahiro Yamada <yamada.masahiro@...ionext.com>
To:     Nicholas Piggin <npiggin@...il.com>
Cc:     Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Arnd Bergmann <arnd@...db.de>,
        Greg KH <gregkh@...uxfoundation.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Will Deacon <will@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Ben Hutchings <ben@...adent.org.uk>
Subject: Re: a bug in genksysms/CONFIG_MODVERSIONS w/ __attribute__((foo))?

Hi.

On Tue, Aug 27, 2019 at 6:59 PM Nicholas Piggin <npiggin@...il.com> wrote:
>
> Nick Desaulniers's on August 27, 2019 8:57 am:
> > On Mon, Aug 26, 2019 at 2:22 PM Nick Desaulniers
> > <ndesaulniers@...gle.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.

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.
If it is good enough for distros, we merge it first,
give them time to migrate over to it, then finally remove modversions??


> I think we left that on hold because some of the bigger distros were
> heading into releases and we didn't care to cause pain. I wonder if
> we could try again.

I agree.


>
> What's your requirement for versioning?

I added Ben Hutchings to CC.

>
> Thanks,
> Nick



-- 
Best Regards
Masahiro Yamada