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

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Date:   Mon, 26 Aug 2019 15:57:39 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Will Deacon <will@...nel.org>, Arnd Bergmann <arnd@...db.de>,
        Michal Marek <michal.lkml@...kovi.net>,
        Nicholas Piggin <npiggin@...il.com>
Cc:     LKML <linux-kernel@...r.kernel.org>
Subject: Re: a bug in genksysms/CONFIG_MODVERSIONS w/ __attribute__((foo))?

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/
-- 
Thanks,
~Nick Desaulniers