linux-kernel - Re: [PATCH v4 3/3] modversions: treat symbol CRCs as 32 bit quantities on 64 bit archs

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Date:   Thu, 19 Jan 2017 09:22:38 +0000
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Jessica Yu <jeyu@...hat.com>,
        Rusty Russell <rusty@...tcorp.com.au>,
        "Suzuki K. Poulose" <suzuki.poulose@....com>,
        Will Deacon <will.deacon@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Arnd Bergmann <arnd@...db.de>,
        Al Viro <viro@...iv.linux.org.uk>,
        ppc-dev <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: [PATCH v4 3/3] modversions: treat symbol CRCs as 32 bit
 quantities on 64 bit archs

On 19 January 2017 at 00:15, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Wed, Jan 18, 2017 at 2:37 PM, Ard Biesheuvel
> <ard.biesheuvel@...aro.org> wrote:
>>
>> For a ballpark number of 10,000 CRCs in the core kernel, this would
>> increase the size of the image by 40 KB for 32-bit architectures (and
>> if saving 40 KB is essential, chances are you won't be using
>> modversions in the first place).
>
> As you say, I don't think the space issue is much of a problem.
>
> I'm more worried about the replacement of one crazy model that has
> problems due to linker subtlety with _another_ one.
>
> Your genksyms.c change is not exactly obvious. I looked at it, and my
> brain just shut down. Why both the
>
>   LONG(0x%08lx);
>
> _and_ the
>
>   "%s__crc_%s = 0x%08lx;\n"
>
> in the linker script? I'm sure there's a good reason, but I'd like to
> see a more explicit explanation fo what the generated linker script
> does and what the rules are.
>

This is simply because modpost still uses the value of the symbol
rather than the value it points to to generate the /other/ side of the
comparison (i.e., Module.symvers etc)

I will look into updating modpost to dereference the symbol as well,
and update the RFC patch accordingly.