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From: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Date: Thu, 19 Jan 2017 09:22:38 +0000
Message-ID: <CAKv+Gu_5hL617uMzT37sn6-1R_9fwoRnmr_jE=zDB1XyHGvDAQ@mail.gmail.com>
Subject: Re: [PATCH v4 3/3] modversions: treat symbol CRCs as 32 bit
 quantities on 64 bit archs
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Michael Ellerman <mpe@ellerman.id.au>, Jessica Yu <jeyu@redhat.com>,
        Rusty Russell <rusty@rustcorp.com.au>,
        "Suzuki K. Poulose" <suzuki.poulose@arm.com>,
        Will Deacon <will.deacon@arm.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>, Arnd Bergmann <arnd@arndb.de>,
        Al Viro <viro@zeniv.linux.org.uk>,
        ppc-dev <linuxppc-dev@lists.ozlabs.org>
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On 19 January 2017 at 00:15, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
> On Wed, Jan 18, 2017 at 2:37 PM, Ard Biesheuvel
> <ard.biesheuvel@linaro.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.