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From:   Arnd Bergmann <arnd@arndb.de>
Date:   Wed, 19 Jan 2022 11:17:30 +0100
Message-ID: <CAK8P3a3b+37jZBE_OSPg7zdrZeoton0X8cka6bxp5AGK3kD+yA@mail.gmail.com>
Subject: Re: [PATCH] crypto: memneq: avoid implicit unaligned accesses
To:     Ard Biesheuvel <ardb@kernel.org>
Cc:     "open list:HARDWARE RANDOM NUMBER GENERATOR CORE" 
        <linux-crypto@vger.kernel.org>,
        Herbert Xu <herbert@gondor.apana.org.au>,
        Arnd Bergmann <arnd@arndb.de>
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On Wed, Jan 19, 2022 at 10:31 AM Ard Biesheuvel <ardb@kernel.org> wrote:
>
> The C standard does not support dereferencing pointers that are not
> aligned with respect to the pointed-to type, and doing so is technically
> undefined behavior, even if the underlying hardware supports it.
>
> This means that conditionally dereferencing such pointers based on
> whether CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS=y is not the right thing
> to do, and actually results in alignment faults on ARM, which are fixed
> up on a slow path. Instead, we should use the unaligned accessors in
> such cases: on architectures that don't care about alignment, they will
> result in identical codegen whereas, e.g., codegen on ARM will avoid
> doubleword loads and stores but use ordinary ones, which are able to
> tolerate misalignment.
>
> Link: https://lore.kernel.org/linux-crypto/CAHk-=wiKkdYLY0bv+nXrcJz3NH9mAqPAafX7PpW5EwVtxsEu7Q@mail.gmail.com/
> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>

Reviewed-by: Arnd Bergmann <arnd@arndb.de>