linux-kernel - Re: [PATCH 07/10] crypto: Use ARCH_DMA_MINALIGN instead of ARCH_KMALLOC

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Date:   Thu, 7 Apr 2022 17:28:10 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Herbert Xu <herbert@...dor.apana.org.au>
Cc:     Ard Biesheuvel <ardb@...nel.org>, Will Deacon <will@...nel.org>,
        Marc Zyngier <maz@...nel.org>, Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-mm@...ck.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org,
        "David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH 07/10] crypto: Use ARCH_DMA_MINALIGN instead of
 ARCH_KMALLOC_MINALIGN

On Thu, Apr 07, 2022 at 07:40:59PM +0800, Herbert Xu wrote:
> On Thu, Apr 07, 2022 at 12:01:02PM +0100, Catalin Marinas wrote:
> > The only issue is whether the compiler gets confused by a pointer to a
> > structure with a smaller alignment than alignof(struct ...). I don't see
> > a performance or correctness issue on arm64 here. It would be a problem
> > if instead of 16 we went down to 8 or 4 due to unaligned accesses but
> > from 128 to 64 (or even 16), I don't think it matters.
> 
> The issue is that there's code in the Crypto API which assumes
> that all pointers returned by kmalloc are aligned to CRYPTO_MINALIGN,
> if you break that then all that code would need to be modified.

I'm not familiar with the crypto API, trying to make sense of it now ;).

I can see in many cases that the kmalloc() caller aligns the requested
size to something like crypto_tfm_ctx_alignment(). So this would
guarantee a kmalloc() object aligned to CRYPTO_MINALIGN.

> However, I think it's better to change the code that assumes
> CRYPTO_MINALIGN guarantees DMA alignment.

I saw Ard already started to refactor some of these. But in the meantime
are there cases where the crypto code does a kmalloc() of less than
CRYPTO_MINALIGN and expects it to be CRYPTO_MINALIGN aligned?

-- 
Catalin