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

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Message-ID: <Yk/8QExHlggU8KgC@gondor.apana.org.au>
Date:   Fri, 8 Apr 2022 17:11:28 +0800
From:   Herbert Xu <herbert@...dor.apana.org.au>
To:     Catalin Marinas <catalin.marinas@....com>
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 Fri, Apr 08, 2022 at 10:04:54AM +0100, Catalin Marinas wrote:
>
> My point is that if the crypto code kmallocs a size aligned to
> crypto_tfm_ctx_alignment() (and CRYPTO_MINALIGN), the slab allocator
> will return memory aligned to CRYPTO_MINALIGN even if
> ARCH_KMALLOC_MINALIGN is smaller.

No we don't align the size to CRYPTO_MINALIGN at all.  We simply
assume that this is the alignment returned by kmalloc.

> Would the crypto code, say, do a kmalloc(64) and expect a 128 byte
> alignment (when CRYPTO_MINALIGN == 128)? Or does it align the size to
> CRYPTO_MINALIGN and do a kmalloc(128) directly? If it's the latter, I
> don't think there's a problem.

It's the former.

I think you can still make the change you want, but first you need
to modify the affected drivers to specify their actual alignment
requirement explicitly through cra_alignmask and then use the
correct methods to access the context pointer.

Basically these drivers have been broken from day one, but their
brokenness has been hidden by the extra-large KMALLOC_MINALIGN
value on arm.  So to reduce the KMALLOC_MINALIGN value, you have
to modify the drivers and set the cra_alignmask value.

Cheers,
-- 
Email: Herbert Xu <herbert@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt