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

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Date:   Fri, 15 Apr 2022 10:51:40 +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 Memory Management List <linux-mm@...ck.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <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 15, 2022 at 03:51:54PM +0800, Herbert Xu wrote:
> On Fri, Apr 15, 2022 at 09:49:12AM +0200, Ard Biesheuvel wrote:
> > I'm not sure I understand what would go wrong if that assumption no
> > longer holds.
> 
> It's very simple, we don't do anything to the pointer returned
> by kmalloc before returning it as a tfm or other object with
> an alignment of CRYPTO_MINALIGN.  IOW if kmalloc starts returning
> pointers that are not aligned to CRYPTO_MINALIGN then we'd be
> lying to the compiler.

I agree that it would be lying to the compiler, but I don't think this
matters for arm64 where the CPU can do unaligned accesses just fine. We
don't even end up with unaligned accesses here. Let's say we have:

struct x {
	...
} __attribute__ ((__aligned__ (128)));

and the kmalloc(sizeof(struct x)) returns a 64-byte aligned pointer. The
compiler-generated code won't have any problem on arm64 accessing the
struct x members. As I said a few times, it's not affecting any other
architecture and not breaking arm64 either.

Anyway, let's agree to disagree. I'll look into keeping CRYPTO_MINALIGN
as ARCH_KMALLOC_MINALIGN and introduce a CRYPTO_DMA_MINALIGN (or just
use ARCH_DMA_MINALIGN directly) together with something like Linus'
dma_kmalloc() in places where an object aligned to ARCH_DMA_MINALIGN is
needed in the crypto code.

-- 
Catalin