Re: [PATCH 07/10] crypto: Use ARCH_DMA_MINALIGN instead of ARCH_KMALLOC_MINALIGN

[Catalin Marinas <catalin.marinas@arm.com>]

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