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Date:   Fri, 15 Apr 2022 13:31:32 +0100
From:   Catalin Marinas <catalin.marinas@arm.com>
To:     Herbert Xu <herbert@gondor.apana.org.au>
Cc:     Ard Biesheuvel <ardb@kernel.org>, Will Deacon <will@kernel.org>,
        Marc Zyngier <maz@kernel.org>, Arnd Bergmann <arnd@arndb.de>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Linux Memory Management List <linux-mm@kvack.org>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        "David S. Miller" <davem@davemloft.net>
Subject: Re: [PATCH 07/10] crypto: Use ARCH_DMA_MINALIGN instead of
 ARCH_KMALLOC_MINALIGN
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On Fri, Apr 15, 2022 at 10:51:40AM +0100, Catalin Marinas wrote:
> 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.

This needs a clarification. For the above structure, kmalloc() will
return a 128-byte aligned pointer since sizeof(x) is a multiple of 128.
The potential problem is if you have something like:

	kmalloc(sizeof(struct x) + 64);

The above could end up as a kmalloc(192) which is available with an
ARCH_KMALLOC_MINALIGN of 64. If that's a real use-case, I can change the
slab patch to not create the 192 (or 48 if we go for an even smaller
ARCH_KMALLOC_MINALIGN) caches and we'd always have ARCH_DMA_MINALIGN
guarantee if the structure itself is correctly aligned. No lying to the
compiler.

-- 
Catalin