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

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Message-ID: <YllirUEhaxCDaV3X@arm.com>
Date:   Fri, 15 Apr 2022 13:18:53 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Ard Biesheuvel <ardb@...nel.org>
Cc:     Herbert Xu <herbert@...dor.apana.org.au>,
        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 10:05:21AM +0200, Ard Biesheuvel wrote:
> On Fri, 15 Apr 2022 at 09:52, Herbert Xu <herbert@...dor.apana.org.au> 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 guess that should be fixable. GIven that this is about padding
> rather than alignment, we could do something like
> 
> struct crypto_request {
>   union {
>       struct {
>         ... fields ...
>       };
>       u8 __padding[ARCH_DMA_MINALIGN];
>    };
>     void __ctx[]  __align(CRYPTO_MINALIGN);
> };
> 
> And then hopefully, we can get rid of the padding once we fix drivers
> doing non-cache coherent inbound DMA into those structures.

But if we keep CRYPTO_MINALIGN as 128, don't we get the padding
automatically?

struct crypto_request {
	...
	void *__ctx[] CRYPTO_MINALIGN_ATTR;
};

__alignof__(struct crypto_request) == 128;
sizeof(struct crypto_request) == N * 128

The same alignment and size is true for a structure like:

struct crypto_alg {
	...
} CRYPTO_MINALIGN_ATTR;

Any kmalloc() of sizeof(the above structures) will return a pointer
aligned to 128, irrespective of what ARCH_KMALLOC_MINALIGN is.

The problem is if you have a structure without any alignment attribute
(just ABI default), making its sizeof() smaller than ARCH_DMA_MINALIGN.
In this case kmalloc() could return a pointer aligned to something
smaller. Is this the case in the crypto code today? I can see it uses
the right alignment annotations already, no need for kmalloc() hacks.

-- 
Catalin