Re: [PATCH] crypto - shash: reduce minimum alignment of shash_desc structure

[Eric Biggers <ebiggers@kernel.org>]

On Thu, Jan 07, 2021 at 01:41:28PM +0100, Ard Biesheuvel wrote:
> Unlike many other structure types defined in the crypto API, the
> 'shash_desc' structure is permitted to live on the stack, which
> implies its contents may not be accessed by DMA masters. (This is
> due to the fact that the stack may be located in the vmalloc area,
> which requires a different virtual-to-physical translation than the
> one implemented by the DMA subsystem)
> 
> Our definition of CRYPTO_MINALIGN_ATTR is based on ARCH_KMALLOC_MINALIGN,
> which may take DMA constraints into account on architectures that support
> non-cache coherent DMA such as ARM and arm64. In this case, the value is
> chosen to reflect the largest cacheline size in the system, in order to
> ensure that explicit cache maintenance as required by non-coherent DMA
> masters does not affect adjacent, unrelated slab allocations. On arm64,
> this value is currently set at 128 bytes.
> 
> This means that applying CRYPTO_MINALIGN_ATTR to struct shash_desc is both
> unnecessary (as it is never used for DMA), and undesirable, given that it
> wastes stack space (on arm64, performing the alignment costs 112 bytes in
> the worst case, and the hole between the 'tfm' and '__ctx' members takes
> up another 120 bytes, resulting in an increased stack footprint of up to
> 232 bytes.) So instead, let's switch to the minimum SLAB alignment, which
> does not take DMA constraints into account.
> 
> Note that this is a no-op for x86.
> 
> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> ---
>  include/crypto/hash.h | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/include/crypto/hash.h b/include/crypto/hash.h
> index af2ff31ff619..13f8a6a54ca8 100644
> --- a/include/crypto/hash.h
> +++ b/include/crypto/hash.h
> @@ -149,7 +149,7 @@ struct ahash_alg {
>  
>  struct shash_desc {
>  	struct crypto_shash *tfm;
> -	void *__ctx[] CRYPTO_MINALIGN_ATTR;
> +	void *__ctx[] __aligned(ARCH_SLAB_MINALIGN);
>  };
>  
>  #define HASH_MAX_DIGESTSIZE	 64
> @@ -162,9 +162,9 @@ struct shash_desc {
>  
>  #define HASH_MAX_STATESIZE	512
>  
> -#define SHASH_DESC_ON_STACK(shash, ctx)				  \
> -	char __##shash##_desc[sizeof(struct shash_desc) +	  \
> -		HASH_MAX_DESCSIZE] CRYPTO_MINALIGN_ATTR; \
> +#define SHASH_DESC_ON_STACK(shash, ctx)					     \
> +	char __##shash##_desc[sizeof(struct shash_desc) + HASH_MAX_DESCSIZE] \
> +		__aligned(__alignof__(struct shash_desc));		     \
>  	struct shash_desc *shash = (struct shash_desc *)__##shash##_desc

Looks good to me, but it would be helpful if the comment above the definition of
CRYPTO_MINALIGN in include/linux/crypto.h was updated.

- Eric