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SubjectRe: [PATCH 1/2] crypto: ccp - Reduce stack frame size with KASAN
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On 03/28/2017 04:58 AM, Arnd Bergmann wrote:> The newly added AES GCM 
implementation uses one of the largest stack frames
> in the kernel, around 1KB on normal 64-bit kernels, and 1.6KB when
> CONFIG_KASAN
> is enabled:
>
> drivers/crypto/ccp/ccp-ops.c: In function 'ccp_run_aes_gcm_cmd':
> drivers/crypto/ccp/ccp-ops.c:851:1: error: the frame size of 1632 bytes
> is larger than 1536 bytes [-Werror=frame-larger-than=]
>
> This is problematic for multiple reasons:
>
> - The crypto functions are often used in deep call chains, e.g. behind
> mm, fs and dm layers, making it more likely to run into an actual stack
> overflow
>
> - Using this much stack space is an indicator that the code is not
> written to be as efficient as it could be.

I'm not sure I agree that A -> B, but I will certainly look into this.

> - While this goes unnoticed at the moment in mainline with the frame size
> warning being disabled when KASAN is in use, I would like to enable
> the warning again, and the current code is slightly above my arbitrary
> pick for a limit of 1536 bytes (I already did patches for every other
> driver exceeding this).

I've got my stack frame size (also) set to 1536, and would have paid
more attention
had a warning occurred due to my code.

> A more drastic refactoring of the driver might be needed to reduce the
> stack usage more substantially, but this patch is fairly simple and
> at least addresses the third one of the problems I mentioned, reducing the
> stack size by about 150 bytes and bringing it below the warning limit
> I picked.

Again, I'll devote some time to this.

> diff --git a/drivers/crypto/ccp/ccp-dev.h b/drivers/crypto/ccp/ccp-dev.h
> index 3a45c2af2fbd..c5ea0796a891 100644
> --- a/drivers/crypto/ccp/ccp-dev.h
> +++ b/drivers/crypto/ccp/ccp-dev.h
> @@ -432,24 +432,24 @@ struct ccp_dma_info {
> unsigned int offset;
> unsigned int length;
> enum dma_data_direction dir;
> -};
> +} __packed __aligned(4);

My gcc 4.8 doesn't understand __aligned(). Shouldn't we use
#pragma(4) here?


> struct ccp_dm_workarea {
> struct device *dev;
> struct dma_pool *dma_pool;
> - unsigned int length;
>
> u8 *address;
> struct ccp_dma_info dma;
> + unsigned int length;
> };
>
> struct ccp_sg_workarea {
> struct scatterlist *sg;
> int nents;
> + unsigned int dma_count;
>
> struct scatterlist *dma_sg;
> struct device *dma_dev;
> - unsigned int dma_count;
> enum dma_data_direction dma_dir;
>
> unsigned int sg_used;

I'm okay with rearranging, but I'm going to submit an alternative patch.

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