Re: [v2 PATCH 2/3] crypto: scatterwalk - Add memcpy_sglist

[Eric Biggers <ebiggers@kernel.org>]

On Fri, Mar 07, 2025 at 11:36:19AM +0800, Herbert Xu wrote:
> Add memcpy_sglist which copies one SG list to another.
> 
> Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
> ---
>  crypto/scatterwalk.c         | 27 +++++++++++++++++++++++++++
>  include/crypto/scatterwalk.h |  3 +++
>  2 files changed, 30 insertions(+)
> 
> diff --git a/crypto/scatterwalk.c b/crypto/scatterwalk.c
> index 20a28c6d94da..8225801488d5 100644
> --- a/crypto/scatterwalk.c
> +++ b/crypto/scatterwalk.c
> @@ -86,6 +86,33 @@ void memcpy_to_sglist(struct scatterlist *sg, unsigned int start,
>  }
>  EXPORT_SYMBOL_GPL(memcpy_to_sglist);
>  
> +void memcpy_sglist(struct scatterlist *dst, struct scatterlist *src,
> +		   unsigned int nbytes)
> +{
> +	struct scatter_walk swalk;
> +	struct scatter_walk dwalk;
> +
> +	if (unlikely(nbytes == 0)) /* in case sg == NULL */
> +		return;
> +
> +	scatterwalk_start(&swalk, src);
> +	scatterwalk_start(&dwalk, dst);
> +
> +	do {
> +		unsigned int slen, dlen;
> +		unsigned int len;
> +
> +		slen = scatterwalk_next(&swalk, nbytes);
> +		dlen = scatterwalk_next(&dwalk, nbytes);
> +		len = min(slen, dlen);
> +		memcpy(dwalk.addr, swalk.addr, len);
> +		scatterwalk_done_dst(&dwalk, len);
> +		scatterwalk_done_src(&swalk, len);
> +		nbytes -= len;
> +	} while (nbytes);
> +}
> +EXPORT_SYMBOL_GPL(memcpy_sglist);

Actually this new function is useless as-is, since it invokes undefined behavior
when the source and destination coincide (which can happen even when src ==
dst), and all the potential callers need to handle that case.  I'm working on a
fixed version.

- Eric