[PATCH] net: fix checksum with unaligned buffer

[Morten Brørup]

> From: Morten Brørup [mailto:mb at smartsharesystems.com]
> Sent: Friday, 17 June 2022 10.45
> 
> With this patch, the checksum can be calculated on an unligned part of
> a packet buffer.
> I.e. the buf parameter is no longer required to be 16 bit aligned.
> 
> The DPDK invariant that packet buffers must be 16 bit aligned remains
> unchanged.
> This invariant also defines how to calculate the 16 bit checksum on an
> unaligned part of a packet buffer.
> 
> Bugzilla ID: 1035
> Cc: stable at dpdk.org
> 
> Signed-off-by: Morten Brørup <mb at smartsharesystems.com>
> ---
>  lib/net/rte_ip.h | 17 +++++++++++++++--
>  1 file changed, 15 insertions(+), 2 deletions(-)
> 
> diff --git a/lib/net/rte_ip.h b/lib/net/rte_ip.h
> index b502481670..8e301d9c26 100644
> --- a/lib/net/rte_ip.h
> +++ b/lib/net/rte_ip.h
> @@ -162,9 +162,22 @@ __rte_raw_cksum(const void *buf, size_t len,
> uint32_t sum)
>  {
>  	/* extend strict-aliasing rules */
>  	typedef uint16_t __attribute__((__may_alias__)) u16_p;
> -	const u16_p *u16_buf = (const u16_p *)buf;
> -	const u16_p *end = u16_buf + len / sizeof(*u16_buf);
> +	const u16_p *u16_buf;
> +	const u16_p *end;
> +
> +	/* if buffer is unaligned, keeping it byte order independent */
> +	if (unlikely((uintptr_t)buf & 1)) {
> +		uint16_t first = 0;
> +		if (unlikely(len == 0))
> +			return 0;
> +		((unsigned char *)&first)[1] = *(const unsigned char *)buf;
> +		sum += first;
> +		buf = (const void *)((uintptr_t)buf + 1);
> +		len--;
> +	}
> 
> +	u16_buf = (const u16_p *)buf;
> +	end = u16_buf + len / sizeof(*u16_buf);
>  	for (; u16_buf != end; ++u16_buf)
>  		sum += *u16_buf;
> 
> --
> 2.17.1

@Emil, can you please test this patch with an unaligned buffer on your application to confirm that it produces the expected result.