Re: [dpdk-dev] [PATCH 3/4] net: introduce functions to verify L4 checksums

[Ferruh Yigit <ferruh.yigit@intel.com>]

On 4/27/2021 2:57 PM, Olivier Matz wrote:
> Since commit d5df2ae0428a ("net: fix unneeded replacement of TCP
> checksum 0"), the functions rte_ipv4_udptcp_cksum() and
> rte_ipv6_udptcp_cksum() can return either 0x0000 or 0xffff when used to
> verify a packet containing a valid checksum.
> 
> Since these functions should be used to calculate the checksum to set in
> a packet, introduce 2 new helpers for checksum verification. They return
> 0 if the checksum is valid in the packet.
> 
> Use this new helper in net/tap driver.
> 
> Signed-off-by: Olivier Matz <olivier.matz@6wind.com>
> ---
>  drivers/net/tap/rte_eth_tap.c |   7 +-
>  lib/net/rte_ip.h              | 124 +++++++++++++++++++++++++++-------
>  2 files changed, 104 insertions(+), 27 deletions(-)
> 
> diff --git a/drivers/net/tap/rte_eth_tap.c b/drivers/net/tap/rte_eth_tap.c
> index 71282e8065..b14d5a1d55 100644
> --- a/drivers/net/tap/rte_eth_tap.c
> +++ b/drivers/net/tap/rte_eth_tap.c
> @@ -365,11 +365,12 @@ tap_verify_csum(struct rte_mbuf *mbuf)
>  					return;
>  				}
>  			}
> -			cksum = rte_ipv4_udptcp_cksum(l3_hdr, l4_hdr);
> +			cksum_ok = !rte_ipv4_udptcp_cksum_verify(l3_hdr,
> +								 l4_hdr);
>  		} else { /* l3 == RTE_PTYPE_L3_IPV6, checked above */
> -			cksum = rte_ipv6_udptcp_cksum(l3_hdr, l4_hdr);
> +			cksum_ok = !rte_ipv6_udptcp_cksum_verify(l3_hdr,
> +								 l4_hdr);
>  		}
> -		cksum_ok = (cksum == 0) || (cksum == 0xffff);
>  		mbuf->ol_flags |= cksum_ok ?
>  			PKT_RX_L4_CKSUM_GOOD : PKT_RX_L4_CKSUM_BAD;
>  	}
> diff --git a/lib/net/rte_ip.h b/lib/net/rte_ip.h
> index 8c189009b0..ef84bcc5bf 100644
> --- a/lib/net/rte_ip.h
> +++ b/lib/net/rte_ip.h
> @@ -344,20 +344,10 @@ rte_ipv4_phdr_cksum(const struct rte_ipv4_hdr *ipv4_hdr, uint64_t ol_flags)
>  }
>  
>  /**
> - * Process the IPv4 UDP or TCP checksum.
> - *
> - * The IP and layer 4 checksum must be set to 0 in the packet by
> - * the caller.
> - *
> - * @param ipv4_hdr
> - *   The pointer to the contiguous IPv4 header.
> - * @param l4_hdr
> - *   The pointer to the beginning of the L4 header.
> - * @return
> - *   The complemented checksum to set in the IP packet.
> + * @internal Calculate the non-complemented IPv4 L4 checksum
>   */
>  static inline uint16_t
> -rte_ipv4_udptcp_cksum(const struct rte_ipv4_hdr *ipv4_hdr, const void *l4_hdr)
> +__rte_ipv4_udptcp_cksum(const struct rte_ipv4_hdr *ipv4_hdr, const void *l4_hdr)
>  {
>  	uint32_t cksum;
>  	uint32_t l3_len, l4_len;
> @@ -374,16 +364,62 @@ rte_ipv4_udptcp_cksum(const struct rte_ipv4_hdr *ipv4_hdr, const void *l4_hdr)
>  	cksum += rte_ipv4_phdr_cksum(ipv4_hdr, 0);
>  
>  	cksum = ((cksum & 0xffff0000) >> 16) + (cksum & 0xffff);
> -	cksum = (~cksum) & 0xffff;
> +
> +	return (uint16_t)cksum;
> +}
> +
> +/**
> + * Process the IPv4 UDP or TCP checksum.
> + *
> + * The IP and layer 4 checksum must be set to 0 in the packet by
> + * the caller.
> + *
> + * @param ipv4_hdr
> + *   The pointer to the contiguous IPv4 header.
> + * @param l4_hdr
> + *   The pointer to the beginning of the L4 header.
> + * @return
> + *   The complemented checksum to set in the IP packet.
> + */
> +static inline uint16_t
> +rte_ipv4_udptcp_cksum(const struct rte_ipv4_hdr *ipv4_hdr, const void *l4_hdr)
> +{
> +	uint16_t cksum = __rte_ipv4_udptcp_cksum(ipv4_hdr, l4_hdr);
> +
> +	cksum = ~cksum;
> +
>  	/*
> -	 * Per RFC 768:If the computed checksum is zero for UDP,
> +	 * Per RFC 768: If the computed checksum is zero for UDP,
>  	 * it is transmitted as all ones
>  	 * (the equivalent in one's complement arithmetic).
>  	 */
>  	if (cksum == 0 && ipv4_hdr->next_proto_id == IPPROTO_UDP)
>  		cksum = 0xffff;
>  
> -	return (uint16_t)cksum;
> +	return cksum;
> +}
> +
> +/**
> + * Validate the IPv4 UDP or TCP checksum.
> + *
> + * @param ipv4_hdr
> + *   The pointer to the contiguous IPv4 header.
> + * @param l4_hdr
> + *   The pointer to the beginning of the L4 header.
> + * @return
> + *   Return 0 if the checksum is correct, else -1.
> + */
> +__rte_experimental
> +static inline int
> +rte_ipv4_udptcp_cksum_verify(const struct rte_ipv4_hdr *ipv4_hdr,
> +			     const void *l4_hdr)
> +{
> +	uint16_t cksum = __rte_ipv4_udptcp_cksum(ipv4_hdr, l4_hdr);
> +
> +	if (cksum != 0xffff)
> +		return -1;
> +
> +	return 0;

There is behavior change in tap verify, I am asking just to verify if expected,

Previously for UDP, if calculated checksum is '0', the 'rte_ipv4_udptcp_cksum()'
returns 0xFFFF.
And 0xFFFF is taken as good checksum by tap PMD.

With new 'rte_ipv4_udptcp_cksum_verify()', if calculated checksum is '0' it will
be taken as bad checksum.

I don't know if calculated checksum with valid checksum in place can return 0.


Also for TCP, 'rte_ipv4_udptcp_cksum_verify()' doesn't have inversion (cksum =
~cksum;) seems changing pass/fail status of the checksum, unless I am not
missing anything here.


>  }
>  
>  /**
> @@ -448,6 +484,25 @@ rte_ipv6_phdr_cksum(const struct rte_ipv6_hdr *ipv6_hdr, uint64_t ol_flags)
>  	return __rte_raw_cksum_reduce(sum);
>  }
>  
> +/**
> + * @internal Calculate the non-complemented IPv4 L4 checksum
> + */
> +static inline uint16_t
> +__rte_ipv6_udptcp_cksum(const struct rte_ipv6_hdr *ipv6_hdr, const void *l4_hdr)
> +{
> +	uint32_t cksum;
> +	uint32_t l4_len;
> +
> +	l4_len = rte_be_to_cpu_16(ipv6_hdr->payload_len);
> +
> +	cksum = rte_raw_cksum(l4_hdr, l4_len);
> +	cksum += rte_ipv6_phdr_cksum(ipv6_hdr, 0);
> +
> +	cksum = ((cksum & 0xffff0000) >> 16) + (cksum & 0xffff);
> +
> +	return (uint16_t)cksum;
> +}
> +
>  /**
>   * Process the IPv6 UDP or TCP checksum.
>   *
> @@ -464,16 +519,10 @@ rte_ipv6_phdr_cksum(const struct rte_ipv6_hdr *ipv6_hdr, uint64_t ol_flags)
>  static inline uint16_t
>  rte_ipv6_udptcp_cksum(const struct rte_ipv6_hdr *ipv6_hdr, const void *l4_hdr)
>  {
> -	uint32_t cksum;
> -	uint32_t l4_len;
> -
> -	l4_len = rte_be_to_cpu_16(ipv6_hdr->payload_len);
> +	uint16_t cksum = __rte_ipv6_udptcp_cksum(ipv6_hdr, l4_hdr);
>  
> -	cksum = rte_raw_cksum(l4_hdr, l4_len);
> -	cksum += rte_ipv6_phdr_cksum(ipv6_hdr, 0);
> +	cksum = ~cksum;
>  
> -	cksum = ((cksum & 0xffff0000) >> 16) + (cksum & 0xffff);
> -	cksum = (~cksum) & 0xffff;
>  	/*
>  	 * Per RFC 768: If the computed checksum is zero for UDP,
>  	 * it is transmitted as all ones
> @@ -482,7 +531,34 @@ rte_ipv6_udptcp_cksum(const struct rte_ipv6_hdr *ipv6_hdr, const void *l4_hdr)
>  	if (cksum == 0 && ipv6_hdr->proto == IPPROTO_UDP)
>  		cksum = 0xffff;
>  
> -	return (uint16_t)cksum;
> +	return cksum;
> +}
> +
> +/**
> + * Validate the IPv6 UDP or TCP checksum.
> + *
> + * The function accepts a 0 checksum, since it can exceptionally happen. See 8.1
> + * (Upper-Layer Checksums) in RFC 8200.
> + *
> + * @param ipv6_hdr
> + *   The pointer to the contiguous IPv6 header.
> + * @param l4_hdr
> + *   The pointer to the beginning of the L4 header.
> + * @return
> + *   Return 0 if the checksum is correct, else -1.
> + */
> +__rte_experimental
> +static inline int
> +rte_ipv6_udptcp_cksum_verify(const struct rte_ipv6_hdr *ipv6_hdr,
> +			     const void *l4_hdr)
> +{
> +	uint16_t cksum;
> +
> +	cksum = __rte_ipv6_udptcp_cksum(ipv6_hdr, l4_hdr);
> +	if (cksum != 0xffff)
> +		return -1;
> +
> +	return 0;
>  }

Nitpicking but, 'rte_ipv4_udptcp_cksum_verify()' is almost same with this
function ('rte_ipv6_udptcp_cksum_verify()') but they have different line
spacing, can be good to have similar syntax for both.