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

[Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>]

On 6/8/21 3:29 PM, Olivier Matz wrote:
> Hi Ferruh, Andrew,
> 
> On Tue, Jun 08, 2021 at 01:23:33PM +0300, Andrew Rybchenko wrote:
>> On 4/30/21 6:42 PM, Ferruh Yigit wrote:
>>> 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.
> 
> rte_ipv4_udptcp_cksum() cannot return 0xFFFF: this is only possible if
> all data is 0. Before verifying a udp packet, the user must check that
> it is not 0 (which means no checksum). In tcp, "Data offset" is never
> 0. Moreover, port=0 is a reserved value for both udp and tcp.
> 
>>> 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.
>>
>> Yes, it looks suspicious to me as well.
>>
>> Olivier, could you clarify, please.
> 
> Before commit d5df2ae0428a ("net: fix unneeded replacement of TCP checksum 0"),
> the behavior was:
> 
>   // rte_ipv4_udptcp_cksum() is 0xffff if checksum is valid
>   // so cksum is 0 if checksum is valid
>   cksum = ~rte_ipv4_udptcp_cksum(l3_hdr, l4_hdr);
>   // ol_flags is set to PKT_RX_L4_CKSUM_GOOD if checksum is valid
>   mbuf->ol_flags |= cksum ? PKT_RX_L4_CKSUM_BAD : PKT_RX_L4_CKSUM_GOOD;
> 
> After commit d5df2ae0428a ("net: fix unneeded replacement of TCP checksum 0"),
> it is broken:
> 
>   // rte_ipv4_udptcp_cksum() is 0 (tcp) or 0xffff (udp) if checksum is valid
>   // so cksum is 0xffff (tcp) or 0 (udp) if checksum is valid
>   cksum = ~rte_ipv4_udptcp_cksum(l3_hdr, l4_hdr);
>   // ol_flags is set to BAD (tcp) or GOOD (udp) if checksum is valid
>   mbuf->ol_flags |= cksum ? PKT_RX_L4_CKSUM_BAD : PKT_RX_L4_CKSUM_GOOD;
> 
> After patch 2/4 ("net/tap: fix Rx cksum flags on TCP packets"), the
> correct behavior is restored:
> 
>   // cksum is 0 (tcp) or 0xffff (udp) if checksum is valid
>   // note: 0xffff for tcp cannot happen (there is at least 1 bit set in the header)
>   // note: 0 for udp cannot happen (it is replaced by in rte_ipv4_udptcp_cksum())
>   cksum = rte_ipv4_udptcp_cksum(l3_hdr, l4_hdr);
>   // cksum_ok is 1 if checksum is valid
>   cksum_ok = (cksum == 0) || (cksum == 0xffff);
>   // ol_flags is set to GOOD if checksum is valid
>   mbuf->ol_flags |= cksum_ok ? PKT_RX_L4_CKSUM_GOOD : PKT_RX_L4_CKSUM_BAD;
> 
> After this patch [3/4] ("net: introduce functions to verify L4 checksums"),
> it is simplified by using rte_ipv4_udptcp_cksum_verify():
> 
>   // cksum_ok is 1 if checksum is valid
>   cksum_ok = !rte_ipv4_udptcp_cksum_verify(l3_hdr, l4_hdr);
>   // ol_flags is set to GOOD if checksum is valid
>   mbuf->ol_flags |= cksum_ok ? PKT_RX_L4_CKSUM_GOOD : PKT_RX_L4_CKSUM_BAD;
> 

Many thanks for the detailed explanations. It replies to all my
questions (even not asked, but kept in my head).

Andrew.

> Olivier
> 
>>
>>>>  }
>>>>  
>>>>  /**
>>>> @@ -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.
>>>
>>