Re: [dpdk-dev] [PATCH] net/tap: Allow all-zero checksum for UDP over IPv4

["Morten Brørup" <mb@smartsharesystems.com>]

> From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Ferruh Yigit
> Sent: Tuesday, November 10, 2020 3:47 PM
> 
> On 11/9/2020 2:22 PM, Michael Pfeiffer wrote:
> > Unlike TCP, UDP checksums are optional and may be zero to indicate "not
> > set" [RFC 768] (except for IPv6, where this prohibited [RFC 8200]). Add
> > this special case to the checksum offload emulation in net/tap.
> >
> > Signed-off-by: Michael Pfeiffer <michael.pfeiffer@tu-ilmenau.de>
> > ---
> >   drivers/net/tap/rte_eth_tap.c | 13 +++++++++++--
> >   1 file changed, 11 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/net/tap/rte_eth_tap.c
> b/drivers/net/tap/rte_eth_tap.c
> > index 2f8abb12c..e486b41c5 100644
> > --- a/drivers/net/tap/rte_eth_tap.c
> > +++ b/drivers/net/tap/rte_eth_tap.c
> > @@ -303,6 +303,7 @@ tap_verify_csum(struct rte_mbuf *mbuf)
> >   	uint16_t cksum = 0;
> >   	void *l3_hdr;
> >   	void *l4_hdr;
> > +	struct rte_udp_hdr *udp_hdr;
> >
> >   	if (l2 == RTE_PTYPE_L2_ETHER_VLAN)
> >   		l2_len += 4;
> > @@ -349,10 +350,18 @@ tap_verify_csum(struct rte_mbuf *mbuf)
> >   		/* Don't verify checksum for multi-segment packets. */
> >   		if (mbuf->nb_segs > 1)
> >   			return;
> > -		if (l3 == RTE_PTYPE_L3_IPV4)
> > +		if (l3 == RTE_PTYPE_L3_IPV4) {
> > +			if (l4 == RTE_PTYPE_L4_UDP) {
> > +				udp_hdr = (struct rte_udp_hdr *)l4_hdr;
> > +				if (udp_hdr->dgram_cksum == 0) {
> > +					mbuf->ol_flags |= PKT_RX_L4_CKSUM_NONE;
> > +					return;
> > +				}
> > +			}
> >   			cksum = ~rte_ipv4_udptcp_cksum(l3_hdr, l4_hdr);
> > -		else if (l3 == RTE_PTYPE_L3_IPV6)
> > +		} else if (l3 == RTE_PTYPE_L3_IPV6) {
> >   			cksum = ~rte_ipv6_udptcp_cksum(l3_hdr, l4_hdr);
> > +		}
> >   		mbuf->ol_flags |= cksum ?
> >   			PKT_RX_L4_CKSUM_BAD :
> >   			PKT_RX_L4_CKSUM_GOOD;
> >
> 
> While checking this I stuck with following part:
> 
>   cksum = ~rte_ipv6_udptcp_cksum(l3_hdr, l4_hdr);
>   ...
>    mbuf->ol_flags |= cksum ?
>    	PKT_RX_L4_CKSUM_BAD :
>   	PKT_RX_L4_CKSUM_GOOD;
> 
> 
> Is this correct, or am I missing something, can intention be '!' here
> instead of
> '~' ?

It is correct. The packet's checksum is calculated by rte_ipv6_udptcp_cksum(), and it should be 0xFFFF. The '~' operation makes cksum 0 iff the calculated checksum is 0xFFFF.