RE: [PATCH V2 net-next] net: hns: Fix to conditionally convey RX checksum flag to stack

[Salil Mehta]

> -----Original Message-----
> From: David Miller [mailto:davem@xxxxxxxxxxxxx]
> Sent: Wednesday, November 30, 2016 7:26 PM
> To: Salil Mehta
> Cc: Zhuangyuzeng (Yisen); mehta.salil.lnk@xxxxxxxxx;
> netdev@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; Linuxarm
> Subject: Re: [PATCH V2 net-next] net: hns: Fix to conditionally convey
> RX checksum flag to stack
> 
> From: Salil Mehta <salil.mehta@xxxxxxxxxx>
> Date: Tue, 29 Nov 2016 13:09:45 +0000
> 
> > +	/* We only support checksum for IPv4,UDP(over IPv4 or IPv6),
> TCP(over
> > +	 * IPv4 or IPv6) and SCTP but we support many L3(IPv4, IPv6,
> MPLS,
> > +	 * PPPoE etc) and L4(TCP, UDP, GRE, SCTP, IGMP, ICMP etc.)
> protocols.
> > +	 * We want to filter out L3 and L4 protocols early on for which
> checksum
> > +	 * is not supported.
>  ...
> > +	 */
> > +	l3id = hnae_get_field(flag, HNS_RXD_L3ID_M, HNS_RXD_L3ID_S);
> > +	l4id = hnae_get_field(flag, HNS_RXD_L4ID_M, HNS_RXD_L4ID_S);
> > +	if ((l3id != HNS_RX_FLAG_L3ID_IPV4) &&
> > +	    ((l3id != HNS_RX_FLAG_L3ID_IPV6) ||
> > +	     (l4id != HNS_RX_FLAG_L4ID_UDP)) &&
> > +	    ((l3id != HNS_RX_FLAG_L3ID_IPV6) ||
> > +	     (l4id != HNS_RX_FLAG_L4ID_TCP)) &&
> > +	    (l4id != HNS_RX_FLAG_L4ID_SCTP))
> > +		return;
> 
> I have a hard time understanding this seemingly overcomplicated
> check.
> 
> It looks like if L3 is IPV4 it will accept any underlying L4 protocol,
> but is that what is really intended?  That doesn't match what this new
> comment states.
I agree that it is bit difficult to read. Earlier, I was banking on the
register(mistakenly, its hardware implementation err ) to de-multiplex
the checksum error type. The register supported indication of just IPv4
Header Checksum Error as well (which meant it could carry any L4 protocol).
IPv6 does not have similar Header checksum error. Therefore, to check
if it is just IPv4 Header checksum error or any supported L4 transport
(UDP or TCP) error over IPv4 or IPv6 I had to bank upon above complex
check

Below suggested solution check would have been insufficient for
example, if packet had IPv4/IGMP and there was a checksum error in IPv4
header.

Comment states:
" We only support checksum for IPv4, UDP(over IPv4 or IPv6), 
  TCP(over IPv4 or IPv6) and SCTP"
   1) Checksum of IPv4 (IPv4 header)
   2) UDP(over IPv4 or IPv6)
   3) TCP(over IPv4 or IPv6)
   4) SCTP (over IPv4 or IPv6)*

(*) I should have put IPv4/IPv6 check for SCTP in the code
    and made it clear in the comment as well?

> 
> My understanding is that the chip supports checksums for:
> 
> 	UDP/IPV4
> 	UDP/IPV6
> 	TCP/IPV4
> 	TCP/IPV6
> 	SCTP/IPV4
> 	SCTP/IPV6

Hardware also supports checksum of IPv4 Header.

> 
> So the simplest thing is to validate each level one at a time:
> 
> 	if (l3 != IPV4 && l3 != IPV6)
> 		return;
> 	if (l4 != UDP && l4 != TCP && l4 != SCTP)
> 		return;
I guess above check will fail to catch cases like IPv4/IGMP, when there
is a bad checksum in The IPv4 header. 

But maybe now since we don't have any method to de-multiplex the kind of
checksum error (cannot depend upon register) we can have below code
re-arrangement:

hns_nic_rx_checksum() {
      /* check supported L3 protocol */
	if (l3 != IPV4 && l3 != IPV6)
		return;
      /* check if L3 protocols error */
      if (l3e)
	 	return;

      /* check if the packets are fragmented */
	If (l3frags)
		Return;

      /* check supported L4 protocol */
 	if (l4 != UDP && l4 != TCP && l4 != SCTP)
 		return;
      /* check if any L4 protocol error */
      if (l3e)
	 	return;

      /* packet with valid checksum - covey to stack */
      skb->ip_summed = CHECKSUM_UNNECESSARY
}

Hope I am not missing something here. Please correct my understanding
if there is any gap here. Thanks!

Best regards
Salil