On Mon, Jun 27, 2016 at 3:53 PM, Cong Wang <xiyou.wangc...@gmail.com> wrote: > On Mon, Jun 27, 2016 at 3:04 PM, Tom Herbert <t...@herbertland.com> wrote: >> On Mon, Jun 27, 2016 at 2:49 PM, Cong Wang <xiyou.wangc...@gmail.com> wrote: >>> On Mon, Jun 27, 2016 at 2:47 PM, Tom Herbert <t...@herbertland.com> wrote: >>>> On Mon, Jun 27, 2016 at 2:44 PM, Cong Wang <xiyou.wangc...@gmail.com> >>>> wrote: >>>>> On Mon, Jun 27, 2016 at 2:08 PM, Or Gerlitz <gerlitz...@gmail.com> wrote: >>>>>> On Mon, Jun 27, 2016 at 9:22 PM, Cong Wang <xiyou.wangc...@gmail.com> >>>>>> wrote: >>>>>>> The stack doesn't trust the complete csum by hardware >>>>>>> even when it is correct. >>>>>> >>>>>> Can you explain that a little further? >>>>> >>>>> Sure, here is the code in __skb_checksum_complete(): >>>>> >>>>> /* skb->csum holds pseudo checksum */ >>>>> sum = csum_fold(csum_add(skb->csum, csum)); >>>>> if (likely(!sum)) { >>>>> if (unlikely(skb->ip_summed == CHECKSUM_COMPLETE) && >>>>> !skb->csum_complete_sw) >>>>> netdev_rx_csum_fault(skb->dev); >>>>> } >>>>> >>>>> So when sum == 0, it means the checksum is correct. And >>>>> we already set ->ip_summed to CHECKSUM_COMPLETE >>>>> after check_csum(), and ->csum_complete_sw is initialized >>>>> to 0 when we allocate the skb. This is why we trigger >>>>> netdev_rx_csum_fault(). >>>>> >>>> Yes, but this also means that the driver gave the stack a checksum >>>> complete value that was incorrect. That's an error. >>> >>> That is the whole purpose of commit f8c6455bb04b944edb69e, >>> isn't it? >> >> No. Unless you've uncovered some other bug, what is probably happening >> is that driver receives a packet with a checksum complete value. It >> records the value in the skbuff and marks it as CHECKSUM_COMPLETE. >> Subsequently, the stack tries to validate a transport layer checksum, >> and the validation fails (checksum does not sum to zero). The stack >> will then call __skb_checksum_complete from >> __skb_checksum_validate_complete. In this case the stack computes that >> transport checksum by hand and sees that transport checksum is valid-- >> so that means that the original value in checksum complete was not >> correct, it is not set to the computed checksum of the whole packet. >> This is an important error because it catches issues where checksum is >> not correctly being pulled up. > > I see, the comments in mlx4 driver said: > > /* Although the stack expects checksum which doesn't include the pseudo > * header, the HW adds it. To address that, we are subtracting the pseudo > * header checksum from the checksum value provided by the HW. > */ > > which seems imply it calculates a correct checksum for the whole > packet here, but the stack disagrees. Therefore skb->csum is not > still not what the stack expects. > Right, skb->csum is not what the stack expects. When it does the computation over the same data it arrives at a different value than what the driver sets. With this error pops that means the checksum in the packet is correct, but driver or something in the stack messed up skb->csum.
> Given skb_checksum_simple_validate() always pass a null pseudo > header, it looks like either the fix-up for pseudo header is not needed > at all for ICMP case, OR we need to call skb_checksum_validate() > for ICMPv4 case. Hmm... Pseudo header is not part of IPv4 checksum calculation so skb_checksum_simple_validate is correct. Seems like a good chance driver is doing fix-up wrong for ICMP. Tom