Messages in this thread | | | Date | Sat, 30 Apr 2016 14:29:40 -0700 | From | Ben Greear <> | Subject | Re: [PATCH 3.2 085/115] veth: don’t modify ip summed; doing so treats packets with bad checks ums as good. |
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On 04/30/2016 02:13 PM, Vijay Pandurangan wrote: > On Sat, Apr 30, 2016 at 4:59 PM, Ben Greear <greearb@candelatech.com> wrote: >> >> >> On 04/30/2016 12:54 PM, Tom Herbert wrote: >>> >>> We've put considerable effort into cleaning up the checksum interface >>> to make it as unambiguous as possible, please be very careful to >>> follow it. Broken checksum processing is really hard to detect and >>> debug. >>> >>> CHECKSUM_UNNECESSARY means that some number of _specific_ checksums >>> (indicated by csum_level) have been verified to be correct in a >>> packet. Blindly promoting CHECKSUM_NONE to CHECKSUM_UNNECESSARY is >>> never right. If CHECKSUM_UNNECESSARY is set in such a manner but the >>> checksum it would refer to has not been verified and is incorrect this >>> is a major bug. >> >> >> Suppose I know that the packet received on a packet-socket has >> already been verified by a NIC that supports hardware checksumming. >> >> Then, I want to transmit it on a veth interface using a second >> packet socket. I do not want veth to recalculate the checksum on >> transmit, nor to validate it on the peer veth on receive, because I do >> not want to waste the CPU cycles. I am assuming that my app is not >> accidentally corrupting frames, so the checksum can never be bad. >> >> How should the checksumming be configured for the packets going into >> the packet-socket from user-space? > > > It seems like that only the receiver should decide whether or not to > checksum packets on the veth, not the sender. > > How about: > > We could add a receiving socket option for "don't checksum packets > received from a veth when the other side has marked them as > elide-checksum-suggested" (similar to UDP_NOCHECKSUM), and a sending > socket option for "mark all data sent via this socket to a veth as > elide-checksum-suggested". > > So the process would be: > > Writer: > 1. open read socket > 2. open write socket, with option elide-checksum-for-veth-suggested > 3. write data > > Reader: > 1. open read socket with "follow-elide-checksum-suggestions-on-veth" > 2. read data > > The kernel / module would then need to persist the flag on all packets > that traverse a veth, and drop these data when they leave the veth > module.
I'm not sure this works completely. In my app, the packet flow might be:
eth0 <-> raw-socket <-> user-space-bridge <-> raw-socket <-> vethA <-> vethB <-> [kernel router/bridge logic ...] <-> eth1
There may be no sockets on the vethB port. And reader/writer is not a good way to look at it since I am implementing a bi-directional bridge in user-space and each packet-socket is for both rx and tx.
>> Also, I might want to send raw frames that do have >> broken checksums (lets assume a real NIC, not veth), and I want them >> to hit the wire with those bad checksums. >> >> >> How do I configure the checksumming in this case? > > > Correct me if I'm wrong but I think this is already possible now. You > can have packets with incorrect checksum hitting the wire as is. What > you cannot do is instruct the receiving end to ignore the checksum > from the sending end when using a physical device (and something I > think we should mimic on the sending device).
Yes, it does work currently (or, last I checked)...I just want to make sure it keeps working.
Thanks, Ben
-- Ben Greear <greearb@candelatech.com> Candela Technologies Inc http://www.candelatech.com
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