netdev - Re: UDP implementation and the MSG

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Date:   Tue, 26 Jan 2021 22:25:53 -0500
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     oliver.graute@...il.com
Cc:     Network Development <netdev@...r.kernel.org>,
        Alexey Kuznetsov <kuznet@....inr.ac.ru>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        Jakub Sitnicki <jakub@...udflare.com>,
        Paolo Abeni <pabeni@...hat.com>, sagi@...htbitslabs.com,
        Christoph Hellwig <hch@....de>
Subject: Re: UDP implementation and the MSG_MORE flag

On Tue, Jan 26, 2021 at 5:00 PM Willem de Bruijn
<willemdebruijn.kernel@...il.com> wrote:
>
> On Tue, Jan 26, 2021 at 4:54 PM Willem de Bruijn
> <willemdebruijn.kernel@...il.com> wrote:
> >
> > On Tue, Jan 26, 2021 at 9:58 AM Oliver Graute <oliver.graute@...il.com> wrote:
> > >
> > > Hello,
> > >
> > > we observe some unexpected behavior in the UDP implementation of the
> > > linux kernel.
> > >
> > > Some UDP packets send via the loopback interface are dropped in the
> > > kernel on the receive side when using sendto with the MSG_MORE flag.
> > > Every drop increases the InCsumErrors in /proc/self/net/snmp. Some
> > > example code to reproduce it is appended below.
> > >
> > > In the code we tracked it down to this code section. ( Even a little
> > > further but its unclear to me wy the csum() is wrong in the bad case)
> > >
> > > udpv6_recvmsg()
> > > ...
> > > if (checksum_valid || udp_skb_csum_unnecessary(skb)) {
> > >                 if (udp_skb_is_linear(skb))
> > >                         err = copy_linear_skb(skb, copied, off, &msg->msg_iter);
> > >                 else
> > >                         err = skb_copy_datagram_msg(skb, off, msg, copied);
> > >         } else {
> > >                 err = skb_copy_and_csum_datagram_msg(skb, off, msg);
> > >                 if (err == -EINVAL) {
> > >                         goto csum_copy_err;
> > >                 }
> > >         }
> > > ...
> > >
> >
> > Thanks for the report with a full reproducer.
> >
> > I don't have a full answer yet, but can reproduce this easily.
> >
> > The third program, without MSG_MORE, builds an skb with
> > CHECKSUM_PARTIAL in __ip_append_data. When looped to the receive path
> > that ip_summed means no additional validation is needed. As encoded in
> > skb_csum_unnecessary.
> >
> > The first and second programs are essentially the same, bar for a
> > slight difference in length. In both cases packet length is very short
> > compared to the loopback device MTU. Because of MSG_MORE, these
> > packets have CHECKSUM_NONE.
> >
> > On receive in
> >
> >   __udp4_lib_rcv()
> >     udp4_csum_init()
> >       err = skb_checksum_init_zero_check()
> >
> > The second program validates and sets ip_summed = CHECKSUM_COMPLETE
> > and csum_valid = 1.
> > The first does not, though err == 0.
> >
> > This appears to succeed consistently for packets <= 68B of payload,
> > fail consistently otherwise. It is not clear to me yet what causes
> > this distinction.
>
> This is from
>
> "
> /* For small packets <= CHECKSUM_BREAK perform checksum complete directly
>  * in checksum_init.
>  */
> #define CHECKSUM_BREAK 76
> "
>
> So the small packet gets checksummed immediately in
> __skb_checksum_validate_complete, but the larger one does not.
>
> Question is why the copy_and_checksum you pointed to seems to fail checksum.

Manually calling __skb_checksum_complete(skb) in
skb_copy_and_csum_datagram_msg succeeds, so it is the
skb_copy_and_csum_datagram that returns an incorrect csum.

Bisection shows that this is a regression in 5.0, between

65d69e2505bb datagram: introduce skb_copy_and_hash_datagram_iter helper (fail)
d05f443554b3 iov_iter: introduce hash_and_copy_to_iter helper
950fcaecd5cc datagram: consolidate datagram copy to iter helpers
cb002d074dab iov_iter: pass void csum pointer to csum_and_copy_to_iter (pass)

That's a significant amount of code change. I'll take a closer look,
but checkpointing state for now..