netdev - XDP breakage with virtio due to 6870de435b90c083ae0f3f7f341287976ef56f03

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Date:   Sun, 22 Apr 2018 16:47:48 -0600
From:   David Ahern <dsahern@...il.com>
To:     tehnerd@...nerd.com, Jason Wang <jasowang@...hat.com>,
        daniel@...earbox.net
Cc:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: XDP breakage with virtio due to
 6870de435b90c083ae0f3f7f341287976ef56f03

This commit breaks my FIB forwarding program:

commit 6870de435b90c083ae0f3f7f341287976ef56f03
Author: Nikita V. Shirokov <tehnerd@...nerd.com>
Date:   Tue Apr 17 21:42:20 2018 -0700

    bpf: make virtio compatible w/ bpf_xdp_adjust_tail

    w/ bpf_xdp_adjust_tail helper xdp's data_end pointer could be changed as
    well (only "decrease" of pointer's location is going to be supported).
    changing of this pointer will change packet's size.
    for virtio driver we need to adjust XDP_PASS handling by recalculating
    length of the packet if it was passed to the TCP/IP stack

    Reviewed-by: Jason Wang <jasowang@...hat.com>
    Signed-off-by: Nikita V. Shirokov <tehnerd@...nerd.com>
    Signed-off-by: Daniel Borkmann <daniel@...earbox.net>

###

Some of the packets (e.g., ARP or those without a resolved neighbor) are
passed to the networking stack. What shows up are clearly broken packets:

# tcpdump -n -i eth1
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth1, link-type EN10MB (Ethernet), capture size 262144 bytes
15:45:29.693238 [|ARP]
	0x0000:  0001 0800 0604 0001 42c0 ce2f 3fa9 0a64  ........B../?..d
15:45:30.710327 [|ARP]
	0x0000:  0001 0800 0604 0001 42c0 ce2f 3fa9 0a64  ........B../?..d
15:45:31.734296 [|ARP]
	0x0000:  0001 0800 0604 0001 42c0 ce2f 3fa9 0a64  ........B../?..d
15:45:32.908720 IP6 truncated-ip6 - 12 bytes
missing!fe80::40c0:ceff:fe2f:3fa9 > ff02::1:ff00:2: ICMP6, neighbor
solicitation[|icmp6]
15:45:33.910530 IP6 truncated-ip6 - 12 bytes missing!2001:db8:1::64 >
ff02::1:ff00:2: ICMP6, neighbor solicitation[|icmp6]
15:45:34.934437 IP6 truncated-ip6 - 12 bytes missing!2001:db8:1::64 >
ff02::1:ff00:2: ICMP6, neighbor solicitation[|icmp6]
15:45:35.958394 IP6 truncated-ip6 - 12 bytes missing!2001:db8:1::64 >
ff02::1:ff00:2: ICMP6, neighbor solicitation[|icmp6]

Reverting the mentioned patch fixes the problem.