commit d55bef5059dd057bd077155375c581b49d25be7e
author Dimitris Michailidis <dmichail@google.com> Fri Oct 19 17:07:13 2018 -0700
committer David S. Miller <davem@davemloft.net> Sat Oct 20 01:13:42 2018 -0700
tree 56269ac3365bde89b860aac7f5224e7054c6b226
parent 48995423143a097527802e28d7add20e5a27677a

net: fix pskb_trim_rcsum_slow() with odd trim offset

We've been getting checksum errors involving small UDP packets, usually
59B packets with 1 extra non-zero padding byte. netdev_rx_csum_fault()
has been complaining that HW is providing bad checksums. Turns out the
problem is in pskb_trim_rcsum_slow(), introduced in commit 88078d98d1bb
("net: pskb_trim_rcsum() and CHECKSUM_COMPLETE are friends").

The source of the problem is that when the bytes we are trimming start
at an odd address, as in the case of the 1 padding byte above,
skb_checksum() returns a byte-swapped value. We cannot just combine this
with skb->csum using csum_sub(). We need to use csum_block_sub() here
that takes into account the parity of the start address and handles the
swapping.

Matches existing code in __skb_postpull_rcsum() and esp_remove_trailer().

Fixes: 88078d98d1bb ("net: pskb_trim_rcsum() and CHECKSUM_COMPLETE are friends")
Signed-off-by: Dimitris Michailidis <dmichail@google.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>