netdev - Re: [PATCH net] net: fix pskb_trim_rcsum

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Message-ID: <a297644f-e3d5-779e-3498-e145b6fd6c3c@gmail.com>
Date:   Fri, 19 Oct 2018 17:13:59 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Dimitris Michailidis <dmichail@...gle.com>, edumazet@...gle.com,
        davem@...emloft.net
Cc:     netdev@...r.kernel.org
Subject: Re: [PATCH net] net: fix pskb_trim_rcsum_slow() with odd trim offset



On 10/19/2018 05:07 PM, Dimitris Michailidis wrote:
> 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@...gle.com>

Thanks a lot Dimitris for finding this.

Reviewed-by: Eric Dumazet <edumazet@...gle.com>