netdev - Re: [PATCH] net: Reduce skb_warn_bad

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Date:   Mon, 7 Aug 2017 00:42:32 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Tonghao Zhang <xiangxia.m.yue@...il.com>
Cc:     Eric Dumazet <eric.dumazet@...il.com>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Willem de Bruijn <willemb@...gle.com>,
        Pravin B Shelar <pshelar@....org>
Subject: Re: [PATCH] net: Reduce skb_warn_bad_offload() noise.

> The openvswitch kernel module calls the __skb_gso_segment()(and sets
> tx_path = false) when passing packets to userspace. The UFO will set
> the ip_summed to CHECKSUM_NONE. There are a lot of warn logs. The warn
> log is shown as below. I guess we should revert the patch.

Indeed, the software UFO code computes the checksum and
sets ip_summed to CHECKSUM_NONE, as is correct on the
egress path.

Commit 6e7bc478c9a0 ("net: skb_needs_check() accepts
CHECKSUM_NONE for tx") revised the tx_path case in
skb_needs_check to avoid the warning exactly for the UFO case.

We cannot just make an exception for CHECKSUM_NONE in the
!tx_path case, as the entire statement then becomes false:

       return skb->ip_summed == CHECKSUM_NONE;

Since on egress CHECKSUM_UNNECESSARY is equivalent to
CHECKSUM_NONE, it should be fine to update the UFO code
to set that, instead:

@@ -235,7 +235,7 @@ static struct sk_buff *udp4_ufo_fragment(struct
sk_buff *skb,
        if (uh->check == 0)
                uh->check = CSUM_MANGLED_0;

-       skb->ip_summed = CHECKSUM_NONE;
+       skb->ip_summed = CHECKSUM_UNNECESSARY;