netdev - Re: [PATCH v2 RFC] 8139cp: Fix GSO MSS handling

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Date:	Sun, 27 Sep 2015 22:37:49 -0700
From:	Tom Herbert <tom@...bertland.com>
To:	David Woodhouse <dwmw2@...radead.org>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	Francois Romieu <romieu@...zoreil.com>,
	David Miller <davem@...emloft.net>,
	Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: [PATCH v2 RFC] 8139cp: Fix GSO MSS handling

On Thu, Sep 24, 2015 at 5:31 AM, David Woodhouse <dwmw2@...radead.org> wrote:
> On Thu, 2015-09-24 at 05:05 -0700, Eric Dumazet wrote:
>>
>> Right, netif_skb_features() only has the following checks :
>>
>> if (gso_segs > dev->gso_max_segs || gso_segs < dev->gso_min_segs)
>>     features &= ~NETIF_F_GSO_MASK;
>>
>> But now we have .ndo_features_check() we could remove this generic
>> check from fast path.
>
> Perhaps so, yes.
>
> Any thoughts on the other reason I was staring at this same code path
> this week? I am able to reliably feed inappropriate packets to a
> NETIF_F_IP_CSUM-capable device with the test program at
> http://bombadil.infradead.org/~dwmw2/raw.c (and equivalent code paths
> via virtio_net, tun and others).
>
> They're *supposed* to get checksummed by software if the device can't
> cope, but netif_skb_features() returns the wrong answer, so we fail to
> do that and they're fed with CHECKSUM_PARTIAL to a device which can't
> handle them. Causing a WARN() or a BUG() or a silent corruption,
> depending on the driver.
>
Which drivers are doing this? It is up to the driver to determine
whether a particular packet being sent can have checksum offloaded to
the device. If it cannot offload the checksum it must call
skb_checksum_help.

Tom

> --
> dwmw2
>
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