netdev - Re: [PATCH net] net: Generalize ndo_gso_check to ndo_features

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Date:	Tue, 23 Dec 2014 23:52:38 -0500 (EST)
From:	David Miller <davem@...emloft.net>
To:	jesse@...ira.com
Cc:	netdev@...r.kernel.org, therbert@...gle.com,
	joestringer@...ira.com, edumazet@...gle.com
Subject: Re: [PATCH net] net: Generalize ndo_gso_check to ndo_features_check

From: Jesse Gross <jesse@...ira.com>
Date: Mon, 22 Dec 2014 08:03:43 -0800

> GSO isn't the only offload feature with restrictions that
> potentially can't be expressed with the current features mechanism.
> Checksum is another although it's a general issue that could in
> theory apply to anything. Even if it may be possible to
> implement these restrictions in other ways, it can result in
> duplicate code or inefficient per-packet behavior.
> 
> This generalizes ndo_gso_check so that drivers can remove any
> features that don't make sense for a given packet, similar to
> netif_skb_features(). It also converts existing driver
> restrictions to the new format, completing the work that was
> done to support tunnel protocols since the issues apply to
> checksums as well.
> 
> CC: Tom Herbert <therbert@...gle.com>
> CC: Joe Stringer <joestringer@...ira.com>
> CC: Eric Dumazet <edumazet@...gle.com>
> Signed-off-by: Jesse Gross <jesse@...ira.com>
> Fixes: 04ffcb255f22 ("net: Add ndo_gso_check")

I don't think this fixes the case which was the main impetus for Eric
Dumazet's patch.

The r8152 USB networking driver supports TSO, but has a length
restriction, and we weren't software segmenting when netif_needs_gso()
returns true, exactly because we didn't clear TSO from the feature
flags.

We really need to sort this out.
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