netdev - Re: skb_shinfo(skb)->nr_frags > 0 while skb_is

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Date:	Tue, 6 Oct 2009 18:21:31 -0700
From:	Stephen Hemminger <shemminger@...tta.com>
To:	John Wright <john.wright@...com>
Cc:	netdev@...r.kernel.org, Michael Chan <mchan@...adcom.com>,
	Bob Montgomery <bob.montgomery@...com>
Subject: Re: skb_shinfo(skb)->nr_frags > 0 while skb_is_gso(skb) == 0?

On Tue, 6 Oct 2009 19:03:15 -0600
John Wright <john.wright@...com> wrote:

> Hello,
> 
> Bob Montgomery and I are debugging an OOPS in the bnx2 driver.  The
> driver OOPSes in bnx2_tx_int(), getting a NULL pointer dereference when
> checking if the skb is GSO.  (This is on 2.6.29, before is_gso was
> cached in the tx_buf (commit d62fda08), but bear with me - while kernels
> with that commit might not crash in the same place, I think we have
> discovered a bug that would manifest itself another way.)
> 
> So, first, a question for someone who knows more about sk_buff's than I:
> is it reasonable/legal for an skb for which skb_is_gso(skb) == 0 to also
> have skb_shinfo(skb)->nr_frags > 0?

Yes, if driver support Scatter/Gather and Checksum offload,
TCP (especially splice) will hand fragmented frames to device.

Don't know what assumptions driver is making that could cause your
issue.
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