netdev - Re: Moving frags and SKBTX_DEV

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Message-ID: <1400097121.7973.126.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Wed, 14 May 2014 12:52:01 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Zoltan Kiss <zoltan.kiss@...rix.com>
Cc:	netdev@...r.kernel.org,
	"xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>,
	kvm@...r.kernel.org, David Miller <davem@...emloft.net>
Subject: Re: Moving frags and SKBTX_DEV_ZEROCOPY skbs

On Wed, 2014-05-14 at 20:41 +0100, Zoltan Kiss wrote:
> On 14/05/14 15:23, Eric Dumazet wrote:
> > On Wed, 2014-05-14 at 14:40 +0100, Zoltan Kiss wrote:
> >> Hi,
> >>
> >> Recently I've investigated issues around SKBTX_DEV_ZEROCOPY skbs where
> >> the frags list were modified. I came across this function skb_shift(),
> >> which moves frags between skbs. And there are a lot more of such kind,
> >> skb_split or skb_try_coalesce, for example.
> >> It could be a dangerous thing if a frag is referenced from an skb which
> >> doesn't have the original destructor_arg, and to avoid that
> >> skb_orphan_frags should be called. Although probably these functions are
> >> not normally touched in usual usecases, I think it would be useful to
> >> review core skb functions proactively and add an skb_orphan_frags
> >> everywhere where the frags could be referenced from other places.
> >> Any opinion about this?
> >
> >
> > For skb_shift(), it is currently used from tcp stack only, where
> > this SKBTX_DEV_ZEROCOPY thing is not used, so I do not think there is a
> > bug for the moment.
> It is called from tcp_input.c, which suggests it can be called on 
> incoming TCP packets.

Nope. 

We split outgoing packets, stored in the socket write queue.
These packets are locally generated by tcp_sendmsg() and tcp_sendpage(),
no way we use SKBTX_DEV_ZEROCOPY yet.

This split happens when we receive an ACK, that's why it is in
tcp_input.c



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