netdev - Re: [stable regression] tcp: make connect() mem charging friendly

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Message-ID: <1429038767.3211.50.camel@decadent.org.uk>
Date:	Tue, 14 Apr 2015 20:12:47 +0100
From:	Ben Hutchings <ben@...adent.org.uk>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	netdev <netdev@...r.kernel.org>, stable <stable@...r.kernel.org>,
	Eric Dumazet <edumazet@...gle.com>, 782515@...s.debian.org
Subject: Re: [stable regression] tcp: make connect() mem charging friendly

On Tue, 2015-04-14 at 05:19 -0700, Eric Dumazet wrote:
> On Tue, 2015-04-14 at 08:32 +0100, Ben Hutchings wrote:
> > Commit 355a901e6cf1b2b763ec85caa2a9f04fbcc4ab4a ("tcp: make connect()
> > mem charging friendly") was backported to various stable branches:
> > 
> > v3.10.73: e64a85197b3f tcp: make connect() mem charging friendly
> > v3.12.40: d06381e8aac5 tcp: make connect() mem charging friendly
> > v3.14.37: 5a8e8f482b4a tcp: make connect() mem charging friendly
> > v3.18.11: e8f117f002ca tcp: make connect() mem charging friendly
> > v3.13.11-ckt19: de023863df9d tcp: make connect() mem charging friendly
> > v3.16.7-ckt9: bea5f6ef9fcb tcp: make connect() mem charging friendly
> > 
> > On the 3.16 branch, this has resulted in a regression for TCP Fast Open:
> > <https://bugs.debian.org/782515>.  The BUG() at the top of
> > tcp_transmit_skb() fires as tcp_skb_pcount(skb) == 0.
> > 
> > tcp_send_syn_data() does:
> > 
> > 	memcpy(syn_data->cb, syn->cb, sizeof(syn->cb));
> > 
> > Since commit cd7d8498c9a5 ("tcp: change tcp_skb_pcount() location") this
> > is sufficient to set the GSO segment count correctly.  But in older
> > branches (< 3.18) the GSO segment count in skb_shared_info is used and
> > is no longer copied by tcp_send_syn_data().
> > 
> > All the versions listed above, except v3.18.11, appear to have suffered
> > this regression.
> > 
> > Ben.
> > 
> 
> Hi Ben
> 
> Sorry to ear that. Have you already fixed this, or are you asking for
> help ?

That was as far as I got this morning.  I can probably come up with a
fix later if you don't sooner.

Ben.

-- 
Ben Hutchings
Editing code like this is akin to sticking plasters on the bleeding stump
of a severed limb. - me, 29 June 1999

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