netdev - Re: [bisected] xfrm: TCP connection initiating PMTU discovery stalls on v3.

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Message-ID: <2044516.PfymfTnqxW@h2o.as.studentenwerk.mhn.de>
Date:	Mon, 15 Dec 2014 19:04:43 +0100
From:	Wolfgang Walter <linux@...m.de>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Thomas Jarosch <thomas.jarosch@...ra2net.com>,
	netdev@...r.kernel.org, Eric Dumazet <edumazet@...gle.com>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	Steffen Klassert <steffen.klassert@...unet.com>
Subject: Re: [bisected] xfrm: TCP connection initiating PMTU discovery stalls on v3.

Hello Eric!

Am Freitag, 12. Dezember 2014, 16:15:40 schrieb Eric Dumazet:
> On Sat, 2014-12-13 at 00:47 +0100, Wolfgang Walter wrote:
> > I can't disable it as the driver will not allow it:
> > # ethtool -K eth0 tx off
> > Cannot change tx-checksumming
> > Could not change any device features
> 
> Sounds a bug in itself :(

I think the "gso disabled for interface" case is broken because:

tcp_sendmsg() sets tcp_gso_segs to zero

tcp_sendmsg() calls tcp_push_one() or __tcp_push_pending_frames()

tcp_push_one() and  __tcp_push_pending_frames() call tcp_write_xmit()

tcp_write_xmit() call tcp_init_tso_segs()

tcp_init_tso_segs() calls tcp_set_skb_tso_segs() because !tso_segs is true

tcp_init_tso_segs() creates a gso-packet if packet size is larger than 
mss_now.


I think tcp_init_tso_segs() assumed that tcp_init_tso_segs() checks if the 
socket supports gso.

I didn't check the other callers of tcp_init_tso_segs().


Regards,
-- 
Wolfgang Walter
Studentenwerk München
Anstalt des öffentlichen Rechts
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