From: Eric Dumazet <eric.dumazet@gmail.com>
To: David Miller <davem@davemloft.net>
Cc: subramanian.vijay@gmail.com, therbert@google.com, netdev@vger.kernel.org
Subject: Re: Bug in computing data_len in tcp_sendmsg?
Date: Fri, 02 Dec 2011 19:36:29 +0100 [thread overview]
Message-ID: <1322850989.2762.47.camel@edumazet-laptop> (raw)
In-Reply-To: <20111202.131304.981034697414242002.davem@davemloft.net>
Le vendredi 02 décembre 2011 à 13:13 -0500, David Miller a écrit :
> From: Eric Dumazet <eric.dumazet@gmail.com>
> Date: Fri, 02 Dec 2011 17:05:35 +0100
>
> > Le vendredi 02 décembre 2011 à 16:44 +0100, Eric Dumazet a écrit :
> >
> >> [PATCH net-next] tcp: fix tcp_trim_head()
> >>
> >> commit f07d960df3 (tcp: avoid frag allocation for small frames)
> >> breaked assumption in tcp stack that skb is either linear (data_len ==
> >> 0), either fully fragged (data_len == len)
> >>
> >> Thanks to Vijay for providing a very detailed explanation.
> >>
> >> Reported-by: Vijay Subramanian <subramanian.vijay@gmail.com>
> >> Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
> >> ---
> >
> > Another problem is the possible misalignement of skb->data if/when we
> > receive an ACK of an odd (not a 4 multiple) tcp sequence.
> >
> > So when/if packet is restransmited, tcp header (and IP header as well)
> > could be misaligned.
> >
> > Hmm, I probably miss something obvious, since this problem could happen
> > for linear skbs before my patch ?
>
> Unfortunately, even if netfilter or the packet scheduler pulled data, this
> misalignment wouldn't happen before your change.
>
> Maybe we should revert your frag allocation avoidance change until we can
> sort this out.
The patch I posted should solve the problem Vijay spotted.
(It really does, I tested it, allowing mtu probing)
What I ask now is following problem (even prior to my frag allocation
patch) :
1) We allocate a linear skb (SG being off) to cook a tcp frame of length
XXX bytes.
2) We send it.
3) We receive an ACK for first 31 bytes.
4) We trim 31 bytes from the head of skb. (skb_pull(skb, 31))
skb->data is now not anymore aligned to a 4 bytes boundary.
5) Later, we need to retransmit skb (XXX minus 31 bytes already ACKed)
We push TCP header, and skb->data is not aligned.
TCP header is not aligned anymore. x86 doesnt care, but what about
other arches with misalign traps ?
next prev parent reply other threads:[~2011-12-02 18:36 UTC|newest]
Thread overview: 29+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-12-01 1:48 Bug in computing data_len in tcp_sendmsg? Tom Herbert
2011-12-01 3:42 ` Eric Dumazet
2011-12-01 4:06 ` Tom Herbert
2011-12-01 4:16 ` David Miller
2011-12-01 4:17 ` Eric Dumazet
2011-12-01 5:09 ` Tom Herbert
2011-12-01 9:15 ` Eric Dumazet
2011-12-01 17:29 ` Jesse Brandeburg
2011-12-01 18:04 ` David Miller
2011-12-01 20:37 ` Eric Dumazet
2011-12-01 22:18 ` Vijay Subramanian
2011-12-01 22:30 ` Eric Dumazet
2011-12-02 6:18 ` Vijay Subramanian
2011-12-02 11:59 ` Eric Dumazet
2011-12-02 15:44 ` Eric Dumazet
2011-12-02 16:05 ` Eric Dumazet
2011-12-02 18:13 ` David Miller
2011-12-02 18:36 ` Eric Dumazet [this message]
2011-12-02 18:40 ` David Miller
2011-12-02 20:22 ` Eric Dumazet
2011-12-02 20:24 ` David Miller
2011-12-02 20:45 ` Eric Dumazet
2011-12-02 20:48 ` Eric Dumazet
2011-12-02 21:30 ` David Miller
2011-12-03 15:09 ` Eric Dumazet
2011-12-04 7:39 ` [PATCH] tcp: take care of misalignments Eric Dumazet
2011-12-04 18:21 ` David Miller
2011-12-04 18:51 ` Eric Dumazet
2011-12-05 23:45 ` David Miller
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