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Subject[PATCH] tcp: fix potential corner case issue in segmentation (Was: Re: [PATCH] Do not use TSO/GSO when there is urgent data)
On Fri, 21 Nov 2008, Petr Tesarik wrote:

> This patch fixes http://bugzilla.kernel.org/show_bug.cgi?id=12014
>
> Since most (if not all) implementations of TSO and even the in-kernel
> software GSO do not update the urgent pointer when splitting a large
> segment, it is necessary to turn off TSO/GSO for all outgoing traffic
> with the URG pointer set.

Good observation, I totally missed this possibility of T/GSO while
looking.

> Looking at tcp_current_mss (and the preceding comment) I even think
> this was the original intention. However, this approach is insufficient,
> because TSO/GSO is turned off only for newly created frames, not for
> frames which were already pending at the arrival of a message with
> MSG_OOB set. These frames were created when TSO/GSO was enabled,
> so they may be large, and they will have the urgent pointer set
> in tcp_transmit_skb().
>
> With this patch, such large packets will be fragmented again before
> going to the transmit routine.

I wonder if there's some corner case which still fails to fragment
in tcp_retransmit_xmit's in skb->len <= cur_mss case if cur_mss
grew very recently (and therefore skb-len now fits to a single segment).

Patch below, changed subject is due to patchwork as requested by DaveM
(if you'd find that strange).

> As a side note, at least the following NICs are known to screw up
> the urgent pointer in the TCP header when doing TSO:
>
> Intel 82566MM (PCI ID 8086:1049)
> Intel 82566DC (PCI ID 8086:104b)
> Intel 82541GI (PCI ID 8086:1076)
> Broadcom NetXtreme II BCM5708 (PCI ID 14e4:164c)

Heh, it might already be longer than the list we would get from the
working ones. Not very likely too many people consider urg too
seriously to get it right.

> Signed-off-by: Petr Tesarik <ptesarik@suse.cz>
> CC: Jan Sembera <jsembera@suse.cz>
> CC: Ilpo Jarvinen <ilpo.jarvinen@helsinki.fi>
>
> --
> tcp_output.c | 7 +++++--
> 1 file changed, 5 insertions(+), 2 deletions(-)
>
> diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
> --- a/net/ipv4/tcp_output.c
> +++ b/net/ipv4/tcp_output.c
> @@ -722,7 +722,8 @@ static void tcp_queue_skb(struct sock *sk, struct sk_buff
> *skb)
> static void tcp_set_skb_tso_segs(struct sock *sk, struct sk_buff *skb,
> unsigned int mss_now)
> {
> - if (skb->len <= mss_now || !sk_can_gso(sk)) {
> + if (skb->len <= mss_now || !sk_can_gso(sk) ||
> + tcp_urg_mode(tcp_sk(sk))) {
> /* Avoid the costly divide in the normal
> * non-TSO case.
> */
> @@ -1163,7 +1164,9 @@ static int tcp_init_tso_segs(struct sock *sk, struct
> sk_buff *skb,
> {
> int tso_segs = tcp_skb_pcount(skb);
>
> - if (!tso_segs || (tso_segs > 1 && tcp_skb_mss(skb) != mss_now)) {
> + if (!tso_segs ||
> + (tso_segs > 1 && (tcp_skb_mss(skb) != mss_now ||
> + tcp_urg_mode(tcp_sk(sk))))) {
> tcp_set_skb_tso_segs(sk, skb, mss_now);
> tso_segs = tcp_skb_pcount(skb);
> }

It's a bit intrusive but I couldn't immediately come up with alternative
that would have worked (came up with some not working ones :-)).

Acked-by: Ilpo Järvinen <ilpo.jarvinen@helsinki.fi>


--
i.


--

[PATCH] tcp: fix potential corner case issue in segmentation

If cur_mss grew very recently so that the previously G/TSOed skb
now fits well into a single segment, it might get send up in
parts unless we calculate # of segments again. Whether it actually
would break something more than that I don't know. But this would
create a more significant problem with the urg t/gso problem that
was found by Petr Tesarik <ptesarik@suse.cz>.

Compile tested only.

Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@helsinki.fi>
---
net/ipv4/tcp_output.c | 2 ++
1 files changed, 2 insertions(+), 0 deletions(-)

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index a524627..129f46b 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -1944,6 +1944,8 @@ int tcp_retransmit_skb(struct sock *sk, struct sk_buff *skb)
if (skb->len > cur_mss) {
if (tcp_fragment(sk, skb, cur_mss, cur_mss))
return -ENOMEM; /* We'll try again later. */
+ } else {
+ tcp_init_tso_segs(sk, skb, cur_mss);
}

/* Collapse two adjacent packets if worthwhile and we can. */
--
1.5.2.2
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