Messages in this thread | | | Date | Fri, 21 Nov 2008 19:49:02 +0200 (EET) | From | "Ilpo Järvinen" <> | Subject | Re: [PATCH] tcp: fix potential corner case issue in segmentation (Was: Re: [PATCH] Do not use TSO/GSO when there is urgent data) |
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On Fri, 21 Nov 2008, Petr Tesarik wrote:
> Dne Friday 21 of November 2008 14:07:32 Ilpo Järvinen napsal(a): > > 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). > > This shouldn't be a problem, because TSO only applies to packets which are > larger than MSS, so the problematic case is when cur_mss gets smaller, not > when it grows. In other words, the original implementation of > tcp_retransmit_xmit() could never make use of TSO/GSO, anyway...
I disagree:
1. mss == x 2. skb->len == 2*x => Send using TSO/GSO 3. mss = 2*x (e.g, mtu probe completes) 4. rexmit skb, nothing resets TSO/GSO fields though now skb->len <= mss, thus it will get sent as two, smaller than what's necessary, packets using TSO/GSO and will not get into that fixed place of yours.
Or did I miss something?
> >[...] > > > 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 :-)). > > Yes, I also noticed that. We could add some more code to tcp_mark_urg(), e.g. > walk sk_write_queue and adjust the pending SKBs there... > > Is it OK to simply set all skb->gso_segs to zero, and let the next call to > tcp_init_tso_segs redo them?
If we walk backwards we could consider short-circuit the walk at 16-bit urg field limit. I wouldn't mind if users of such obscure feature pay the price but the final decision is up to Dave of course.
> I mean, will tcp_init_tso_segs() be always > called on all SKBs which are in the write queue at the time tcp_mark_urg() is > called?
I realized there is more breakage: That tcp_current_mss tcp_urg_mode is too late as well, it won't work either until we're already past the relevant seqno range... It basically starts working at snd_up upwards rather than working on snd_up-2^16..snd_up region.
In addition, it's quite impossible to tso/gso successfully past ceil_to_mss(snd_up - 2^16) boundary anyway because we don't have enough bits in the urgent field to tell at which point the fields should get set (unless there would be some out-of-band communication channel for the urgent sequence number).
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