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    SubjectRe: [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:

    > 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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