[PATCH net-2.6 0/3]: TCP fixes

This fixes Bugzilla #10384

tcp_simple_retransmit does L increment without any checking
whatsoever for overflowing S+L when Reno is in use.

The simplest scenario I can currently think of is rather
complex in practice (there might be some more straightforward
cases though). Ie., if mss is reduced during mtu probing, it
may end up marking everything lost and if some duplicate ACKs
arrived prior to that sacked_out will be non-zero as well,
leading to S+L > packets_out, tcp_clean_rtx_queue on the next
cumulative ACK or tcp_fastretrans_alert on the next duplicate
ACK will fix the S counter.

More straightforward (but questionable) solution would be to
just call tcp_reset_reno_sack() in tcp_simple_retransmit but
it would negatively impact the probe's retransmission, ie.,
the retransmissions would not occur if some duplicate ACKs
had arrived.

So I had to add reno sacked_out reseting to CA_Loss state
when the first cumulative ACK arrives (this stale sacked_out
might actually be the explanation for the reports of left_out
overflows in kernel prior to 2.6.23 and S+L overflow reports
of 2.6.24). However, this alone won't be enough to fix kernel
before 2.6.24 because it is building on top of the commit
1b6d427bb7e ([TCP]: Reduce sacked_out with reno when purging
write_queue) to keep the sacked_out from overflowing.

Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@helsinki.fi>
Reported-by: Alessandro Suardi <alessandro.suardi@gmail.com>
---
include/net/tcp.h | 2 ++
net/ipv4/tcp_input.c | 24 ++++++++++++++++++------
net/ipv4/tcp_output.c | 3 +++
3 files changed, 23 insertions(+), 6 deletions(-)

diff --git a/include/net/tcp.h b/include/net/tcp.h
index 723b368..8f5fc52 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -760,6 +760,8 @@ static inline unsigned int tcp_packets_in_flight(const struct tcp_sock *tp)
return tp->packets_out - tcp_left_out(tp) + tp->retrans_out;
}

+extern int tcp_limit_reno_sacked(struct tcp_sock *tp);
+
/* If cwnd > ssthresh, we may raise ssthresh to be half-way to cwnd.
* The exception is rate halving phase, when cwnd is decreasing towards
* ssthresh.
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 58cad8b..8cace40 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -1625,13 +1625,11 @@ out:
return flag;
}

-/* If we receive more dupacks than we expected counting segments
- * in assumption of absent reordering, interpret this as reordering.
- * The only another reason could be bug in receiver TCP.
+/* Limits sacked_out so that sum with lost_out isn't ever larger than
+ * packets_out. Returns zero if sacked_out adjustement wasn't necessary.
*/
-static void tcp_check_reno_reordering(struct sock *sk, const int addend)
+int tcp_limit_reno_sacked(struct tcp_sock *tp)
{
- struct tcp_sock *tp = tcp_sk(sk);
u32 holes;

holes = max(tp->lost_out, 1U);
@@ -1639,8 +1637,20 @@ static void tcp_check_reno_reordering(struct sock *sk, const int addend)

if ((tp->sacked_out + holes) > tp->packets_out) {
tp->sacked_out = tp->packets_out - holes;
- tcp_update_reordering(sk, tp->packets_out + addend, 0);
+ return 1;
}
+ return 0;
+}
+
+/* If we receive more dupacks than we expected counting segments
+ * in assumption of absent reordering, interpret this as reordering.
+ * The only another reason could be bug in receiver TCP.
+ */
+static void tcp_check_reno_reordering(struct sock *sk, const int addend)
+{
+ struct tcp_sock *tp = tcp_sk(sk);
+ if (tcp_limit_reno_sacked(tp))
+ tcp_update_reordering(sk, tp->packets_out + addend, 0);
}

/* Emulate SACKs for SACKless connection: account for a new dupack. */
@@ -2600,6 +2610,8 @@ static void tcp_fastretrans_alert(struct sock *sk, int pkts_acked, int flag)
case TCP_CA_Loss:
if (flag & FLAG_DATA_ACKED)
icsk->icsk_retransmits = 0;
+ if (tcp_is_reno(tp) && flag & FLAG_SND_UNA_ADVANCED)
+ tcp_reset_reno_sack(tp);
if (!tcp_try_undo_loss(sk)) {
tcp_moderate_cwnd(tp);
tcp_xmit_retransmit_queue(sk);
diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 6e25540..441fdd3 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -1808,6 +1808,9 @@ void tcp_simple_retransmit(struct sock *sk)
if (!lost)
return;

+ if (tcp_is_reno(tp))
+ tcp_limit_reno_sacked(tp);
+
tcp_verify_left_out(tp);

/* Don't muck with the congestion window here.
--
1.5.2.2

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Hi Dave,

Here are some TCP fixes that resulted from the last weeks reports
& debug. The first one is quite likely to hit but it's considerably
harder to get it print an overflow warning, while I've seen two
reports about the second one (one of them is for 2.6.24.y), please
ignore the earlier version of the tcp_simple_retransmit patch. The
FRTO patch is once again result of code review rather than some
report but seems necessary to avoid detection of some not that
likely cases as spurious RTOs when there were some other losses
in the same window with the probe. Nevertheless, it should be safe
to return non-FRTO behavior.

....So far, they're just compile tested. I'll see if I find some time
to boot them on the evening.

There might still be one fackets_out miscounter awaiting detection
with debug patch (I hope Georgi catches it) because none of these
seem an obvious reason for triggery of the !sacked_out && fackets_out
trap.

All of them are also valid for stables but please note that it won't
be too easy, at least for the first & second patch, because of recent
changes (especially if stable != 2.6.24.y). I'll try to do the
adapted versions later on for the stable.

--
i.


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Fixes a long-standing bug which makes NewReno recovery crippled.
With GSO the whole head skb was marked as LOST which is in
violation of NewReno procedure that only wants to mark one packet
and ended up breaking our TCP code by causing counter overflow
because our code was built on top of assumption about valid
NewReno procedure. This manifested as triggering a WARN_ON for
the overflow in a number of places.

It seems relatively safe alternative to just do nothing if
tcp_fragment fails due to oom because another duplicate ACK is
likely to be received soon and the fragmentation will be retried.

Special thanks goes to Soeren Sonnenburg <kernel@nn7.de> who was
lucky enough to be able to reproduce this so that the warning
for the overflow was hit. It's not as easy task as it seems even
if this bug happens quite often because the amount of outstanding
data is pretty significant for the mismarkings to lead to an
overflow.

Because it's very late in 2.6.25-rc cycle (if this even makes in
time), I didn't want to touch anything with SACK enabled here.
Fragmenting might be useful for it as well but it's more or less
a policy decision rather than mandatory fix. Thus there's no need
to rush and we can postpone considering tcp_fragment with SACK
for 2.6.26.

In 2.6.24 and earlier, this very same bug existed but the effect
is slightly different because of a small changes in the if
conditions that fit to the patch's context. With them nothing
got lost marker and thus no retransmissions happened.

Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@helsinki.fi>
---
net/ipv4/tcp_input.c | 22 ++++++++++++++++++----
1 files changed, 18 insertions(+), 4 deletions(-)

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 6e46b4c..58cad8b 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -2138,7 +2138,9 @@ static void tcp_mark_head_lost(struct sock *sk, int packets, int fast_rexmit)
{
struct tcp_sock *tp = tcp_sk(sk);
struct sk_buff *skb;
- int cnt;
+ int cnt, oldcnt;
+ int err;
+ unsigned int mss;

BUG_TRAP(packets <= tp->packets_out);
if (tp->lost_skb_hint) {
@@ -2157,13 +2159,25 @@ static void tcp_mark_head_lost(struct sock *sk, int packets, int fast_rexmit)
tp->lost_skb_hint = skb;
tp->lost_cnt_hint = cnt;

+ if (after(TCP_SKB_CB(skb)->end_seq, tp->high_seq))
+ break;
+
+ oldcnt = cnt;
if (tcp_is_fack(tp) || tcp_is_reno(tp) ||
(TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED))
cnt += tcp_skb_pcount(skb);

- if (((!fast_rexmit || (tp->lost_out > 0)) && (cnt > packets)) ||
- after(TCP_SKB_CB(skb)->end_seq, tp->high_seq))
- break;
+ if ((!fast_rexmit || (tp->lost_out > 0)) && (cnt > packets)) {
+ if (tcp_is_sack(tp) || (oldcnt >= packets))
+ break;
+
+ mss = skb_shinfo(skb)->gso_size;
+ err = tcp_fragment(sk, skb, (packets - oldcnt) * mss, mss);
+ if (err < 0)
+ break;
+ cnt = packets;
+ }
+
if (!(TCP_SKB_CB(skb)->sacked & (TCPCB_SACKED_ACKED|TCPCB_LOST))) {
TCP_SKB_CB(skb)->sacked |= TCPCB_LOST;
tp->lost_out += tcp_skb_pcount(skb);
--
1.5.2.2

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On Mon, 7 Apr 2008, Ilpo Järvinen wrote:
[color=blue]
> Fixes a long-standing bug which makes NewReno recovery crippled.
> With GSO the whole head skb was marked as LOST which is in
> violation of NewReno procedure that only wants to mark one packet
> and ended up breaking our TCP code by causing counter overflow
> because our code was built on top of assumption about valid
> NewReno procedure. This manifested as triggering a WARN_ON for
> the overflow in a number of places.
>
> It seems relatively safe alternative to just do nothing if
> tcp_fragment fails due to oom because another duplicate ACK is
> likely to be received soon and the fragmentation will be retried.
>
> Special thanks goes to Soeren Sonnenburg <kernel@nn7.de> who was
> lucky enough to be able to reproduce this so that the warning
> for the overflow was hit. It's not as easy task as it seems even
> if this bug happens quite often because the amount of outstanding
> data is pretty significant for the mismarkings to lead to an
> overflow.
>
> Because it's very late in 2.6.25-rc cycle (if this even makes in
> time), I didn't want to touch anything with SACK enabled here.
> Fragmenting might be useful for it as well but it's more or less
> a policy decision rather than mandatory fix. Thus there's no need
> to rush and we can postpone considering tcp_fragment with SACK
> for 2.6.26.
>
> In 2.6.24 and earlier, this very same bug existed but the effect
> is slightly different because of a small changes in the if
> conditions that fit to the patch's context. With them nothing
> got lost marker and thus no retransmissions happened.
>
> Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@helsinki.fi>
> ---
> net/ipv4/tcp_input.c | 22 ++++++++++++++++++----
> 1 files changed, 18 insertions(+), 4 deletions(-)
>
> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> index 6e46b4c..58cad8b 100644
> --- a/net/ipv4/tcp_input.c
> +++ b/net/ipv4/tcp_input.c
> @@ -2138,7 +2138,9 @@ static void tcp_mark_head_lost(struct sock *sk, int packets, int fast_rexmit)
> {
> struct tcp_sock *tp = tcp_sk(sk);
> struct sk_buff *skb;
> - int cnt;
> + int cnt, oldcnt;
> + int err;
> + unsigned int mss;
>
> BUG_TRAP(packets <= tp->packets_out);
> if (tp->lost_skb_hint) {
> @@ -2157,13 +2159,25 @@ static void tcp_mark_head_lost(struct sock *sk, int packets, int fast_rexmit)
> tp->lost_skb_hint = skb;
> tp->lost_cnt_hint = cnt;
>
> + if (after(TCP_SKB_CB(skb)->end_seq, tp->high_seq))
> + break;
> +
> + oldcnt = cnt;
> if (tcp_is_fack(tp) || tcp_is_reno(tp) ||
> (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED))
> cnt += tcp_skb_pcount(skb);
>
> - if (((!fast_rexmit || (tp->lost_out > 0)) && (cnt > packets)) ||
> - after(TCP_SKB_CB(skb)->end_seq, tp->high_seq))
> - break;
> + if ((!fast_rexmit || (tp->lost_out > 0)) && (cnt > packets)) {[/color]

....Nah, this won't work too well. I'll repost soon.
[color=blue]
> + if (tcp_is_sack(tp) || (oldcnt >= packets))
> + break;
> +
> + mss = skb_shinfo(skb)->gso_size;
> + err = tcp_fragment(sk, skb, (packets - oldcnt) * mss, mss);
> + if (err < 0)
> + break;
> + cnt = packets;
> + }
> +
> if (!(TCP_SKB_CB(skb)->sacked & (TCPCB_SACKED_ACKED|TCPCB_LOST))) {
> TCP_SKB_CB(skb)->sacked |= TCPCB_LOST;
> tp->lost_out += tcp_skb_pcount(skb);
>[/color]

--
i.