netdev - [PATCH] tcp: avoid a possible divide by zero

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Date:	Tue, 07 Dec 2010 22:28:08 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Ben Hutchings <bhutchings@...arflare.com>,
	David Miller <davem@...emloft.net>
Cc:	Martin Steigerwald <Martin@...htvoll.de>,
	netdev <netdev@...r.kernel.org>
Subject: [PATCH] tcp: avoid a possible divide by zero

Le mardi 07 décembre 2010 à 21:02 +0000, Ben Hutchings a écrit :
> On Tue, 2010-12-07 at 17:12 +0100, Eric Dumazet wrote:
> > Le mardi 07 décembre 2010 à 16:39 +0100, Martin Steigerwald a écrit :
> > 
> > > A participant of a linux performance training I hold found a bug with 
> > > window scaling which did not receive any reply as well:
> > > 
> > > Bug 20312 -  System freeze with multiples of 32 in 
> > > /proc/sys/net/ipv4/tcp_adv_win_scale
> > > https://bugzilla.kernel.org/show_bug.cgi?id=20312
> > > 
> > 
> > User bug ?
> > 
> > Documentation/networking/ip-sysctl.txt
> > 
> > tcp_adv_win_scale - INTEGER
> > 	Count buffering overhead as bytes/2^tcp_adv_win_scale 
> > 	(if tcp_adv_win_scale > 0) or bytes-bytes/2^(-tcp_adv_win_scale),
> > 	if it is <= 0.
> > 	Default: 2
> > 
> > Given we use 32bit numbers, using values outside of [-31 ... 31] makes litle sense.
> > 
> > We could add sysctl range limit, but user should not mess with 
> > /proc/sys/net/ipv4/parameters unless he knows what he is doing ?
> [...]
> 
> For mere humans, the range is not quite os obvious.  Which is why this
> has been fixed in net-2.6 (as noted on that bug report now).
> 

Thanks

Great, I feel we are going to fix all sysctls, one by one then :(

lkml removed from Cc


[PATCH] tcp: avoid a possible divide by zero

sysctl_tcp_tso_win_divisor might be set to zero while one cpu runs in
tcp_tso_should_defer(). Make sure we dont allow a divide by zero by
reading sysctl_tcp_tso_win_divisor once.

Signed-off-by: Eric Dumazet <eric.dumazet@...il.com>
---
 net/ipv4/tcp_output.c |    6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 05b1ecf..0281223 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -1513,6 +1513,7 @@ static int tcp_tso_should_defer(struct sock *sk, struct sk_buff *skb)
 	struct tcp_sock *tp = tcp_sk(sk);
 	const struct inet_connection_sock *icsk = inet_csk(sk);
 	u32 send_win, cong_win, limit, in_flight;
+	int win_divisor;
 
 	if (TCP_SKB_CB(skb)->flags & TCPHDR_FIN)
 		goto send_now;
@@ -1544,13 +1545,14 @@ static int tcp_tso_should_defer(struct sock *sk, struct sk_buff *skb)
 	if ((skb != tcp_write_queue_tail(sk)) && (limit >= skb->len))
 		goto send_now;
 
-	if (sysctl_tcp_tso_win_divisor) {
+	win_divisor = sysctl_tcp_tso_win_divisor;
+	if (win_divisor) {
 		u32 chunk = min(tp->snd_wnd, tp->snd_cwnd * tp->mss_cache);
 
 		/* If at least some fraction of a window is available,
 		 * just use it.
 		 */
-		chunk /= sysctl_tcp_tso_win_divisor;
+		chunk /= win_divisor;
 		if (limit >= chunk)
 			goto send_now;
 	} else {


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