Re: kernel 2.6.39 eats multicast packets

[Eric Dumazet <eric.dumazet@gmail.com>]

Le vendredi 17 juin 2011 à 10:32 +0200, Knut Tidemann a écrit :
> Hello.
> 
> We're seeing an issue where a listening UDP socket in a multicast group  
> doesn't receive some multicast packets.
>  From simple testing it seems that the first packet from a new host is not  
> passed through the kernel and down to the socket, but the next packets  
> are. The packets can be
> seen with a tool such as tcpdump, but they never reach the user space  
> socket. It is worth noting, that the packet loss does not occur when  
> sending to and from the same host,
> to a multicast address. The address and port we have been using in our  
> tests are 224.0.1.75:5060. I've also attached the testing code at the end  
> of this email. The issue was also present in 3.0-rc1.
> 
> This issue is not present in 2.6.38 and I've bisected the issue to the  
> following commit:
> 
> ----
> b23dd4fe42b455af5c6e20966b7d6959fa8352ea is the first bad commit
> commit b23dd4fe42b455af5c6e20966b7d6959fa8352ea
> Author: David S. Miller <davem@davemloft.net>
> Date:   Wed Mar 2 14:31:35 2011 -0800
> 
>      ipv4: Make output route lookup return rtable directly.
> 
>      Instead of on the stack.
> 
>      Signed-off-by: David S. Miller <davem@davemloft.net>
> ----
> 

Knut, this is awesome, your bug report is perfect and was really helpful
to let me fix the bug in maybe 15 minutes, including reboot and tests ;)

Many thanks !

[PATCH] ipv4: fix multicast losses

Knut Tidemann found that first packet of a multicast flow was not
correctly received, and bisected the regression to commit b23dd4fe42b4
(Make output route lookup return rtable directly.)

Special thanks to Knut, who provided a very nice bug report, including
sample programs to demonstrate the bug.

Reported-and-bisectedby: Knut Tidemann <knut.andre.tidemann@jotron.com>
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
---
 net/ipv4/route.c |    4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/net/ipv4/route.c b/net/ipv4/route.c
index 045f0ec..aa13ef1 100644
--- a/net/ipv4/route.c
+++ b/net/ipv4/route.c
@@ -1902,9 +1902,7 @@ static int ip_route_input_mc(struct sk_buff *skb, __be32 daddr, __be32 saddr,
 
 	hash = rt_hash(daddr, saddr, dev->ifindex, rt_genid(dev_net(dev)));
 	rth = rt_intern_hash(hash, rth, skb, dev->ifindex);
-	err = 0;
-	if (IS_ERR(rth))
-		err = PTR_ERR(rth);
+	return IS_ERR(rth) ? PTR_ERR(rth) : 0;
 
 e_nobufs:
 	return -ENOBUFS;