netdev - Re: [PATCH] net: l2tp: fix reversed udp6 checksum flags

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Date:	Fri, 29 Apr 2016 03:25:55 +0800
From:	Wang Shanker <shankerwangmiao@...il.com>
To:	James Chapman <jchapman@...alix.com>
Cc:	netdev@...r.kernel.org, Tom Herbert <therbert@...gle.com>,
	"David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH] net: l2tp: fix reversed udp6 checksum flags

I think this is a logic error, rather than a change to the default
UDP checksum setting. As expected, take rx for example, the flag 
`L2TP_ATTR_UDP_ZERO_CSUM6_RX` is not set by default, and udp6 
checksum will be checked by default. The fact is that, not setting
`L2TP_ATTR_UDP_ZERO_CSUM6_RX` leads to ignoring udp6 checksum. Such
a behavior does not correspond to the name 
“L2TP_ATTR_UDP_ZERO_CSUM6_RX”. As a result, I call it a logic error.

> 在 2016年4月29日，02:46，James Chapman <jchapman@...alix.com> 写道：
> 
> Some additional background on this: Wang found this when configuring
> l2tp tunnels using "ip l2tp" between two systems and then one system
> was upgraded. The tunnel failed to pass data because one side had UDP
> checksums enabled and the other now had them disabled. It seems kernel
> changes related to UDP checksums resulted in a change to the default
> UDP checksum setting for L2TP tunnels when using IPv6. Unfortunately,
> iproute2 doesn't let the user configure L2TP UDP checksum settings, so
> without this fix, some users may see problems depending on the kernel
> version differences on the L2TP peers. One for stable?
> 
> Acked-by: James Chapman <jchapman@...alix.com>
> 
> On 28 April 2016 at 18:29, Wang Shanker <shankerwangmiao@...il.com> wrote:
>> This patch fixes a bug which causes the behavior of whether to ignore
>> udp6 checksum of udp6 encapsulated l2tp tunnel contrary to what
>> userspace program requests.
>> 
>> When the flag `L2TP_ATTR_UDP_ZERO_CSUM6_RX` is set by userspace, it is
>> expected that udp6 checksums of received packets of the l2tp tunnel
>> to create should be ignored. In `l2tp_netlink.c`:
>> `l2tp_nl_cmd_tunnel_create()`, `cfg.udp6_zero_rx_checksums` is set
>> according to the flag, and then passed to `l2tp_core.c`:
>> `l2tp_tunnel_create()` and then `l2tp_tunnel_sock_create()`. In
>> `l2tp_tunnel_sock_create()`, `udp_conf.use_udp6_rx_checksums` is set
>> the same to `cfg.udp6_zero_rx_checksums`. However, if we want the
>> checksum to be ignored, `udp_conf.use_udp6_rx_checksums` should be set
>> to `false`, i.e. be set to the contrary. Similarly, the same should be
>> done to `udp_conf.use_udp6_tx_checksums`.
>> 
>> Signed-off-by: Miao Wang <shankerwangmiao@...il.com>
>> ---
>> net/l2tp/l2tp_core.c | 4 ++--
>> 1 file changed, 2 insertions(+), 2 deletions(-)
>> 
>> diff --git a/net/l2tp/l2tp_core.c b/net/l2tp/l2tp_core.c
>> index afca2eb..6edfa99 100644
>> --- a/net/l2tp/l2tp_core.c
>> +++ b/net/l2tp/l2tp_core.c
>> @@ -1376,9 +1376,9 @@ static int l2tp_tunnel_sock_create(struct net *net,
>>                        memcpy(&udp_conf.peer_ip6, cfg->peer_ip6,
>>                               sizeof(udp_conf.peer_ip6));
>>                        udp_conf.use_udp6_tx_checksums =
>> -                           cfg->udp6_zero_tx_checksums;
>> +                         ! cfg->udp6_zero_tx_checksums;
>>                        udp_conf.use_udp6_rx_checksums =
>> -                           cfg->udp6_zero_rx_checksums;
>> +                         ! cfg->udp6_zero_rx_checksums;
>>                } else
>> #endif
>>                {
>> --
>> 2.5.2
>>