Re: [PATCH] ip: add udp_csum, udp6_csum_tx, udp6_csum_rx control flags to ip l2tp add tunnel

[Wang Shanker <shankerwangmiao@gmail.com>]

> 在 2016年4月27日，23:33，Wang Shanker <shankerwangmiao@gmail.com> 写道：
> 
> 
> 
>> 在 2016年4月27日，20:21，James Chapman <jchapman@katalix.com> 写道：
>> 
>> On 26 April 2016 at 15:15, Wang Shanker <shankerwangmiao@gmail.com> wrote:
>>> Hi, all
>>> 
>>> It’s my first time to contribute to such an important open source project. Things began when I upgraded my server, called "Server A", form ubuntu 14.04 to 16.04, which is shipped with new kernel version, 4.4. After upgrade, I soon found a l2tp tunnel between this server and another linux server, called "Server B", via ipv6 broke down. Here is the network topology:
>>> 
>>> +----------+                    +----------+
>>> | Server A | -- IPV6 Network -- | Server B |
>>> +----------+                    +----------+
>>> 
>>> The l2tp tunnel was encapsulated in udp datagrams. All the configuration was normal and could work after I reverted the kernel on Server A to original version.
>>> 
>>> Here is what i did to create the tunnel:
>>> 
>>> ```
>>> on Server A:
>>> 
>>> ip l2tp add tunnel tunnel_id 86 peer_tunnel_id 86 remote 2001:db8::aaaa local 2001:db8::bbbb udp_sport 1086 udp_dport 1086
>>> ip l2tp add session name l2tpeth0 tunnel_id 86 session_id 86 peer_session_id 86
>>> ip l s l2tpeth0 up
>>> 
>>> on Server B:
>>> 
>>> ip l2tp add tunnel tunnel_id 86 peer_tunnel_id 86 local 2001:db8::aaaa remote 2001:db8::bbbb udp_sport 1086 udp_dport 1086
>>> ip l2tp add session name l2tpeth0 tunnel_id 86 session_id 86 peer_session_id 86
>>> ip l s l2tpeth0 up
>>> 
>>> ```
>>> 
>>> When I used tcpdump to diagnose the problem, I got such result:
>>> 
>>> ```
>>> on Server A:
>>> 
>>> arping -i l2tpeth0 -0 1.2.3.4
>>> 
>>> on Server B:
>>> 
>>> tcpdump -i eth0 -n port 1086  -v
>>> 
>>> 21:35:57.818810 IP6 (flowlabel 0x8f028, hlim 64, next-header UDP (17) payload length: 62) 2001:db8::aaaa.1086 > 2001:db8::bbbb.1086: [bad udp cksum 0x0000 -> 0x1140!] UDP, length 54
>>> 21:35:58.820572 IP6 (flowlabel 0x8f028, hlim 64, next-header UDP (17) payload length: 62) 2001:db8::aaaa.1086 > 2001:db8::bbbb.1086: [bad udp cksum 0x0000 -> 0x1140!] UDP, length 54
>>> 21:35:59.822216 IP6 (flowlabel 0x8f028, hlim 64, next-header UDP (17) payload length: 62) 2001:db8::aaaa.1086 > 2001:db8::bbbb.1086: [bad udp cksum 0x0000 -> 0x1140!] UDP, length 54
>>> 
>>> ```
>>> 
>>> After looking into kernel source, I found out that in this commit a new feature to set udp6 checksum to zero in commit 6b649fe, which added `L2TP_ATTR_UDP_ZERO_CSUM6_TX` and `L2TP_ATTR_UDP_ZERO_CSUM6_TX`.
>>> 
>>> As a result, I added `udp_csum`, `udp6_csum_tx`, `udp6_csum_rx` control flags to `ip l2tp add tunnel` to control those attributes about checksum.
>>> 
>>> Using this to create the tunnel instead on Server A:
>>> 
>>> ```
>>> ip l2tp add tunnel tunnel_id 86 peer_tunnel_id 86 remote 2001:db8::aaaa local 2001:db8::bbbb udp_sport 1086 udp_dport 1086 udp6_csum_tx on udp6_csum_rx on
>>> ```
>>> 
>>> I finally got:
>>> 
>>> ```
>>> on Server A:
>>> 
>>> arping -i l2tpeth0 -0 1.2.3.4
>>> 
>>> on Server B:
>>> 
>>> tcpdump -i eth0 -n port 1086  -v
>>> 
>>> 22:07:03.844297 IP6 (flowlabel 0x8f028, hlim 64, next-header UDP (17) payload length: 62) 2001:db8::aaaa.1086 > 2001:db8::bbbb.1086: [udp sum ok] UDP, length 54
>>> 22:07:04.845717 IP6 (flowlabel 0x8f028, hlim 64, next-header UDP (17) payload length: 62) 2001:db8::aaaa.1086 > 2001:db8::bbbb.1086: [udp sum ok] UDP, length 54
>>> 22:07:05.847965 IP6 (flowlabel 0x8f028, hlim 64, next-header UDP (17) payload length: 62) 2001:db8::aaaa.1086 > 2001:db8::bbbb.1086: [udp sum ok] UDP, length 54
>>> 
>>> tcpdump -i l2tpeth0 -v
>>> 
>>> 22:10:35.691326 ARP, Ethernet (len 6), IPv4 (len 4), Request who-has 1.2.3.4 tell 0.0.0.0, length 28
>>> 22:10:36.693627 ARP, Ethernet (len 6), IPv4 (len 4), Request who-has 1.2.3.4 tell 0.0.0.0, length 28
>>> 22:10:37.695010 ARP, Ethernet (len 6), IPv4 (len 4), Request who-has 1.2.3.4 tell 0.0.0.0, length 28
>>> 22:10:38.697121 ARP, Ethernet (len 6), IPv4 (len 4), Request who-has 1.2.3.4 tell 0.0.0.0, length 28
>>> 
>>> ```
>>> 
>>> It seems to work. However, is it the real point that should be fixed and does my patch fix it well? I need your suggestion.
>> 
>> This seems reasonable to me. It is good to provide user control of
>> L2TP checksum options.
>> 
>> However, there is a problem with your patch. The netlink attributes
>> you are accessing to control checksums are flags, not u8 values.
> 
> I’m not so familiar with kernel code. However, in `<linux/l2tp.h>` : 
> 
> ```
> 
> /*
> * ATTR types defined for L2TP
> */
> enum {
> 	L2TP_ATTR_NONE,			/* no data */
> // ...
> 	L2TP_ATTR_IP6_DADDR,		/* struct in6_addr */
> 	L2TP_ATTR_UDP_ZERO_CSUM6_TX,	/* u8 */
> 	L2TP_ATTR_UDP_ZERO_CSUM6_RX,	/* u8 */
> // ...
> 
> }
> ```
> 
> isn’t L2TP_ATTR_UDP_ZERO_CSUM6_TX a u8 value? Or should I use `addattr` instead of `addattr8`? 
> 
>> 
>> Maybe the default checksum setting for such l2tp tunnels should be
>> changed in the l2tp kernel code to match the previous behaviour where
>> IPv6 checksums were disabled?
> 
> I think so. However, I’m confused with those code.
> 
> From the name of the attrs `L2TP_ATTR_UDP_ZERO_CSUM6_TX` and `L2TP_ATTR_UDP_ZERO_CSUM6_RX`, I 
> can tell that when those flags are set, the checksum will be zero. Also, according to the 
> comment of commit 6b649fe in kernel source, “Added new L2TP configuration options to allow 
> TX and RX of zero checksums in IPv6. Default is not to use them.”, checksums shouldn't have
> been zero by default. However, in fact, they are. I think there may be some bugs in kernel
> source. 

I think I’ve got the bug. Here is the patch for kernel

---
 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
                {
-- 


>> 
>>> 
>>> 
>>> ---
>>> ip/ipl2tp.c | 45 +++++++++++++++++++++++++++++++++++++++++++++
>>> 1 file changed, 45 insertions(+)
>>> 
>>> diff --git a/ip/ipl2tp.c b/ip/ipl2tp.c
>>> index 3c8ee93..67a6482 100644
>>> --- a/ip/ipl2tp.c
>>> +++ b/ip/ipl2tp.c
>>> @@ -56,6 +56,8 @@ struct l2tp_parm {
>>> 
>>>       uint16_t pw_type;
>>>       uint16_t mtu;
>>> +       int udp6_csum_tx:1;
>>> +       int udp6_csum_rx:1;
>>>       int udp_csum:1;
>>>       int recv_seq:1;
>>>       int send_seq:1;
>>> @@ -117,6 +119,9 @@ static int create_tunnel(struct l2tp_parm *p)
>>>       if (p->encap == L2TP_ENCAPTYPE_UDP) {
>>>               addattr16(&req.n, 1024, L2TP_ATTR_UDP_SPORT, p->local_udp_port);
>>>               addattr16(&req.n, 1024, L2TP_ATTR_UDP_DPORT, p->peer_udp_port);
>>> +               addattr8 (&req.n, 1024, L2TP_ATTR_UDP_CSUM, p->udp_csum);
>>> +               addattr8 (&req.n, 1024, L2TP_ATTR_UDP_ZERO_CSUM6_TX, p->udp6_csum_tx);
>>> +               addattr8 (&req.n, 1024, L2TP_ATTR_UDP_ZERO_CSUM6_RX, p->udp6_csum_rx);
>>>       }
>>> 
>>>       if (rtnl_talk(&genl_rth, &req.n, NULL, 0) < 0)
>>> @@ -282,6 +287,14 @@ static int get_response(struct nlmsghdr *n, void *arg)
>>>               p->l2spec_len = rta_getattr_u8(attrs[L2TP_ATTR_L2SPEC_LEN]);
>>> 
>>>       p->udp_csum = !!attrs[L2TP_ATTR_UDP_CSUM];
>>> +       if (attrs[L2TP_ATTR_UDP_ZERO_CSUM6_TX])
>>> +         p->udp6_csum_tx = rta_getattr_u8(attrs[L2TP_ATTR_UDP_ZERO_CSUM6_TX]);
>>> +       else
>>> +         p->udp6_csum_tx = 1;
>>> +       if (attrs[L2TP_ATTR_UDP_ZERO_CSUM6_RX])
>>> +         p->udp6_csum_rx = rta_getattr_u8(attrs[L2TP_ATTR_UDP_ZERO_CSUM6_RX]);
>>> +       else
>>> +         p->udp6_csum_rx = 1;
>>>       if (attrs[L2TP_ATTR_COOKIE])
>>>               memcpy(p->cookie, RTA_DATA(attrs[L2TP_ATTR_COOKIE]),
>>>                      p->cookie_len = RTA_PAYLOAD(attrs[L2TP_ATTR_COOKIE]));
>>> @@ -470,6 +483,9 @@ static void usage(void)
>>>       fprintf(stderr, "          tunnel_id ID peer_tunnel_id ID\n");
>>>       fprintf(stderr, "          [ encap { ip | udp } ]\n");
>>>       fprintf(stderr, "          [ udp_sport PORT ] [ udp_dport PORT ]\n");
>>> +       fprintf(stderr, "          [ udp_csum { on | off } ]\n");
>>> +       fprintf(stderr, "          [ udp6_csum_tx { on | off } ]\n");
>>> +       fprintf(stderr, "          [ udp6_csum_rx { on | off } ]\n");
>>>       fprintf(stderr, "Usage: ip l2tp add session [ name NAME ]\n");
>>>       fprintf(stderr, "          tunnel_id ID\n");
>>>       fprintf(stderr, "          session_id ID peer_session_id ID\n");
>>> @@ -500,6 +516,8 @@ static int parse_args(int argc, char **argv, int cmd, struct l2tp_parm *p)
>>>       /* Defaults */
>>>       p->l2spec_type = L2TP_L2SPECTYPE_DEFAULT;
>>>       p->l2spec_len = 4;
>>> +       p->udp6_csum_rx = 1;
>>> +       p->udp6_csum_tx = 1;
>>> 
>>>       while (argc > 0) {
>>>               if (strcmp(*argv, "encap") == 0) {
>>> @@ -569,6 +587,33 @@ static int parse_args(int argc, char **argv, int cmd, struct l2tp_parm *p)
>>>                       if (get_u16(&uval, *argv, 0))
>>>                               invarg("invalid port\n", *argv);
>>>                       p->peer_udp_port = uval;
>>> +               } else if (strcmp(*argv, "udp_csum") == 0) {
>>> +                       NEXT_ARG();
>>> +                       if (strcmp(*argv, "on") == 0) {
>>> +                               p->udp_csum = 1;
>>> +                       } else if (strcmp(*argv, "off") == 0) {
>>> +                               p->udp_csum = 0;
>>> +                       } else {
>>> +                               invarg("invalid option for udp_csum\n", *argv);
>>> +                       }
>>> +               } else if (strcmp(*argv, "udp6_csum_rx") == 0) {
>>> +                       NEXT_ARG();
>>> +                       if (strcmp(*argv, "on") == 0) {
>>> +                               p->udp6_csum_rx = 1;
>>> +                       } else if (strcmp(*argv, "off") == 0) {
>>> +                               p->udp6_csum_rx = 0;
>>> +                       } else {
>>> +                               invarg("invalid option for udp6_csum_rx\n", *argv);
>>> +                       }
>>> +               } else if (strcmp(*argv, "udp6_csum_tx") == 0) {
>>> +                       NEXT_ARG();
>>> +                       if (strcmp(*argv, "on") == 0) {
>>> +                               p->udp6_csum_tx = 1;
>>> +                       } else if (strcmp(*argv, "off") == 0) {
>>> +                               p->udp6_csum_tx = 0;
>>> +                       } else {
>>> +                               invarg("invalid option for udp6_csum_tx\n", *argv);
>>> +                       }
>>>               } else if (strcmp(*argv, "offset") == 0) {
>>>                       __u8 uval;
>>> 
>>> --
>>> 2.5.2