With version 0 of the protocol it was legal to encode the 'Subflow Id' in
the MP_PRIO suboption, to specify which subflow would change its 'Backup'
flag. This has been removed from v1 specification: thus, according to RFC
8684 ยง3.3.8, the resulting 'Length' for MP_PRIO changed from 4 to 3 byte.
Current Linux generates / parses MP_PRIO according to the old spec, using
'Length' equal to 4, and hardcoding 1 as 'Subflow Id'; RFC compliance can
improve if we change 'Length' in other to become 3, leaving a 'Nop' after
the MP_PRIO suboption. In this way the kernel will emit and accept *only*
MP_PRIO suboptions that are compliant to version 1 of the MPTCP protocol.
unpatched 5.11-rc kernel:
[root@bottarga ~]# tcpdump -tnnr unpatched.pcap | grep prio
reading from file unpatched.pcap, link-type LINUX_SLL (Linux cooked v1)
dropped privs to tcpdump
IP 10.0.3.2.48433 > 10.0.1.1.10006: Flags [.], ack 1, win 502, options
[nop,nop,TS val 4032325513 ecr 1876514270,mptcp prio non-backup id 1,mptcp dss
ack 14084896651682217737], length 0
patched 5.11-rc kernel:
[root@bottarga ~]# tcpdump -tnnr patched.pcap | grep prio
reading from file patched.pcap, link-type LINUX_SLL (Linux cooked v1)
dropped privs to tcpdump
IP 10.0.3.2.49735 > 10.0.1.1.10006: Flags [.], ack 1, win 502, options
[nop,nop,TS val 1276737699 ecr 2686399734,mptcp prio non-backup,nop,mptcp dss
ack 18433038869082491686], length 0
Changes since v2:
- when accounting for option space, don't increment 'TCPOLEN_MPTCP_PRIO'
and use 'TCPOLEN_MPTCP_PRIO_ALIGN' instead, thanks to Matthieu Baerts.
Changes since v1:
- refactor patch to avoid using 'TCPOLEN_MPTCP_PRIO' with its old value,
thanks to Geliang Tang.
Fixes: 067065422fcd ("mptcp: add the outgoing MP_PRIO support")
Reviewed-by: Mat Martineau <[email protected]>
Reviewed-by: Matthieu Baerts <[email protected]>
Signed-off-by: Davide Caratti <[email protected]>
---
net/mptcp/options.c | 5 +++--
net/mptcp/protocol.h | 3 ++-
2 files changed, 5 insertions(+), 3 deletions(-)
diff --git a/net/mptcp/options.c b/net/mptcp/options.c
index c9643344a8d7..17ad42c65087 100644
--- a/net/mptcp/options.c
+++ b/net/mptcp/options.c
@@ -699,10 +699,11 @@ static bool mptcp_established_options_mp_prio(struct sock
*sk,
if (!subflow->send_mp_prio)
return false;
- if (remaining < TCPOLEN_MPTCP_PRIO)
+ /* account for the trailing 'nop' option */
+ if (remaining < TCPOLEN_MPTCP_PRIO_ALIGN)
return false;
- *size = TCPOLEN_MPTCP_PRIO;
+ *size = TCPOLEN_MPTCP_PRIO_ALIGN;
opts->suboptions |= OPTION_MPTCP_PRIO;
opts->backup = subflow->request_bkup;
diff --git a/net/mptcp/protocol.h b/net/mptcp/protocol.h
index 1460705aaad0..07ee319f7847 100644
--- a/net/mptcp/protocol.h
+++ b/net/mptcp/protocol.h
@@ -60,7 +60,8 @@
#define TCPOLEN_MPTCP_ADD_ADDR6_BASE_PORT 24
#define TCPOLEN_MPTCP_PORT_LEN 4
#define TCPOLEN_MPTCP_RM_ADDR_BASE 4
-#define TCPOLEN_MPTCP_PRIO 4
+#define TCPOLEN_MPTCP_PRIO 3
+#define TCPOLEN_MPTCP_PRIO_ALIGN 4
#define TCPOLEN_MPTCP_FASTCLOSE 12
/* MPTCP MP_JOIN flags */
--
2.29.2
