FAILED: patch "[PATCH] mptcp: pm: in-kernel: usable client side with C-flag" failed to apply to 6.12-stable tree

The patch below does not apply to the 6.12-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.12.y
git checkout FETCH_HEAD
git cherry-pick -x 4b1ff850e0c1aacc23e923ed22989b827b9808f9
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2025101650-setback-proofs-9687@gregkh' --subject-prefix 'PATCH 6.12.y' HEAD^..

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 4b1ff850e0c1aacc23e923ed22989b827b9808f9 Mon Sep 17 00:00:00 2001
From: "Matthieu Baerts (NGI0)" <[email protected]>
Date: Thu, 25 Sep 2025 12:32:36 +0200
Subject: [PATCH] mptcp: pm: in-kernel: usable client side with C-flag

When servers set the C-flag in their MP_CAPABLE to tell clients not to create subflows to the initial address and port, clients will likely not use their other endpoints. That's because the in-kernel path-manager uses the 'subflow' endpoints to create subflows only to the initial address and port.

If the limits have not been modified to accept ADD_ADDR, the client doesn't try to establish new subflows. If the limits accept ADD_ADDR, the routing routes will be used to select the source IP.

The C-flag is typically set when the server is operating behind a legacy Layer 4 load balancer, or using anycast IP address. Clients having their different 'subflow' endpoints setup, don't end up creating multiple subflows as expected, and causing some deployment issues.

A special case is then added here: when servers set the C-flag in the MPC and directly sends an ADD_ADDR, this single ADD_ADDR is accepted. The 'subflows' endpoints will then be used with this new remote IP and port. This exception is only allowed when the ADD_ADDR is sent immediately after the 3WHS, and makes the client switching to the 'fully established' mode. After that, 'select_local_address()' will not be able to find any subflows, because 'id_avail_bitmap' will be filled in mptcp_pm_create_subflow_or_signal_addr(), when switching to 'fully established' mode.

index 204e1f61212e..584cab90aa6e 100644
@@ -637,9 +637,12 @@ void mptcp_pm_add_addr_received(const struct sock *ssk,
 		} else {
 			__MPTCP_INC_STATS(sock_net((struct sock *)msk), MPTCP_MIB_ADDADDRDROP);
 		}
-	/* id0 should not have a different address */
+	/* - id0 should not have a different address
+	 * - special case for C-flag: linked to fill_local_addresses_vec()
+	 */
 	} else if ((addr->id == 0 && !mptcp_pm_is_init_remote_addr(msk, addr)) ||
-		   (addr->id > 0 && !READ_ONCE(pm->accept_addr))) {
+		   (addr->id > 0 && !READ_ONCE(pm->accept_addr) &&
+		    !mptcp_pm_add_addr_c_flag_case(msk))) {
 		mptcp_pm_announce_addr(msk, addr, true);
 		mptcp_pm_add_addr_send_ack(msk);
 	} else if (mptcp_pm_schedule_work(msk, MPTCP_PM_ADD_ADDR_RECEIVED)) {

index a1787a1344ac..cbe54331e5c7 100644
@@ -1199,6 +1199,14 @@ static inline void mptcp_pm_close_subflow(struct mptcp_sock *msk)
 	spin_unlock_bh(&msk->pm.lock);
 }
 
+static inline bool mptcp_pm_add_addr_c_flag_case(struct mptcp_sock *msk)
+{
+	return READ_ONCE(msk->pm.remote_deny_join_id0) &&
+	       msk->pm.local_addr_used == 0 &&
+	       mptcp_pm_get_add_addr_accept_max(msk) == 0 &&
+	       msk->pm.subflows < mptcp_pm_get_subflows_max(msk);
+}
+
 void mptcp_sockopt_sync_locked(struct mptcp_sock *msk, struct sock *ssk);
 
 static inline struct mptcp_ext *mptcp_get_ext(const struct sk_buff *skb)

commit 4b1ff850e0c1aacc23e923ed22989b827b9808f9 upstream.

When servers set the C-flag in their MP_CAPABLE to tell clients not to create subflows to the initial address and port, clients will likely not use their other endpoints. That's because the in-kernel path-manager uses the 'subflow' endpoints to create subflows only to the initial address and port.

If the limits have not been modified to accept ADD_ADDR, the client doesn't try to establish new subflows. If the limits accept ADD_ADDR, the routing routes will be used to select the source IP.

The C-flag is typically set when the server is operating behind a legacy Layer 4 load balancer, or using anycast IP address. Clients having their different 'subflow' endpoints setup, don't end up creating multiple subflows as expected, and causing some deployment issues.

A special case is then added here: when servers set the C-flag in the MPC and directly sends an ADD_ADDR, this single ADD_ADDR is accepted. The 'subflows' endpoints will then be used with this new remote IP and port. This exception is only allowed when the ADD_ADDR is sent immediately after the 3WHS, and makes the client switching to the 'fully established' mode. After that, 'select_local_address()' will not be able to find any subflows, because 'id_avail_bitmap' will be filled in mptcp_pm_create_subflow_or_signal_addr(), when switching to 'fully established' mode.

[ Conflict in pm.c, because commit 498d7d8b75f1 ("mptcp: pm: remove
'_nl' from mptcp_pm_nl_is_init_remote_addr") renamed an helper in the context, and it is not in this version. The same new code can be applied at the same place.
Conflict in pm_kernel.c, because the modified code has been moved from pm_netlink.c to pm_kernel.c in commit 8617e85e04bd ("mptcp: pm: split in-kernel PM specific code"), which is not in this version. The resolution is easy: simply by applying the patch where 'pm_kernel.c' has been replaced 'pm_netlink.c'. 'patch --merge' managed to apply this modified patch without creating any conflicts. ]

---
 net/mptcp/pm.c         |  7 ++++--
 net/mptcp/pm_netlink.c | 50 +++++++++++++++++++++++++++++++++++++++++-
 net/mptcp/protocol.h   |  8 +++++++
 3 files changed, 62 insertions(+), 3 deletions(-)

index 2c8815daf5b0..1b7541206a70 100644
@@ -226,9 +226,12 @@ void mptcp_pm_add_addr_received(const struct sock *ssk,
 		} else {
 			__MPTCP_INC_STATS(sock_net((struct sock *)msk), MPTCP_MIB_ADDADDRDROP);
 		}
-	/* id0 should not have a different address */
+	/* - id0 should not have a different address
+	 * - special case for C-flag: linked to fill_local_addresses_vec()
+	 */
 	} else if ((addr->id == 0 && !mptcp_pm_nl_is_init_remote_addr(msk, addr)) ||
-		   (addr->id > 0 && !READ_ONCE(pm->accept_addr))) {
+		   (addr->id > 0 && !READ_ONCE(pm->accept_addr) &&
+		    !mptcp_pm_add_addr_c_flag_case(msk))) {
 		mptcp_pm_announce_addr(msk, addr, true);
 		mptcp_pm_add_addr_send_ack(msk);
 	} else if (mptcp_pm_schedule_work(msk, MPTCP_PM_ADD_ADDR_RECEIVED)) {

index 6f191b125978..9653fee227ab 100644
@@ -1172,6 +1172,14 @@ static inline void mptcp_pm_close_subflow(struct mptcp_sock *msk)
 	spin_unlock_bh(&msk->pm.lock);
 }
 
+static inline bool mptcp_pm_add_addr_c_flag_case(struct mptcp_sock *msk)
+{
+	return READ_ONCE(msk->pm.remote_deny_join_id0) &&
+	       msk->pm.local_addr_used == 0 &&
+	       mptcp_pm_get_add_addr_accept_max(msk) == 0 &&
+	       msk->pm.subflows < mptcp_pm_get_subflows_max(msk);
+}
+
 void mptcp_sockopt_sync_locked(struct mptcp_sock *msk, struct sock *ssk);
 
 static inline struct mptcp_ext *mptcp_get_ext(const struct sk_buff *skb)

2.51.0

This is a note to let you know that I've just added the patch titled

mptcp: pm: in-kernel: usable client side with C-flag

to the 6.12-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
mptcp-pm-in-kernel-usable-client-side-with-c-flag.patch
and it can be found in the queue-6.12 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From [email protected] Thu Oct 16 18:57:55 2025
From: "Matthieu Baerts (NGI0)" <[email protected]>
Date: Thu, 16 Oct 2025 18:56:57 +0200
Subject: mptcp: pm: in-kernel: usable client side with C-flag
To: [email protected], [email protected]
Cc: MPTCP Upstream <[email protected]>, "Matthieu Baerts (NGI0)" <[email protected]>, Geliang Tang <[email protected]>, Jakub Kicinski <[email protected]>
Message-ID: <[email protected]>

From: "Matthieu Baerts (NGI0)" <[email protected]>

commit 4b1ff850e0c1aacc23e923ed22989b827b9808f9 upstream.

When servers set the C-flag in their MP_CAPABLE to tell clients not to create subflows to the initial address and port, clients will likely not use their other endpoints. That's because the in-kernel path-manager uses the 'subflow' endpoints to create subflows only to the initial address and port.

If the limits have not been modified to accept ADD_ADDR, the client doesn't try to establish new subflows. If the limits accept ADD_ADDR, the routing routes will be used to select the source IP.

The C-flag is typically set when the server is operating behind a legacy Layer 4 load balancer, or using anycast IP address. Clients having their different 'subflow' endpoints setup, don't end up creating multiple subflows as expected, and causing some deployment issues.

A special case is then added here: when servers set the C-flag in the MPC and directly sends an ADD_ADDR, this single ADD_ADDR is accepted. The 'subflows' endpoints will then be used with this new remote IP and port. This exception is only allowed when the ADD_ADDR is sent immediately after the 3WHS, and makes the client switching to the 'fully established' mode. After that, 'select_local_address()' will not be able to find any subflows, because 'id_avail_bitmap' will be filled in mptcp_pm_create_subflow_or_signal_addr(), when switching to 'fully established' mode.

[ Conflict in pm.c, because commit 498d7d8b75f1 ("mptcp: pm: remove
'_nl' from mptcp_pm_nl_is_init_remote_addr") renamed an helper in the context, and it is not in this version. The same new code can be applied at the same place.
Conflict in pm_kernel.c, because the modified code has been moved from pm_netlink.c to pm_kernel.c in commit 8617e85e04bd ("mptcp: pm: split in-kernel PM specific code"), which is not in this version. The resolution is easy: simply by applying the patch where 'pm_kernel.c' has been replaced 'pm_netlink.c'. 'patch --merge' managed to apply this modified patch without creating any conflicts. ]

---
 net/mptcp/pm.c         |    7 ++++--
 net/mptcp/pm_netlink.c |   50 ++++++++++++++++++++++++++++++++++++++++++++++++-
 net/mptcp/protocol.h   |    8 +++++++
 3 files changed, 62 insertions(+), 3 deletions(-)

@@ -226,9 +226,12 @@ void mptcp_pm_add_addr_received(const st
 		} else {
 			__MPTCP_INC_STATS(sock_net((struct sock *)msk), MPTCP_MIB_ADDADDRDROP);
 		}
-	/* id0 should not have a different address */
+	/* - id0 should not have a different address
+	 * - special case for C-flag: linked to fill_local_addresses_vec()
+	 */
 	} else if ((addr->id == 0 && !mptcp_pm_nl_is_init_remote_addr(msk, addr)) ||
-		   (addr->id > 0 && !READ_ONCE(pm->accept_addr))) {
+		   (addr->id > 0 && !READ_ONCE(pm->accept_addr) &&
+		    !mptcp_pm_add_addr_c_flag_case(msk))) {
 		mptcp_pm_announce_addr(msk, addr, true);
 		mptcp_pm_add_addr_send_ack(msk);
 	} else if (mptcp_pm_schedule_work(msk, MPTCP_PM_ADD_ADDR_RECEIVED)) {

@@ -1172,6 +1172,14 @@ static inline void mptcp_pm_close_subflo
 	spin_unlock_bh(&msk->pm.lock);
 }
 
+static inline bool mptcp_pm_add_addr_c_flag_case(struct mptcp_sock *msk)
+{
+	return READ_ONCE(msk->pm.remote_deny_join_id0) &&
+	       msk->pm.local_addr_used == 0 &&
+	       mptcp_pm_get_add_addr_accept_max(msk) == 0 &&
+	       msk->pm.subflows < mptcp_pm_get_subflows_max(msk);
+}
+
 void mptcp_sockopt_sync_locked(struct mptcp_sock *msk, struct sock *ssk);
 
 static inline struct mptcp_ext *mptcp_get_ext(const struct sk_buff *skb)

Patches currently in stable-queue which might be from [email protected] are

queue-6.12/mptcp-pm-in-kernel-usable-client-side-with-c-flag.patch queue-6.12/selftests-mptcp-join-validate-c-flag-def-limit.patch