This patch tests MPTCP_PM_ADDR_FLAG_SET_ID flag when appending a new
address in mptcp_pm_nl_append_new_local_addr() in pm_netlink.c and in
mptcp_userspace_pm_append_new_local_addr() in pm_userspace.c. If this
flag is set, do not alloc new address ID from id_bitmap, just keep the
userspace set address ID.

Fixes: e5ed101a6028 ("mptcp: userspace pm allow creating id 0 subflow")
Signed-off-by: Geliang Tang <geliang.tang@linux.dev>
---
 net/mptcp/pm_netlink.c   | 7 +++++--
 net/mptcp/pm_userspace.c | 9 +++++++--
 2 files changed, 12 insertions(+), 4 deletions(-)

diff --git a/net/mptcp/pm_netlink.c b/net/mptcp/pm_netlink.c
index dedc5a038b10..8a311b497603 100644
--- a/net/mptcp/pm_netlink.c
+++ b/net/mptcp/pm_netlink.c
@@ -949,7 +949,7 @@ static int mptcp_pm_nl_append_new_local_addr(struct pm_nl_pernet *pernet,
 		}
 	}
 
-	if (!entry->addr.id) {
+	if (!entry->addr.id && !(entry->flags & MPTCP_PM_ADDR_FLAG_SET_ID)) {
 find_next:
 		entry->addr.id = find_next_zero_bit(pernet->id_bitmap,
 						    MPTCP_PM_MAX_ADDR_ID + 1,
@@ -960,9 +960,12 @@ static int mptcp_pm_nl_append_new_local_addr(struct pm_nl_pernet *pernet,
 		}
 	}
 
-	if (!entry->addr.id)
+	if (!entry->addr.id && !(entry->flags & MPTCP_PM_ADDR_FLAG_SET_ID))
 		goto out;
 
+	if (entry->flags & MPTCP_PM_ADDR_FLAG_SET_ID)
+		entry->flags &= ~MPTCP_PM_ADDR_FLAG_SET_ID;
+
 	__set_bit(entry->addr.id, pernet->id_bitmap);
 	if (entry->addr.id > pernet->next_id)
 		pernet->next_id = entry->addr.id;
diff --git a/net/mptcp/pm_userspace.c b/net/mptcp/pm_userspace.c
index 8134dbdc7005..ce8ae7aa5aad 100644
--- a/net/mptcp/pm_userspace.c
+++ b/net/mptcp/pm_userspace.c
@@ -41,8 +41,11 @@ static int mptcp_userspace_pm_append_new_local_addr(struct mptcp_sock *msk,
 	spin_lock_bh(&msk->pm.lock);
 	list_for_each_entry(e, &msk->pm.userspace_pm_local_addr_list, list) {
 		addr_match = mptcp_addresses_equal(&e->addr, &entry->addr, true);
-		if (addr_match && entry->addr.id == 0)
+		if (addr_match && entry->addr.id == 0 &&
+		    !(entry->flags & MPTCP_PM_ADDR_FLAG_SET_ID))
 			entry->addr.id = e->addr.id;
+		else if (entry->flags & MPTCP_PM_ADDR_FLAG_SET_ID)
+			entry->flags &= ~MPTCP_PM_ADDR_FLAG_SET_ID;
 		id_match = (e->addr.id == entry->addr.id);
 		if (addr_match && id_match) {
 			match = e;
@@ -64,10 +67,12 @@ static int mptcp_userspace_pm_append_new_local_addr(struct mptcp_sock *msk,
 		}
 
 		*e = *entry;
-		if (!e->addr.id)
+		if (!e->addr.id && !(e->flags & MPTCP_PM_ADDR_FLAG_SET_ID))
 			e->addr.id = find_next_zero_bit(id_bitmap,
 							MPTCP_PM_MAX_ADDR_ID + 1,
 							1);
+		else if (e->flags & MPTCP_PM_ADDR_FLAG_SET_ID)
+			e->flags &= ~MPTCP_PM_ADDR_FLAG_SET_ID;
 		list_add_tail_rcu(&e->list, &msk->pm.userspace_pm_local_addr_list);
 		msk->pm.local_addr_used++;
 		ret = e->addr.id;
-- 
2.39.2