When userspace PM requires to create an ID 0 subflow in "userspace pm
create id 0 subflow" test like this:

        userspace_pm_add_sf $ns2 10.0.3.2 0

An ID 1 subflow, in fact, is created.

Since in mptcp_pm_nl_append_new_local_addr(), 'id 0' will be treated as
no ID is set by userspace, and will allocate a new ID immediately:

     if (!e->addr.id)
             e->addr.id = find_next_zero_bit(pernet->id_bitmap,
                                             MPTCP_PM_MAX_ADDR_ID + 1,
                                             1);

To solve this issue, a new flag 'MPTCP_PM_ADDR_FLAG_SET_ID' is added
to distinguish between whether userspace has set an ID 0 or whether
userspace has not set any address.

Add a new parameter 'set_id' for mptcp_pm_parse_pm_addr_attr(), and
pass a 'set_id' flag to them. If an address id is set from userspace,
this 'set_id' will be set as true. If 'set_id' is set, then the newly
added flag MPTCP_PM_ADDR_FLAG_SET_ID wiil be set.

Fixes: e5ed101a6028 ("mptcp: userspace pm allow creating id 0 subflow")
Signed-off-by: Geliang Tang <geliang.tang@linux.dev>
---
 include/uapi/linux/mptcp.h |  1 +
 net/mptcp/pm_netlink.c     | 17 +++++++++++++----
 2 files changed, 14 insertions(+), 4 deletions(-)

diff --git a/include/uapi/linux/mptcp.h b/include/uapi/linux/mptcp.h
index 74cfe496891e..ef3663792765 100644
--- a/include/uapi/linux/mptcp.h
+++ b/include/uapi/linux/mptcp.h
@@ -36,6 +36,7 @@
 #define MPTCP_PM_ADDR_FLAG_BACKUP                      (1 << 2)
 #define MPTCP_PM_ADDR_FLAG_FULLMESH                    (1 << 3)
 #define MPTCP_PM_ADDR_FLAG_IMPLICIT                    (1 << 4)
+#define MPTCP_PM_ADDR_FLAG_SET_ID                      (1 << 5)
 
 struct mptcp_info {
 	__u8	mptcpi_subflows;
diff --git a/net/mptcp/pm_netlink.c b/net/mptcp/pm_netlink.c
index 661c226dad18..dedc5a038b10 100644
--- a/net/mptcp/pm_netlink.c
+++ b/net/mptcp/pm_netlink.c
@@ -1159,7 +1159,8 @@ static int mptcp_pm_parse_pm_addr_attr(struct nlattr *tb[],
 				       const struct nlattr *attr,
 				       struct genl_info *info,
 				       struct mptcp_addr_info *addr,
-				       bool require_family)
+				       bool require_family,
+				       bool *set_id)
 {
 	int err, addr_addr;
 
@@ -1174,8 +1175,11 @@ static int mptcp_pm_parse_pm_addr_attr(struct nlattr *tb[],
 	if (err)
 		return err;
 
-	if (tb[MPTCP_PM_ADDR_ATTR_ID])
+	if (tb[MPTCP_PM_ADDR_ATTR_ID]) {
 		addr->id = nla_get_u8(tb[MPTCP_PM_ADDR_ATTR_ID]);
+		if (set_id)
+			*set_id = true;
+	}
 
 	if (!tb[MPTCP_PM_ADDR_ATTR_FAMILY]) {
 		if (!require_family)
@@ -1223,7 +1227,7 @@ int mptcp_pm_parse_addr(struct nlattr *attr, struct genl_info *info,
 
 	memset(addr, 0, sizeof(*addr));
 
-	return mptcp_pm_parse_pm_addr_attr(tb, attr, info, addr, true);
+	return mptcp_pm_parse_pm_addr_attr(tb, attr, info, addr, true, NULL);
 }
 
 int mptcp_pm_parse_entry(struct nlattr *attr, struct genl_info *info,
@@ -1231,11 +1235,13 @@ int mptcp_pm_parse_entry(struct nlattr *attr, struct genl_info *info,
 			 struct mptcp_pm_addr_entry *entry)
 {
 	struct nlattr *tb[MPTCP_PM_ADDR_ATTR_MAX + 1];
+	bool set_id = false;
 	int err;
 
 	memset(entry, 0, sizeof(*entry));
 
-	err = mptcp_pm_parse_pm_addr_attr(tb, attr, info, &entry->addr, require_family);
+	err = mptcp_pm_parse_pm_addr_attr(tb, attr, info, &entry->addr,
+					  require_family, &set_id);
 	if (err)
 		return err;
 
@@ -1248,6 +1254,9 @@ int mptcp_pm_parse_entry(struct nlattr *attr, struct genl_info *info,
 	if (tb[MPTCP_PM_ADDR_ATTR_FLAGS])
 		entry->flags = nla_get_u32(tb[MPTCP_PM_ADDR_ATTR_FLAGS]);
 
+	if (set_id)
+		entry->flags |= MPTCP_PM_ADDR_FLAG_SET_ID;
+
 	if (tb[MPTCP_PM_ADDR_ATTR_PORT])
 		entry->addr.port = htons(nla_get_u16(tb[MPTCP_PM_ADDR_ATTR_PORT]));
 
-- 
2.39.2