There was a support for signal endpoints, but only when the endpoint's
flag was changed during a connection. If an endpoint with the signal and
backup was already present, the MP_JOIN reply was not containing the
backup flag as expected.

That's confusing to have this inconsistent behaviour. On the other hand,
the infrastructure to set the backup flag in the SYN + ACK + MP_JOIN was
already there, it was just never set before. Now when requesting the
local ID from the path-manager, the backup status is also requested.

There is a special case for the ID0: the PM has to return this ID0 if
the local address of the initial subflow is being used, and not the ID
of the related endpoint. Still, it is required to look at the different
endpoints to find if one has been defined for this address with the
backup flag.

Note that when the userspace PM is used, the backup flag can be set if
the local address was already used before with a backup flag, e.g. if
the address was announced with the 'backup' flag, or a subflow was
created with the 'backup' flag.

Fixes: 4596a2c1b7f5 ("mptcp: allow creating non-backup subflows")
Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/507
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
---
Notes:
 - v2:
   - Only set *backup on success (Mat).
   - Support MPJ to/from ID0 with backup.
   - Split the tests in a new dedicated commit to ease the backports.
---
 net/mptcp/pm.c           | 15 +++++++++++----
 net/mptcp/pm_netlink.c   | 23 ++++++++++++++++++++++-
 net/mptcp/pm_userspace.c | 25 +++++++++++++++++++++++--
 net/mptcp/protocol.h     | 11 ++++++++---
 net/mptcp/subflow.c      |  9 +++++++--
 5 files changed, 71 insertions(+), 12 deletions(-)

diff --git a/net/mptcp/pm.c b/net/mptcp/pm.c
index 55406720c607..359738b8826e 100644
--- a/net/mptcp/pm.c
+++ b/net/mptcp/pm.c
@@ -405,7 +405,8 @@ bool mptcp_pm_rm_addr_signal(struct mptcp_sock *msk, unsigned int remaining,
 	return ret;
 }
 
-int mptcp_pm_get_local_id(struct mptcp_sock *msk, struct sock_common *skc)
+int mptcp_pm_get_local_id(struct mptcp_sock *msk, struct sock_common *skc,
+			  bool *backup)
 {
 	struct mptcp_addr_info skc_local;
 	struct mptcp_addr_info msk_local;
@@ -418,12 +419,18 @@ int mptcp_pm_get_local_id(struct mptcp_sock *msk, struct sock_common *skc)
 	 */
 	mptcp_local_address((struct sock_common *)msk, &msk_local);
 	mptcp_local_address((struct sock_common *)skc, &skc_local);
-	if (mptcp_addresses_equal(&msk_local, &skc_local, false))
+	if (mptcp_addresses_equal(&msk_local, &skc_local, false)) {
+		if (mptcp_pm_is_userspace(msk))
+			*backup = mptcp_userspace_pm_is_backup(msk, &skc_local);
+		else
+			*backup = mptcp_pm_nl_is_backup(msk, &skc_local);
+
 		return 0;
+	}
 
 	if (mptcp_pm_is_userspace(msk))
-		return mptcp_userspace_pm_get_local_id(msk, &skc_local);
-	return mptcp_pm_nl_get_local_id(msk, &skc_local);
+		return mptcp_userspace_pm_get_local_id(msk, &skc_local, backup);
+	return mptcp_pm_nl_get_local_id(msk, &skc_local, backup);
 }
 
 int mptcp_pm_get_flags_and_ifindex_by_id(struct mptcp_sock *msk, unsigned int id,
diff --git a/net/mptcp/pm_netlink.c b/net/mptcp/pm_netlink.c
index 7635fac91539..796800a7fe96 100644
--- a/net/mptcp/pm_netlink.c
+++ b/net/mptcp/pm_netlink.c
@@ -1064,7 +1064,8 @@ static int mptcp_pm_nl_create_listen_socket(struct sock *sk,
 	return err;
 }
 
-int mptcp_pm_nl_get_local_id(struct mptcp_sock *msk, struct mptcp_addr_info *skc)
+int mptcp_pm_nl_get_local_id(struct mptcp_sock *msk, struct mptcp_addr_info *skc,
+			     bool *backup)
 {
 	struct mptcp_pm_addr_entry *entry;
 	struct pm_nl_pernet *pernet;
@@ -1076,6 +1077,7 @@ int mptcp_pm_nl_get_local_id(struct mptcp_sock *msk, struct mptcp_addr_info *skc
 	list_for_each_entry_rcu(entry, &pernet->local_addr_list, list) {
 		if (mptcp_addresses_equal(&entry->addr, skc, entry->addr.port)) {
 			ret = entry->addr.id;
+			*backup = !!(entry->flags & MPTCP_PM_ADDR_FLAG_BACKUP);
 			break;
 		}
 	}
@@ -1094,6 +1096,7 @@ int mptcp_pm_nl_get_local_id(struct mptcp_sock *msk, struct mptcp_addr_info *skc
 	entry->ifindex = 0;
 	entry->flags = MPTCP_PM_ADDR_FLAG_IMPLICIT;
 	entry->lsk = NULL;
+	*backup = false;
 	ret = mptcp_pm_nl_append_new_local_addr(pernet, entry, true);
 	if (ret < 0)
 		kfree(entry);
@@ -1101,6 +1104,24 @@ int mptcp_pm_nl_get_local_id(struct mptcp_sock *msk, struct mptcp_addr_info *skc
 	return ret;
 }
 
+bool mptcp_pm_nl_is_backup(struct mptcp_sock *msk, struct mptcp_addr_info *skc)
+{
+	struct pm_nl_pernet *pernet = pm_nl_get_pernet_from_msk(msk);
+	struct mptcp_pm_addr_entry *entry;
+	bool backup = false;
+
+	rcu_read_lock();
+	list_for_each_entry_rcu(entry, &pernet->local_addr_list, list) {
+		if (mptcp_addresses_equal(&entry->addr, skc, entry->addr.port)) {
+			backup = !!(entry->flags & MPTCP_PM_ADDR_FLAG_BACKUP);
+			break;
+		}
+	}
+	rcu_read_unlock();
+
+	return backup;
+}
+
 #define MPTCP_PM_CMD_GRP_OFFSET       0
 #define MPTCP_PM_EV_GRP_OFFSET        1
 
diff --git a/net/mptcp/pm_userspace.c b/net/mptcp/pm_userspace.c
index f0a4590506c6..4e3829f96c88 100644
--- a/net/mptcp/pm_userspace.c
+++ b/net/mptcp/pm_userspace.c
@@ -137,7 +137,7 @@ int mptcp_userspace_pm_get_flags_and_ifindex_by_id(struct mptcp_sock *msk,
 }
 
 int mptcp_userspace_pm_get_local_id(struct mptcp_sock *msk,
-				    struct mptcp_addr_info *skc)
+				    struct mptcp_addr_info *skc, bool *backup)
 {
 	struct mptcp_pm_addr_entry *entry = NULL, *e, new_entry;
 	__be16 msk_sport =  ((struct inet_sock *)
@@ -151,13 +151,16 @@ int mptcp_userspace_pm_get_local_id(struct mptcp_sock *msk,
 		}
 	}
 	spin_unlock_bh(&msk->pm.lock);
-	if (entry)
+	if (entry) {
+		*backup = !!(entry->flags & MPTCP_PM_ADDR_FLAG_BACKUP);
 		return entry->addr.id;
+	}
 
 	memset(&new_entry, 0, sizeof(struct mptcp_pm_addr_entry));
 	new_entry.addr = *skc;
 	new_entry.addr.id = 0;
 	new_entry.flags = MPTCP_PM_ADDR_FLAG_IMPLICIT;
+	*backup = false;
 
 	if (new_entry.addr.port == msk_sport)
 		new_entry.addr.port = 0;
@@ -165,6 +168,24 @@ int mptcp_userspace_pm_get_local_id(struct mptcp_sock *msk,
 	return mptcp_userspace_pm_append_new_local_addr(msk, &new_entry, true);
 }
 
+bool mptcp_userspace_pm_is_backup(struct mptcp_sock *msk,
+				  struct mptcp_addr_info *skc)
+{
+	struct mptcp_pm_addr_entry *entry;
+	bool backup = false;
+
+	spin_lock_bh(&msk->pm.lock);
+	list_for_each_entry(entry, &msk->pm.userspace_pm_local_addr_list, list) {
+		if (mptcp_addresses_equal(&entry->addr, skc, false)) {
+			backup = !!(entry->flags & MPTCP_PM_ADDR_FLAG_BACKUP);
+			break;
+		}
+	}
+	spin_unlock_bh(&msk->pm.lock);
+
+	return backup;
+}
+
 int mptcp_pm_nl_announce_doit(struct sk_buff *skb, struct genl_info *info)
 {
 	struct nlattr *token = info->attrs[MPTCP_PM_ATTR_TOKEN];
diff --git a/net/mptcp/protocol.h b/net/mptcp/protocol.h
index 6b6b76152db5..e6cc7111f843 100644
--- a/net/mptcp/protocol.h
+++ b/net/mptcp/protocol.h
@@ -1111,9 +1111,14 @@ bool mptcp_pm_add_addr_signal(struct mptcp_sock *msk, const struct sk_buff *skb,
 			      bool *drop_other_suboptions);
 bool mptcp_pm_rm_addr_signal(struct mptcp_sock *msk, unsigned int remaining,
 			     struct mptcp_rm_list *rm_list);
-int mptcp_pm_get_local_id(struct mptcp_sock *msk, struct sock_common *skc);
-int mptcp_pm_nl_get_local_id(struct mptcp_sock *msk, struct mptcp_addr_info *skc);
-int mptcp_userspace_pm_get_local_id(struct mptcp_sock *msk, struct mptcp_addr_info *skc);
+int mptcp_pm_get_local_id(struct mptcp_sock *msk, struct sock_common *skc,
+			  bool *backup);
+int mptcp_pm_nl_get_local_id(struct mptcp_sock *msk, struct mptcp_addr_info *skc,
+			     bool *backup);
+int mptcp_userspace_pm_get_local_id(struct mptcp_sock *msk, struct mptcp_addr_info *skc,
+				    bool *backup);
+bool mptcp_pm_nl_is_backup(struct mptcp_sock *msk, struct mptcp_addr_info *skc);
+bool mptcp_userspace_pm_is_backup(struct mptcp_sock *msk, struct mptcp_addr_info *skc);
 int mptcp_pm_dump_addr(struct sk_buff *msg, struct netlink_callback *cb);
 int mptcp_pm_nl_dump_addr(struct sk_buff *msg,
 			  struct netlink_callback *cb);
diff --git a/net/mptcp/subflow.c b/net/mptcp/subflow.c
index be406197b1c4..e60ba0327e4d 100644
--- a/net/mptcp/subflow.c
+++ b/net/mptcp/subflow.c
@@ -87,6 +87,7 @@ static struct mptcp_sock *subflow_token_join_request(struct request_sock *req)
 	struct mptcp_subflow_request_sock *subflow_req = mptcp_subflow_rsk(req);
 	struct mptcp_sock *msk;
 	int local_id;
+	bool backup;
 
 	msk = mptcp_token_get_sock(sock_net(req_to_sk(req)), subflow_req->token);
 	if (!msk) {
@@ -94,12 +95,13 @@ static struct mptcp_sock *subflow_token_join_request(struct request_sock *req)
 		return NULL;
 	}
 
-	local_id = mptcp_pm_get_local_id(msk, (struct sock_common *)req);
+	local_id = mptcp_pm_get_local_id(msk, (struct sock_common *)req, &backup);
 	if (local_id < 0) {
 		sock_put((struct sock *)msk);
 		return NULL;
 	}
 	subflow_req->local_id = local_id;
+	subflow_req->request_bkup = backup;
 
 	return msk;
 }
@@ -610,16 +612,19 @@ static int subflow_chk_local_id(struct sock *sk)
 {
 	struct mptcp_subflow_context *subflow = mptcp_subflow_ctx(sk);
 	struct mptcp_sock *msk = mptcp_sk(subflow->conn);
+	bool backup;
 	int err;
 
 	if (likely(subflow->local_id >= 0))
 		return 0;
 
-	err = mptcp_pm_get_local_id(msk, (struct sock_common *)sk);
+	err = mptcp_pm_get_local_id(msk, (struct sock_common *)sk, &backup);
 	if (err < 0)
 		return err;
 
 	subflow_set_local_id(subflow, err);
+	subflow->request_bkup = backup;
+
 	return 0;
 }
 

-- 
2.45.2

Hi Matt,

On Tue, Jul 16, 2024 at 10:53:18PM +0200, Matthieu Baerts (NGI0) wrote:
> There was a support for signal endpoints, but only when the endpoint's
> flag was changed during a connection. If an endpoint with the signal and
> backup was already present, the MP_JOIN reply was not containing the
> backup flag as expected.
> 
> That's confusing to have this inconsistent behaviour. On the other hand,
> the infrastructure to set the backup flag in the SYN + ACK + MP_JOIN was
> already there, it was just never set before. Now when requesting the
> local ID from the path-manager, the backup status is also requested.
> 
> There is a special case for the ID0: the PM has to return this ID0 if
> the local address of the initial subflow is being used, and not the ID
> of the related endpoint. Still, it is required to look at the different
> endpoints to find if one has been defined for this address with the
> backup flag.
> 
> Note that when the userspace PM is used, the backup flag can be set if
> the local address was already used before with a backup flag, e.g. if
> the address was announced with the 'backup' flag, or a subflow was
> created with the 'backup' flag.
> 
> Fixes: 4596a2c1b7f5 ("mptcp: allow creating non-backup subflows")
> Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/507
> Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
> ---
> Notes:
>  - v2:
>    - Only set *backup on success (Mat).
>    - Support MPJ to/from ID0 with backup.
>    - Split the tests in a new dedicated commit to ease the backports.
> ---
>  net/mptcp/pm.c           | 15 +++++++++++----
>  net/mptcp/pm_netlink.c   | 23 ++++++++++++++++++++++-
>  net/mptcp/pm_userspace.c | 25 +++++++++++++++++++++++--
>  net/mptcp/protocol.h     | 11 ++++++++---
>  net/mptcp/subflow.c      |  9 +++++++--
>  5 files changed, 71 insertions(+), 12 deletions(-)
> 
> diff --git a/net/mptcp/pm.c b/net/mptcp/pm.c
> index 55406720c607..359738b8826e 100644
> --- a/net/mptcp/pm.c
> +++ b/net/mptcp/pm.c
> @@ -405,7 +405,8 @@ bool mptcp_pm_rm_addr_signal(struct mptcp_sock *msk, unsigned int remaining,
>  	return ret;
>  }
>  
> -int mptcp_pm_get_local_id(struct mptcp_sock *msk, struct sock_common *skc)
> +int mptcp_pm_get_local_id(struct mptcp_sock *msk, struct sock_common *skc,
> +			  bool *backup)
>  {
>  	struct mptcp_addr_info skc_local;
>  	struct mptcp_addr_info msk_local;
> @@ -418,12 +419,18 @@ int mptcp_pm_get_local_id(struct mptcp_sock *msk, struct sock_common *skc)
>  	 */
>  	mptcp_local_address((struct sock_common *)msk, &msk_local);
>  	mptcp_local_address((struct sock_common *)skc, &skc_local);
> -	if (mptcp_addresses_equal(&msk_local, &skc_local, false))
> +	if (mptcp_addresses_equal(&msk_local, &skc_local, false)) {
> +		if (mptcp_pm_is_userspace(msk))
> +			*backup = mptcp_userspace_pm_is_backup(msk, &skc_local);
> +		else
> +			*backup = mptcp_pm_nl_is_backup(msk, &skc_local);
> +
>  		return 0;
> +	}
>  
>  	if (mptcp_pm_is_userspace(msk))
> -		return mptcp_userspace_pm_get_local_id(msk, &skc_local);
> -	return mptcp_pm_nl_get_local_id(msk, &skc_local);
> +		return mptcp_userspace_pm_get_local_id(msk, &skc_local, backup);
> +	return mptcp_pm_nl_get_local_id(msk, &skc_local, backup);
>  }

I think that instead of mixing "backup" into get_local_id() interface, it is much
better to add a new interface is_backup() for PM in pm.c:

bool mptcp_pm_is_backup(struct mptcp_sock *msk, struct sock_common *skc)
{
       struct mptcp_addr_info skc_local;

       mptcp_local_address((struct sock_common *)skc, &skc_local);

       if (mptcp_pm_is_userspace(msk))
               return mptcp_userspace_pm_is_backup(msk, &skc_local);
       return mptcp_pm_nl_is_backup(msk, &skc_local);
}

No need to test whether the two addresses msk_local and skc_local are the same
in mptcp_pm_is_backup(), just invoke mptcp_pm_nl_is_backup() or
mptcp_userspace_pm_is_backup() with skc_local.

>  
>  int mptcp_pm_get_flags_and_ifindex_by_id(struct mptcp_sock *msk, unsigned int id,
> diff --git a/net/mptcp/pm_netlink.c b/net/mptcp/pm_netlink.c
> index 7635fac91539..796800a7fe96 100644
> --- a/net/mptcp/pm_netlink.c
> +++ b/net/mptcp/pm_netlink.c
> @@ -1064,7 +1064,8 @@ static int mptcp_pm_nl_create_listen_socket(struct sock *sk,
>  	return err;
>  }
>  
> -int mptcp_pm_nl_get_local_id(struct mptcp_sock *msk, struct mptcp_addr_info *skc)
> +int mptcp_pm_nl_get_local_id(struct mptcp_sock *msk, struct mptcp_addr_info *skc,
> +			     bool *backup)
>  {
>  	struct mptcp_pm_addr_entry *entry;
>  	struct pm_nl_pernet *pernet;
> @@ -1076,6 +1077,7 @@ int mptcp_pm_nl_get_local_id(struct mptcp_sock *msk, struct mptcp_addr_info *skc
>  	list_for_each_entry_rcu(entry, &pernet->local_addr_list, list) {
>  		if (mptcp_addresses_equal(&entry->addr, skc, entry->addr.port)) {
>  			ret = entry->addr.id;
> +			*backup = !!(entry->flags & MPTCP_PM_ADDR_FLAG_BACKUP);
>  			break;
>  		}
>  	}
> @@ -1094,6 +1096,7 @@ int mptcp_pm_nl_get_local_id(struct mptcp_sock *msk, struct mptcp_addr_info *skc
>  	entry->ifindex = 0;
>  	entry->flags = MPTCP_PM_ADDR_FLAG_IMPLICIT;
>  	entry->lsk = NULL;
> +	*backup = false;
>  	ret = mptcp_pm_nl_append_new_local_addr(pernet, entry, true);
>  	if (ret < 0)
>  		kfree(entry);
> @@ -1101,6 +1104,24 @@ int mptcp_pm_nl_get_local_id(struct mptcp_sock *msk, struct mptcp_addr_info *skc
>  	return ret;
>  }
>  
> +bool mptcp_pm_nl_is_backup(struct mptcp_sock *msk, struct mptcp_addr_info *skc)
> +{
> +	struct pm_nl_pernet *pernet = pm_nl_get_pernet_from_msk(msk);
> +	struct mptcp_pm_addr_entry *entry;
> +	bool backup = false;
> +
> +	rcu_read_lock();
> +	list_for_each_entry_rcu(entry, &pernet->local_addr_list, list) {
> +		if (mptcp_addresses_equal(&entry->addr, skc, entry->addr.port)) {
> +			backup = !!(entry->flags & MPTCP_PM_ADDR_FLAG_BACKUP);
> +			break;
> +		}
> +	}
> +	rcu_read_unlock();
> +
> +	return backup;
> +}
> +
>  #define MPTCP_PM_CMD_GRP_OFFSET       0
>  #define MPTCP_PM_EV_GRP_OFFSET        1
>  
> diff --git a/net/mptcp/pm_userspace.c b/net/mptcp/pm_userspace.c
> index f0a4590506c6..4e3829f96c88 100644
> --- a/net/mptcp/pm_userspace.c
> +++ b/net/mptcp/pm_userspace.c
> @@ -137,7 +137,7 @@ int mptcp_userspace_pm_get_flags_and_ifindex_by_id(struct mptcp_sock *msk,
>  }
>  
>  int mptcp_userspace_pm_get_local_id(struct mptcp_sock *msk,
> -				    struct mptcp_addr_info *skc)
> +				    struct mptcp_addr_info *skc, bool *backup)
>  {
>  	struct mptcp_pm_addr_entry *entry = NULL, *e, new_entry;
>  	__be16 msk_sport =  ((struct inet_sock *)
> @@ -151,13 +151,16 @@ int mptcp_userspace_pm_get_local_id(struct mptcp_sock *msk,
>  		}
>  	}
>  	spin_unlock_bh(&msk->pm.lock);
> -	if (entry)
> +	if (entry) {
> +		*backup = !!(entry->flags & MPTCP_PM_ADDR_FLAG_BACKUP);
>  		return entry->addr.id;
> +	}
>  
>  	memset(&new_entry, 0, sizeof(struct mptcp_pm_addr_entry));
>  	new_entry.addr = *skc;
>  	new_entry.addr.id = 0;
>  	new_entry.flags = MPTCP_PM_ADDR_FLAG_IMPLICIT;
> +	*backup = false;
>  
>  	if (new_entry.addr.port == msk_sport)
>  		new_entry.addr.port = 0;
> @@ -165,6 +168,24 @@ int mptcp_userspace_pm_get_local_id(struct mptcp_sock *msk,
>  	return mptcp_userspace_pm_append_new_local_addr(msk, &new_entry, true);
>  }
>  
> +bool mptcp_userspace_pm_is_backup(struct mptcp_sock *msk,
> +				  struct mptcp_addr_info *skc)
> +{
> +	struct mptcp_pm_addr_entry *entry;
> +	bool backup = false;
> +
> +	spin_lock_bh(&msk->pm.lock);
> +	list_for_each_entry(entry, &msk->pm.userspace_pm_local_addr_list, list) {
> +		if (mptcp_addresses_equal(&entry->addr, skc, false)) {
> +			backup = !!(entry->flags & MPTCP_PM_ADDR_FLAG_BACKUP);
> +			break;
> +		}
> +	}
> +	spin_unlock_bh(&msk->pm.lock);
> +
> +	return backup;
> +}
> +
>  int mptcp_pm_nl_announce_doit(struct sk_buff *skb, struct genl_info *info)
>  {
>  	struct nlattr *token = info->attrs[MPTCP_PM_ATTR_TOKEN];
> diff --git a/net/mptcp/protocol.h b/net/mptcp/protocol.h
> index 6b6b76152db5..e6cc7111f843 100644
> --- a/net/mptcp/protocol.h
> +++ b/net/mptcp/protocol.h
> @@ -1111,9 +1111,14 @@ bool mptcp_pm_add_addr_signal(struct mptcp_sock *msk, const struct sk_buff *skb,
>  			      bool *drop_other_suboptions);
>  bool mptcp_pm_rm_addr_signal(struct mptcp_sock *msk, unsigned int remaining,
>  			     struct mptcp_rm_list *rm_list);
> -int mptcp_pm_get_local_id(struct mptcp_sock *msk, struct sock_common *skc);
> -int mptcp_pm_nl_get_local_id(struct mptcp_sock *msk, struct mptcp_addr_info *skc);
> -int mptcp_userspace_pm_get_local_id(struct mptcp_sock *msk, struct mptcp_addr_info *skc);
> +int mptcp_pm_get_local_id(struct mptcp_sock *msk, struct sock_common *skc,
> +			  bool *backup);
> +int mptcp_pm_nl_get_local_id(struct mptcp_sock *msk, struct mptcp_addr_info *skc,
> +			     bool *backup);
> +int mptcp_userspace_pm_get_local_id(struct mptcp_sock *msk, struct mptcp_addr_info *skc,
> +				    bool *backup);
> +bool mptcp_pm_nl_is_backup(struct mptcp_sock *msk, struct mptcp_addr_info *skc);
> +bool mptcp_userspace_pm_is_backup(struct mptcp_sock *msk, struct mptcp_addr_info *skc);
>  int mptcp_pm_dump_addr(struct sk_buff *msg, struct netlink_callback *cb);
>  int mptcp_pm_nl_dump_addr(struct sk_buff *msg,
>  			  struct netlink_callback *cb);
> diff --git a/net/mptcp/subflow.c b/net/mptcp/subflow.c
> index be406197b1c4..e60ba0327e4d 100644
> --- a/net/mptcp/subflow.c
> +++ b/net/mptcp/subflow.c
> @@ -87,6 +87,7 @@ static struct mptcp_sock *subflow_token_join_request(struct request_sock *req)
>  	struct mptcp_subflow_request_sock *subflow_req = mptcp_subflow_rsk(req);
>  	struct mptcp_sock *msk;
>  	int local_id;
> +	bool backup;
>  
>  	msk = mptcp_token_get_sock(sock_net(req_to_sk(req)), subflow_req->token);
>  	if (!msk) {
> @@ -94,12 +95,13 @@ static struct mptcp_sock *subflow_token_join_request(struct request_sock *req)
>  		return NULL;
>  	}
>  
> -	local_id = mptcp_pm_get_local_id(msk, (struct sock_common *)req);
> +	local_id = mptcp_pm_get_local_id(msk, (struct sock_common *)req, &backup);
>  	if (local_id < 0) {
>  		sock_put((struct sock *)msk);
>  		return NULL;
>  	}
>  	subflow_req->local_id = local_id;
> +	subflow_req->request_bkup = backup;

Always invoke is_backup() after get_local_id():
	
	subflow_req->request_bkup = mptcp_pm_is_backup(msk, (struct sock_common *)req);

>  
>  	return msk;
>  }
> @@ -610,16 +612,19 @@ static int subflow_chk_local_id(struct sock *sk)
>  {
>  	struct mptcp_subflow_context *subflow = mptcp_subflow_ctx(sk);
>  	struct mptcp_sock *msk = mptcp_sk(subflow->conn);
> +	bool backup;
>  	int err;
>  
>  	if (likely(subflow->local_id >= 0))
>  		return 0;
>  
> -	err = mptcp_pm_get_local_id(msk, (struct sock_common *)sk);
> +	err = mptcp_pm_get_local_id(msk, (struct sock_common *)sk, &backup);
>  	if (err < 0)
>  		return err;
>  
>  	subflow_set_local_id(subflow, err);
> +	subflow->request_bkup = backup;

	subflow->request_bkup = mptcp_pm_is_backup(msk, (struct sock_common *)sk);

WDYT?

Thanks,
-Geliang

> +
>  	return 0;
>  }
>  
> 
> -- 
> 2.45.2
>

Hi Geliang,

Thank you for the review!

On 17/07/2024 05:04, Geliang Tang wrote:
> Hi Matt,
> 
> On Tue, Jul 16, 2024 at 10:53:18PM +0200, Matthieu Baerts (NGI0) wrote:
>> There was a support for signal endpoints, but only when the endpoint's
>> flag was changed during a connection. If an endpoint with the signal and
>> backup was already present, the MP_JOIN reply was not containing the
>> backup flag as expected.
>>
>> That's confusing to have this inconsistent behaviour. On the other hand,
>> the infrastructure to set the backup flag in the SYN + ACK + MP_JOIN was
>> already there, it was just never set before. Now when requesting the
>> local ID from the path-manager, the backup status is also requested.
>>
>> There is a special case for the ID0: the PM has to return this ID0 if
>> the local address of the initial subflow is being used, and not the ID
>> of the related endpoint. Still, it is required to look at the different
>> endpoints to find if one has been defined for this address with the
>> backup flag.
>>
>> Note that when the userspace PM is used, the backup flag can be set if
>> the local address was already used before with a backup flag, e.g. if
>> the address was announced with the 'backup' flag, or a subflow was
>> created with the 'backup' flag.

(...)

>> diff --git a/net/mptcp/pm.c b/net/mptcp/pm.c
>> index 55406720c607..359738b8826e 100644
>> --- a/net/mptcp/pm.c
>> +++ b/net/mptcp/pm.c
>> @@ -405,7 +405,8 @@ bool mptcp_pm_rm_addr_signal(struct mptcp_sock *msk, unsigned int remaining,
>>  	return ret;
>>  }
>>  
>> -int mptcp_pm_get_local_id(struct mptcp_sock *msk, struct sock_common *skc)
>> +int mptcp_pm_get_local_id(struct mptcp_sock *msk, struct sock_common *skc,
>> +			  bool *backup)
>>  {
>>  	struct mptcp_addr_info skc_local;
>>  	struct mptcp_addr_info msk_local;
>> @@ -418,12 +419,18 @@ int mptcp_pm_get_local_id(struct mptcp_sock *msk, struct sock_common *skc)
>>  	 */
>>  	mptcp_local_address((struct sock_common *)msk, &msk_local);
>>  	mptcp_local_address((struct sock_common *)skc, &skc_local);
>> -	if (mptcp_addresses_equal(&msk_local, &skc_local, false))
>> +	if (mptcp_addresses_equal(&msk_local, &skc_local, false)) {
>> +		if (mptcp_pm_is_userspace(msk))
>> +			*backup = mptcp_userspace_pm_is_backup(msk, &skc_local);
>> +		else
>> +			*backup = mptcp_pm_nl_is_backup(msk, &skc_local);
>> +
>>  		return 0;
>> +	}
>>  
>>  	if (mptcp_pm_is_userspace(msk))
>> -		return mptcp_userspace_pm_get_local_id(msk, &skc_local);
>> -	return mptcp_pm_nl_get_local_id(msk, &skc_local);
>> +		return mptcp_userspace_pm_get_local_id(msk, &skc_local, backup);
>> +	return mptcp_pm_nl_get_local_id(msk, &skc_local, backup);
>>  }
> 
> I think that instead of mixing "backup" into get_local_id() interface, it is much
> better to add a new interface is_backup() for PM in pm.c:
> 
> bool mptcp_pm_is_backup(struct mptcp_sock *msk, struct sock_common *skc)
> {
>        struct mptcp_addr_info skc_local;
> 
>        mptcp_local_address((struct sock_common *)skc, &skc_local);
> 
>        if (mptcp_pm_is_userspace(msk))
>                return mptcp_userspace_pm_is_backup(msk, &skc_local);
>        return mptcp_pm_nl_is_backup(msk, &skc_local);
> }
> 
> No need to test whether the two addresses msk_local and skc_local are the same
> in mptcp_pm_is_backup(), just invoke mptcp_pm_nl_is_backup() or
> mptcp_userspace_pm_is_backup() with skc_local.
I initially wanted to avoid iterating the entry list twice in some
cases, but this special case for the ID0 increases the complexity of the
code. I can indeed switch to a new dedicated helper, thanks!

Cheers,
Matt
-- 
Sponsored by the NGI0 Core fund.