FAILED: patch "[PATCH] mptcp: pm: do not ignore 'subflow' if 'signal' flag is also" failed to apply to 6.1-stable tree

The patch below does not apply to the 6.1-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-6.1.y
git checkout FETCH_HEAD
git cherry-pick -x 85df533a787bf07bf4367ce2a02b822ff1fba1a3
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2024081200-squiggle-idly-2e63@gregkh' --subject-prefix 'PATCH 6.1.y' HEAD^..

Possible dependencies:

85df533a787b ("mptcp: pm: do not ignore 'subflow' if 'signal' flag is also set") c95eb32ced82 ("mptcp: pm: reduce indentation blocks")
528cb5f2a1e8 ("mptcp: pass addr to mptcp_pm_alloc_anno_list") 77e4b94a3de6 ("mptcp: update userspace pm infos")
24430f8bf516 ("mptcp: add address into userspace pm list") fb00ee4f3343 ("mptcp: netlink: respect v4/v6-only sockets") 80638684e840 ("mptcp: get sk from msk directly")

thanks,

greg k-h

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

From 85df533a787bf07bf4367ce2a02b822ff1fba1a3 Mon Sep 17 00:00:00 2001
From: "Matthieu Baerts (NGI0)" <[email protected]>
Date: Wed, 31 Jul 2024 13:05:57 +0200
Subject: [PATCH] mptcp: pm: do not ignore 'subflow' if 'signal' flag is also
set

Up to the 'Fixes' commit, having an endpoint with both the 'signal' and 'subflow' flags, resulted in the creation of a subflow and an address announcement using the address linked to this endpoint. After this commit, only the address announcement was done, ignoring the 'subflow' flag.

That's because the same bitmap is used for the two flags. It is OK to keep this single bitmap, the already selected local endpoint simply have to be re-used, but not via select_local_address() not to look at the just modified bitmap.

Note that it is unusual to set the two flags together: creating a new subflow using a new local address will implicitly advertise it to the other peer. So in theory, no need to advertise it explicitly as well. Maybe there are use-cases -- the subflow might not reach the other peer that way, we can ask the other peer to try initiating the new subflow without delay -- or very likely the user is confused, and put both flags "just to be sure at least the right one is set". Still, if it is allowed, the kernel should do what has been asked: using this endpoint to announce the address and to create a new subflow from it.

An alternative is to forbid the use of the two flags together, but that's probably too late, there are maybe use-cases, and it was working before. This patch will avoid people complaining subflows are not created using the endpoint they added with the 'subflow' and 'signal' flag.

Note that with the current patch, the subflow might not be created in some corner cases, e.g. if the 'subflows' limit was reached when sending the ADD_ADDR, but changed later on. It is probably not worth splitting id_avail_bitmap per target ('signal', 'subflow'), which will add another large field to the msk "just" to track (again) endpoints. Anyway, currently when the limits are changed, the kernel doesn't check if new subflows can be created or removed, because we would need to keep track of the received ADD_ADDR, and more. It sounds OK to assume that the limits should be properly configured before establishing new connections.

index 2be7af377cda..4cae2aa7be5c 100644
@@ -512,8 +512,8 @@ __lookup_addr(struct pm_nl_pernet *pernet, const struct mptcp_addr_info *info)
 
 static void mptcp_pm_create_subflow_or_signal_addr(struct mptcp_sock *msk)
 {
+	struct mptcp_pm_addr_entry *local, *signal_and_subflow = NULL;
 	struct sock *sk = (struct sock *)msk;
-	struct mptcp_pm_addr_entry *local;
 	unsigned int add_addr_signal_max;
 	unsigned int local_addr_max;
 	struct pm_nl_pernet *pernet;

@@ -579,6 +579,9 @@ static void mptcp_pm_create_subflow_or_signal_addr(struct mptcp_sock *msk)
 		msk->pm.add_addr_signaled++;
 		mptcp_pm_announce_addr(msk, &local->addr, false);
 		mptcp_pm_nl_addr_send_ack(msk);
+
+		if (local->flags & MPTCP_PM_ADDR_FLAG_SUBFLOW)
+			signal_and_subflow = local;
 	}
 
 subflow:

@@ -589,9 +592,14 @@ static void mptcp_pm_create_subflow_or_signal_addr(struct mptcp_sock *msk)
 		bool fullmesh;
 		int i, nr;
 
-		local = select_local_address(pernet, msk);
-		if (!local)
-			break;
+		if (signal_and_subflow) {
+			local = signal_and_subflow;
+			signal_and_subflow = NULL;
+		} else {
+			local = select_local_address(pernet, msk);
+			if (!local)
+				break;
+		}
 
 		fullmesh = !!(local->flags & MPTCP_PM_ADDR_FLAG_FULLMESH);