[RFC PATCH net] mptcp: pm: fix ADD_ADDR timer infinite retry on option space insufficient

[Li Xiasong <lixiasong1@huawei.com>]

When TCP option space is insufficient (e.g., IPv6 with tcp_timestamps
enabled), the original code jumped to out_unlock without clearing the
addr_signal flag. This caused mptcp_pm_add_timer to keep rescheduling
indefinitely without sending ADD_ADDR, preventing the endpoint list from
being traversed.

In a pure ACK scenario (indicated by drop_other_suboptions=true), if
the option space is insufficient to carry the ADD_ADDR suboption, it
is appropriate to drop this address signal to allow the timer handler
to move on to other addresses.

Fixes: 00cfd77b9063 ("mptcp: retransmit ADD_ADDR when timeout")
Signed-off-by: Li Xiasong <lixiasong1@huawei.com>
---

Seeking feedback on:

When announcing addresses to the peer, MPTCP sends a pure ACK packet
to carry MPTCP options (ADD_ADDR). In this scenario, if the option space
is insufficient for ADD_ADDR, clearing addr_signal would:

  - Prevent the timer from retrying infinitely
  - Allow the timer to continue traversing and processing other addresses
  - Not block other subflow creation or address announcement operations

Is there any scenario where we should retry later instead of clearing
the address signal/echo flag? However, if a pure ACK doesn't have
enough space for the flag, subsequent packets won't either.

---
 net/mptcp/pm.c | 17 ++++++++---------
 1 file changed, 8 insertions(+), 9 deletions(-)

diff --git a/net/mptcp/pm.c b/net/mptcp/pm.c
index 57a456690406..1d49779c6a1f 100644
--- a/net/mptcp/pm.c
+++ b/net/mptcp/pm.c
@@ -881,19 +881,18 @@ bool mptcp_pm_add_addr_signal(struct mptcp_sock *msk, const struct sk_buff *skb,
 	}
 
 	*echo = mptcp_pm_should_add_signal_echo(msk);
+	add_addr = msk->pm.addr_signal &
+		~(*echo ? BIT(MPTCP_ADD_ADDR_ECHO) : BIT(MPTCP_ADD_ADDR_SIGNAL));
 	port = !!(*echo ? msk->pm.remote.port : msk->pm.local.port);
-
 	family = *echo ? msk->pm.remote.family : msk->pm.local.family;
-	if (remaining < mptcp_add_addr_len(family, *echo, port))
-		goto out_unlock;
 
-	if (*echo) {
-		*addr = msk->pm.remote;
-		add_addr = msk->pm.addr_signal & ~BIT(MPTCP_ADD_ADDR_ECHO);
-	} else {
-		*addr = msk->pm.local;
-		add_addr = msk->pm.addr_signal & ~BIT(MPTCP_ADD_ADDR_SIGNAL);
+	if (remaining < mptcp_add_addr_len(family, *echo, port)) {
+		if (*drop_other_suboptions)
+			WRITE_ONCE(msk->pm.addr_signal, add_addr);
+		goto out_unlock;
 	}
+
+	*addr = *echo ? msk->pm.remote : msk->pm.local;
 	WRITE_ONCE(msk->pm.addr_signal, add_addr);
 	ret = true;
 
-- 
2.34.1