Re: [PATCH] mptcp: only honor zero-length DATA_FIN when a mapping is present

[Paolo Abeni]

On 6/29/26 1:00 PM, Michael Bommarito wrote:
> On Mon, Jun 29, 2026 at 5:50 AM Paolo Abeni <pabeni@xxxxxxxxxx> wrote:
>> Isn't this fixed by commit 5e939544f9d2 ("mptcp: fix uninit-value in
>> mptcp_established_options") ?
> 
> I did the reproduction ~10 days ago on linus's latest, so definitely
> still reproducing.  I think 5e939544f9d2 was on the TX side and this
> is about the RX option path, so they don't overlap on flows either.


Right.

AFAICS the RFC is a little vague about enforcing meaningful flags
combination.

I think it would be better to avoid another conditional while processing
incoming ack. Does the following solve the issue? 

Thanks!

---
diff --git a/net/mptcp/options.c b/net/mptcp/options.c
index 0ca60314d667..b924209a9b74 100644
--- a/net/mptcp/options.c
+++ b/net/mptcp/options.c
@@ -157,7 +157,6 @@ static void mptcp_parse_option(const struct sk_buff *skb,
 		ptr++;
 
 		flags = (*ptr++) & MPTCP_DSS_FLAG_MASK;
-		mp_opt->data_fin = (flags & MPTCP_DSS_DATA_FIN) != 0;
 		mp_opt->dsn64 = (flags & MPTCP_DSS_DSN64) != 0;
 		mp_opt->use_map = (flags & MPTCP_DSS_HAS_MAP) != 0;
 		mp_opt->ack64 = (flags & MPTCP_DSS_ACK64) != 0;
@@ -178,6 +177,7 @@ static void mptcp_parse_option(const struct sk_buff *skb,
 		}
 
 		if (mp_opt->use_map) {
+			mp_opt->data_fin = (flags & MPTCP_DSS_DATA_FIN) != 0;
 			if (mp_opt->dsn64)
 				expected_opsize += TCPOLEN_MPTCP_DSS_MAP64;
 			else