[PATCH net v2 13/15] mptcp: options: fix uninit-value in mptcp_write_data_fin

["Matthieu Baerts (NGI0)" <matttbe@kernel.org>]

When sending a DATA_FIN without data, and because the DATA_FIN occupies
1 octet of the connection-level sequence space [1], it is then required
to add a DSS mapping with specific values.

If the checksum has been negotiated, it also needs to be computed, and
included in the outgoing packet, and thus the initial csum data needs to
be reset to 0 as well. This is no longer the case since commit
cfcceb7a39fc ("tcp: shrink per-packet memset in __tcp_transmit_skb()"),
because the whole ext_copy structure is no longer zeroed by default.

This seems to be the only case where use_map is changed and set
afterwards, so initialising the csum field only in this case, along with
other fields for this specific case.

Fixes: cfcceb7a39fc ("tcp: shrink per-packet memset in __tcp_transmit_skb()")
Cc: stable@vger.kernel.org
Link: https://datatracker.ietf.org/doc/html/rfc8684#section-3.3.3 [1]
Link: https://sashiko.dev/#/patchset/20260812-net-next-mptcp-misc-feat-7-3-v1-0-1905a818f6cb%40kernel.org?part=2
Reviewed-by: Geliang Tang <geliang@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
---
Cc: Kuniyuki Iwashima <kuniyu@google.com>
Cc: Keita Morisaki <kmta1236@gmail.com>
Cc: Jakub Sitnicki <jakub@cloudflare.com>
---
 net/mptcp/options.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/net/mptcp/options.c b/net/mptcp/options.c
index 196a46e7467d..ce0de02f5a3a 100644
--- a/net/mptcp/options.c
+++ b/net/mptcp/options.c
@@ -612,6 +612,7 @@ static void mptcp_write_data_fin(struct mptcp_subflow_context *subflow,
 		ext->data_seq = data_fin_tx_seq;
 		ext->subflow_seq = 0;
 		ext->data_len = 1;
+		ext->csum = 0;
 	} else if (ext->data_seq + ext->data_len == data_fin_tx_seq) {
 		/* If there's an existing DSS mapping and it is the
 		 * final mapping, DATA_FIN consumes 1 additional byte of

-- 
2.55.0