netdev - [PATCH v1 net 1/2] net: update comments for SKB_GSO_TCP_ECN and SKB_GSO_TCP

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [day] [month] [year] [list]

Message-Id: <20260204090310.7528-2-chia-yu.chang@nokia-bell-labs.com>
Date: Wed,  4 Feb 2026 10:03:09 +0100
From: chia-yu.chang@...ia-bell-labs.com
To: parav@...dia.com,
	jasowang@...hat.com,
	mst@...hat.com,
	shenjian15@...wei.com,
	salil.mehta@...wei.com,
	shaojijie@...wei.com,
	saeedm@...dia.com,
	tariqt@...dia.com,
	mbloch@...dia.com,
	leonro@...dia.com,
	linux-rdma@...r.kernel.org,
	netdev@...r.kernel.org,
	davem@...emloft.net,
	edumazet@...gle.com,
	kuba@...nel.org,
	pabeni@...hat.com,
	horms@...nel.org,
	ij@...nel.org,
	ncardwell@...gle.com,
	koen.de_schepper@...ia-bell-labs.com,
	g.white@...lelabs.com,
	ingemar.s.johansson@...csson.com,
	mirja.kuehlewind@...csson.com,
	cheshire@...le.com,
	rs.ietf@....at,
	Jason_Livingood@...cast.com,
	vidhi_goel@...le.com
Cc: Chia-Yu Chang <chia-yu.chang@...ia-bell-labs.com>
Subject: [PATCH v1 net 1/2] net: update comments for SKB_GSO_TCP_ECN and SKB_GSO_TCP_ACCECN

From: Chia-Yu Chang <chia-yu.chang@...ia-bell-labs.com>

This patch updates the documentation of ECN‑related GSO flags, it
clarifies the limitations of SKB_GSO_TCP_ECN and explains how to preserve
the CWR flag (part of the ACE signal) in the Rx path.

For Tx, SKB_GSO_TCP_ECN and SKB_GSO_TCP_ACCECN are used respectively for
RFC3168 ECN and AccECN (RFC9768). SKB_GSO_TCP_ECN indicates that the
first segment has CWR set, while subsequent segments have CWR cleared.
In contrast, SKB_GSO_TCP_ACCECN means that the segment uses AccECN and
therefore its CWR flag must not be modified durging segmentation.

For RX, SKB_GSO_TCP_ECN shall NOT be used, because the stack cannot know
whether the connection uses RFC3168 ECN or AccECN, whereas RFC3168 ECN
offload may clear CWR flag and thus corrupts the ACE signal. Instead, any
segment that arrives with CWR set must use the SKB_GSO_TCP_ACCECN flag
to prevent RFC3168 ECN offload logic from clearing the CWR flag.

Co-developed-by: Ilpo Järvinen <ij@...nel.org>
Signed-off-by: Ilpo Järvinen <ij@...nel.org>
Signed-off-by: Chia-Yu Chang <chia-yu.chang@...ia-bell-labs.com>
---
 include/linux/skbuff.h | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 112e48970338..85ec9632af58 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -671,7 +671,13 @@ enum {
 	/* This indicates the skb is from an untrusted source. */
 	SKB_GSO_DODGY = 1 << 1,
 
-	/* This indicates the tcp segment has CWR set. */
+	/* For TX, this indicates that the first TCP segment has CWR set, and
+	 * any subsequent segment in the same skb has CWR cleared. This flag
+	 * must not be used in RX, because the connection to which the segment
+	 * belongs is not tracked to use RFC3168 or AccECN. Using RFC3168 ECN
+	 * offload may clear CWR and corrupt ACE signal (CWR is part of it).
+	 * Instead, SKB_GSO_TCP_ACCECN shall be used to avoid CWR corruption.
+	 */
 	SKB_GSO_TCP_ECN = 1 << 2,
 
 	__SKB_GSO_TCP_FIXEDID = 1 << 3,
@@ -706,6 +712,13 @@ enum {
 
 	SKB_GSO_FRAGLIST = 1 << 18,
 
+	/* For TX, this indicates that the TCP segment uses the CWR flag as part
+	 * of the ACE signal, and the CWR flag must not be modified in the skb.
+	 * For RX, any incoming segment with CWR set must use this flag so that
+	 * no RFC3168 ECN offload can clear the CWR flag. This is required to
+	 * preserve ACE signal correceness (CWR is part of it) in a forwarding
+	 * scenario, e.g., from one netdevice RX to other netdevice TX
+	 */
 	SKB_GSO_TCP_ACCECN = 1 << 19,
 
 	/* These indirectly map onto the same netdev feature.
-- 
2.34.1