FAILED: patch "[PATCH] veth: fix stale peer max_mtu on XDP program replacement" failed to apply to 6.1-stable tree

[Sasha Levin <sashal@kernel.org>]

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 <stable@vger.kernel.org>.

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 c955325db639e9503941227e14e1f1ccc99442a1
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@vger.kernel.org>' --in-reply-to '20261107055359-failed-6.1-c955325db639@kernel.org' --subject-prefix 'PATCH 6.1.y' 'HEAD^..'

Thanks,
Sasha

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

From c955325db639e9503941227e14e1f1ccc99442a1 Mon Sep 17 00:00:00 2001
From: yanxia <yanxia@cmss.chinamobile.com>
Date: Tue, 20 Oct 2026 10:01:47 +0800
Subject: [PATCH] veth: fix stale peer max_mtu on XDP program replacement

veth_xdp_set() calculates the peer MTU limit from the incoming program's
XDP fragment capability, but only stores it when no program was attached.
Replacing a program with one having a different fragment capability
therefore leaves the peer's max_mtu stale.

Replacing a regular program with an xdp.frags program keeps rejecting
otherwise valid jumbo MTUs. The opposite replacement leaves a larger
limit in place and permits MTUs which a fresh attachment of the regular
program would reject.

Update peer->max_mtu after every successful attachment or replacement.
Keep the GSO feature update conditional on the first attachment.

Found during review of the XDP replacement path and confirmed with
native XDP on an isolated veth pair. Before the fix, direct replacement
rejects MTU 9000 after switching to xdp.frags, and accepts it after
switching back to a regular program. Detaching and attaching instead
produces the expected results in both directions.

UML tests on net reproduce the problem before this change and match the
detach/attach controls afterwards, with both devices up or down and
with either endpoint receiving XDP. Failed replacements preserve the
previous state; same-type replacements and detach also behave as
expected. Small-frame traffic passes in the devices-up cases.

Fixes: 7cda76d858a4 ("veth: Allow jumbo frames in xdp mode")
Cc: stable@vger.kernel.org
Signed-off-by: yanxia <yanxia@cmss.chinamobile.com>
Link: https://patch.msgid.link/20261020020147.790274-1-yanxia@cmss.chinamobile.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
---
 drivers/net/veth.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/drivers/net/veth.c b/drivers/net/veth.c
index 6205742f88d9c..3eca92d3dc60d 100644
--- a/drivers/net/veth.c
+++ b/drivers/net/veth.c
@@ -1627,10 +1627,9 @@ static int veth_xdp_set(struct net_device *dev, struct bpf_prog *prog,
 			}
 		}
 
-		if (!old_prog) {
+		if (!old_prog)
 			peer->hw_features &= ~NETIF_F_GSO_SOFTWARE;
-			peer->max_mtu = max_mtu;
-		}
+		peer->max_mtu = max_mtu;
 
 		xdp_features_set_redirect_target(peer, true);
 	}
-- 
2.53.0