[PATCH net] 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 <redacted>
---
 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 643b97dc..7db389ae 100644
--- a/drivers/net/veth.c
+++ b/drivers/net/veth.c

@@ -1622,10 +1622,9 @@ 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);
 	}

base-commit: 78731b46967e3e7102b5cd5062bef185f3404b5c
-- 
2.43.0