[PATCH net v2 1/2] veth: keep carrier off when resizing channels with a down peer

[Xia Yan <yana06070@163.com>]

veth_set_channels() temporarily turns carrier off while reconfiguring a
running interface. It then turns carrier on for both ends without
checking whether the peer is administratively up. If only one end is up,
changing its channel counts therefore reports LOWER_UP even though the
peer remains down.

For example, after creating a pair with two RX and TX queues per end:

  ip link set vca up
  ethtool -L vca rx 1 tx 1

vca incorrectly reports LOWER_UP while vcb is still down.

Only restore carrier when the peer has IFF_UP, matching veth_open().
The local interface is already known to be running at this point.

Found during LLM-assisted source review and reproduced with a veth pair
in a network namespace. Built and booted UML kernels before and after
the fix; the accompanying test fails in the four mixed up/down cases
before the fix and passes all six cases after it.

Fixes: 4752eeb3d891 ("veth: implement support for set_channel ethtool op")
Cc: stable@vger.kernel.org
Assisted-by: LLM
Signed-off-by: Xia Yan <yana06070@163.com>
---
 drivers/net/veth.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/drivers/net/veth.c b/drivers/net/veth.c
index 643b97d..4e75830 100644
--- a/drivers/net/veth.c
+++ b/drivers/net/veth.c
@@ -1372,9 +1372,10 @@ static int veth_set_channels(struct net_device *dev,
 		 * to identify the range we have to disable
 		 */
 		veth_disable_range_safe(dev, new_rx_count, old_rx_count);
-		netif_carrier_on(dev);
-		if (peer)
+		if (peer && (peer->flags & IFF_UP)) {
+			netif_carrier_on(dev);
 			netif_carrier_on(peer);
+		}
 	}
 
 	/* update XDP supported features */

base-commit: 513444cc62e231a11c9b15ea88de0890da9b2b4f
-- 
2.43.0