[PATCH net v3 1/2] veth: keep carrier off when resizing channels with a down peer

[Xia Yan]

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 network namespaces, without kernel instrumentation or error injection.
With the complete default veth.sh on x86_64 QEMU/KVM (three vCPUs), the
four mixed up/down cases fail before the fix and all six carrier cases
pass after it. All 55 existing checks pass on both kernels.

Fixes: 4752eeb3d891 ("veth: implement support for set_channel ethtool op")
Cc: stable@xxxxxxxxxxxxxxx
Assisted-by: LLM
Signed-off-by: Xia Yan <yana06070@xxxxxxx>
---

Notes:
    Changes in v3:
    - Driver code is unchanged from v2.
    - The companion selftest is now part of veth.sh.

    v2: https://lore.kernel.org/netdev/20261020135544.24441-1-yana06070@xxxxxxx/

    Testing on the v2 baseline 513444cc62e2:
    - Complete default veth.sh on x86_64 QEMU/KVM, three vCPUs:
      before: 57 checks pass, four mixed up/down carrier checks fail;
      after: all 61 checks pass. The 55 existing checks pass on both.
    - bash -n and git diff --check pass.
    - If creating the veth pair fails, the case is skipped without
      masking failures from earlier test cases.

    Both patches also apply to net/main ffdf678dd166
    (application check only, checked on 2026-10-22).

    The QEMU guests failed to power off cleanly (initramfs issue) after
    the tests completed; the exit codes above are the veth.sh results.


 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