[PATCH net v4 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.
The companion veth.sh test reproduces the two mixed up/down failures
before the fix and passes after it.

Fixes: 4752eeb3d891 ("veth: implement support for set_channel ethtool op")
Cc: stable@xxxxxxxxxxxxxxx
Reviewed-by: Hangbin Liu <liuhangbin@xxxxxxxxxx>
Assisted-by: LLM
Signed-off-by: Xia Yan <yana06070@xxxxxxx>
---
Changes in v4:
- Driver code is unchanged. Add Hangbin's Reviewed-by.
- The companion test drops the extra GRO on/off runs, as discussed
  with Hangbin, and keeps up/up and both mixed up/down states.

v3: https://lore.kernel.org/netdev/20261022085842.2527-1-yana06070@xxxxxxx/

Testing on 513444cc62e2, x86_64 QEMU/KVM with three vCPUs:
- Complete default veth.sh:
  before: 56 checks pass, two mixed up/down carrier checks fail;
  after: all 58 checks pass. The 55 existing checks pass on both.
- Script exit status: 1 before, 0 after. No checks were skipped.
- Both guests powered down normally.
- bash -n and git diff --check pass.

 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 643b97dc52456df80de10068ccac9af2ada70256..4e75830061ccd3def0476b19a2f92e818464fce9 100644
--- a/drivers/net/veth.c
+++ b/drivers/net/veth.c
@@ -1372,9 +1372,10 @@ out:
 		 * 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

-- 
2.39.0.windows.2