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

[Hangbin Liu]

On Thu, Oct 22, 2026 at 04:58:41PM +0800, Xia Yan wrote:
> 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
> 

This one looks good to me. I can reproduce the report issue.

Reviewed-by: Hangbin Liu <liuhangbin@xxxxxxxxxx>