Re: [PATCH net v5 1/2] veth: keep carrier off when resizing channels with a down peer

[Jiayuan Chen <jiayuan.chen@linux.dev>]

On 10/29/26 10:42 AM, 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.
> 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@vger.kernel.org
> Reviewed-by: Hangbin Liu <liuhangbin@kylinos.cn>
> Assisted-by: LLM
> Signed-off-by: Xia Yan <yana06070@163.com>



Reviewed-by: Jiayuan Chen <jiayuan.chen@linux.dev>


Thanks, I clearly remember it has been reported somewhere...

> ---
> Changes in v5:
> - Driver code is unchanged.
> - The companion test drops unnecessary quotes around literal up
>    and carries Hangbin's review and test tags.
>
> v4: https://lore.kernel.org/netdev/20261024093007.12981-1-yana06070@163.com/
>
> 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@163.com/
>
> Runtime testing of v4 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.
> - The runtime results above are from v4. The only v5 script change
>    removes quotes around a literal argument.
>
>   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
>