[PATCH net] net: bcmgenet: unshare the skb before writing the control block

[PATCH net] net: bcmgenet: unshare the skb before writing the control block

[Nicolai Buchwitz]

bcmgenet stores first_cb, last_cb and bytes_sent in the skb control
block and reads them back on completion to free the skb and credit BQL.
ether_setup() leaves IFF_TX_SKB_SHARING set, so the stack can resubmit
a shared skb that is still in the ring. The next transmit overwrites
last_cb, so the earlier slots complete without crediting their bytes
and BQL stalls the queue until the watchdog resets it.

Unshare the skb before writing the control block.

Reproduce with pktgen in clone_skb mode, which hands the driver the same
skb while earlier copies are still in the ring:

  modprobe pktgen
  pg=/proc/net/pktgen
  echo "add_device eth0" > $pg/kpktgend_0
  echo "dst_mac AA:BB:CC:DD:EE:FF" > $pg/eth0
  echo "dst 192.0.2.1" > $pg/eth0
  echo "clone_skb 8" > $pg/eth0
  echo "count 0" > $pg/eth0
  echo start > $pg/pgctrl &
  sleep 5
  echo stop > $pg/pgctrl
  wait
  cat /sys/class/net/eth0/queues/tx-0/byte_queue_limits/inflight

inflight stays nonzero on the now idle queue. clone_skb 0 leaves it at
zero.

Fixes: f48bed16a756 ("net: bcmgenet: Free skb after last Tx frag")
Signed-off-by: Nicolai Buchwitz <nb@tipi-net.de>
---
Another finding while testing the genet MTU series. Similar shared-skb
bug as the macb fixes already merged in net [1], here in bcmgenet's Tx
control block rather than the software FCS path.

[1] https://lore.kernel.org/netdev/20261006-nb-macb-shared-skb-net-v1-0-a80641479041@tipi-net.de/
---
 drivers/net/ethernet/broadcom/genet/bcmgenet.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/drivers/net/ethernet/broadcom/genet/bcmgenet.c b/drivers/net/ethernet/broadcom/genet/bcmgenet.c
index e8908916558b..f4cfbff49a1b 100644
--- a/drivers/net/ethernet/broadcom/genet/bcmgenet.c
+++ b/drivers/net/ethernet/broadcom/genet/bcmgenet.c
@@ -2153,6 +2153,16 @@ static netdev_tx_t bcmgenet_xmit(struct sk_buff *skb, struct net_device *dev)
 		goto out;
 	}
 
+	/* We store Tx state in the control block, so the skb must not be
+	 * shared, but ether_setup() leaves IFF_TX_SKB_SHARING set.
+	 */
+	skb = skb_share_check(skb, GFP_ATOMIC);
+	if (!skb) {
+		BCMGENET_STATS64_INC((&ring->stats64), dropped);
+		ret = NETDEV_TX_OK;
+		goto out;
+	}
+
 	/* Retain how many bytes will be sent on the wire, without TSB inserted
 	 * by transmit checksum offload
 	 */

---
base-commit: af32da41b0327b9c6a37856ba82b6760d6c8d10e
change-id: 20261009-nb-genet-shared-skb-net-5747ee5df864

Best regards,
--  
Nicolai Buchwitz <nb@tipi-net.de>

Re: [PATCH net] net: bcmgenet: unshare the skb before writing the control block

[netdev-bot+sinfo]

Hi!

This is an automated message. This series looks like a fix, but its
commit messages seem to be missing some information:

 - What hardware the change was tested on. For driver fixes please
   mention the device (and if relevant firmware version) used for
   testing, or say that the change was not tested on real hardware.

Please do not repost the series just to address the above. Instead,
reply to this email with the missing information, so that reviewers
can take it into account. If the series needs another revision for
other reasons, please include the information in the commit messages
then.

The evaluation is done by an LLM so it may be wrong, if you think
that is the case please reply and explain.

Re: [PATCH net] net: bcmgenet: unshare the skb before writing the control block

[Nicolai Buchwitz]

On 9.10.2026 12:14, netdev-bot+sinfo@kernel.org wrote:
> Hi!
> 
> This is an automated message. This series looks like a fix, but its
> commit messages seem to be missing some information:
> 
>  - What hardware the change was tested on. For driver fixes please
>    mention the device (and if relevant firmware version) used for
>    testing, or say that the change was not tested on real hardware.

Raspberry Pi CM4

> 
> Please do not repost the series just to address the above. Instead,
> reply to this email with the missing information, so that reviewers
> can take it into account. If the series needs another revision for
> other reasons, please include the information in the commit messages
> then.
> 
> The evaluation is done by an LLM so it may be wrong, if you think
> that is the case please reply and explain.