[PATCH iwl-net v1] i40e: don't advertise IFF_SUPP_NOFCS

[PATCH iwl-net v1] i40e: don't advertise IFF_SUPP_NOFCS

[Kohei Enju]

i40e advertises IFF_SUPP_NOFCS, allowing users to use the SO_NOFCS
socket option. However, this option is silently ignored, as the driver
does not check skb->no_fcs, and always enables FCS insertion offload.

Fix this by removing the advertisement of IFF_SUPP_NOFCS.

This behavior can be reproduced with a simple AF_PACKET socket:

  import socket
  s = socket.socket(socket.AF_PACKET, socket.SOCK_RAW)
  s.setsockopt(socket.SOL_SOCKET, 43, 1) # SO_NOFCS
  s.bind(("eth0", 0))
  s.send(b'\xff' * 64)

Previously, send() succeeds but the driver ignores SO_NOFCS.
With this change, send() fails with -EPROTONOSUPPORT, as expected.

Fixes: 41c445ff0f48 ("i40e: main driver core")
Signed-off-by: Kohei Enju <kohei@enjuk.jp>
---
 drivers/net/ethernet/intel/i40e/i40e_main.c | 1 -
 1 file changed, 1 deletion(-)

diff --git a/drivers/net/ethernet/intel/i40e/i40e_main.c b/drivers/net/ethernet/intel/i40e/i40e_main.c
index 3749f32ef95a..a3c52aa6255b 100644
--- a/drivers/net/ethernet/intel/i40e/i40e_main.c
+++ b/drivers/net/ethernet/intel/i40e/i40e_main.c
@@ -13831,7 +13831,6 @@ static int i40e_config_netdev(struct i40e_vsi *vsi)
 	netdev->neigh_priv_len = sizeof(u32) * 4;
 
 	netdev->priv_flags |= IFF_UNICAST_FLT;
-	netdev->priv_flags |= IFF_SUPP_NOFCS;
 	/* Setup netdev TC information */
 	i40e_vsi_config_netdev_tc(vsi, vsi->tc_config.enabled_tc);
 
-- 
2.51.0

Re: [Intel-wired-lan] [PATCH iwl-net v1] i40e: don't advertise IFF_SUPP_NOFCS

[Paul Menzel]

Dear Kohei,


Thank you for your patch.

Am 25.03.26 um 21:50 schrieb Kohei Enju:
> i40e advertises IFF_SUPP_NOFCS, allowing users to use the SO_NOFCS
> socket option. However, this option is silently ignored, as the driver
> does not check skb->no_fcs, and always enables FCS insertion offload.
> 
> Fix this by removing the advertisement of IFF_SUPP_NOFCS.
> 
> This behavior can be reproduced with a simple AF_PACKET socket:
> 
>    import socket
>    s = socket.socket(socket.AF_PACKET, socket.SOCK_RAW)
>    s.setsockopt(socket.SOL_SOCKET, 43, 1) # SO_NOFCS
>    s.bind(("eth0", 0))
>    s.send(b'\xff' * 64)
> 
> Previously, send() succeeds but the driver ignores SO_NOFCS.

Would I check that with tcpdump?

> With this change, send() fails with -EPROTONOSUPPORT, as expected.

Great commit message! Thank you.

One question, did you look into, what is needed to implement working 
SO_NOFC support?

> Fixes: 41c445ff0f48 ("i40e: main driver core")
> Signed-off-by: Kohei Enju <kohei@enjuk.jp>
> ---
>   drivers/net/ethernet/intel/i40e/i40e_main.c | 1 -
>   1 file changed, 1 deletion(-)
> 
> diff --git a/drivers/net/ethernet/intel/i40e/i40e_main.c b/drivers/net/ethernet/intel/i40e/i40e_main.c
> index 3749f32ef95a..a3c52aa6255b 100644
> --- a/drivers/net/ethernet/intel/i40e/i40e_main.c
> +++ b/drivers/net/ethernet/intel/i40e/i40e_main.c
> @@ -13831,7 +13831,6 @@ static int i40e_config_netdev(struct i40e_vsi *vsi)
>   	netdev->neigh_priv_len = sizeof(u32) * 4;
>   
>   	netdev->priv_flags |= IFF_UNICAST_FLT;
> -	netdev->priv_flags |= IFF_SUPP_NOFCS;
>   	/* Setup netdev TC information */
>   	i40e_vsi_config_netdev_tc(vsi, vsi->tc_config.enabled_tc);

Nice catch.


Kind regards,

Paul

Re: [Intel-wired-lan] [PATCH iwl-net v1] i40e: don't advertise IFF_SUPP_NOFCS

[Kohei Enju]

On 03/26 07:05, Paul Menzel wrote:
> Dear Kohei,
> 
> 
> Thank you for your patch.
> 
> Am 25.03.26 um 21:50 schrieb Kohei Enju:
> > i40e advertises IFF_SUPP_NOFCS, allowing users to use the SO_NOFCS
> > socket option. However, this option is silently ignored, as the driver
> > does not check skb->no_fcs, and always enables FCS insertion offload.
> > 
> > Fix this by removing the advertisement of IFF_SUPP_NOFCS.
> > 
> > This behavior can be reproduced with a simple AF_PACKET socket:
> > 
> >    import socket
> >    s = socket.socket(socket.AF_PACKET, socket.SOCK_RAW)
> >    s.setsockopt(socket.SOL_SOCKET, 43, 1) # SO_NOFCS
> >    s.bind(("eth0", 0))
> >    s.send(b'\xff' * 64)
> > 
> > Previously, send() succeeds but the driver ignores SO_NOFCS.
> 
> Would I check that with tcpdump?

On the RX side, it can be possible to observe such frames using tcpdump,
if the RX device supports:
  - rx-all (accepting all frames including bad frames)
  - keeping the FCS (not stripping it)

On the TX side, however, the hardware-generated FCS is not visible to
tcpdump.

In my setup, I verified that the CRC error stats didn't increase on the
RX side when using the script above. If SO_NOFCS were actually honored,
we would expect frames with invalid/custom CRC to be transmitted, and
the RX side device should report CRC errors.

> 
> > With this change, send() fails with -EPROTONOSUPPORT, as expected.
> 
> Great commit message! Thank you.
> 
> One question, did you look into, what is needed to implement working SO_NOFC
> support?

From the driver implementation, it looks like clearing
I40E_TX_DESC_CMD_ICRC when skb->no_fcs is set could enable the expected
behavior.
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/net/ethernet/intel/i40e/i40e_txrx.c#n3936

However, implementing proper support would require careful validation,
and also may impact the TX fast path slightly. So this patch focuses on
just correcting netdev->priv_flags.

> 
> > Fixes: 41c445ff0f48 ("i40e: main driver core")
> > Signed-off-by: Kohei Enju <kohei@enjuk.jp>
> > ---
> >   drivers/net/ethernet/intel/i40e/i40e_main.c | 1 -
> >   1 file changed, 1 deletion(-)
> > 
> > diff --git a/drivers/net/ethernet/intel/i40e/i40e_main.c b/drivers/net/ethernet/intel/i40e/i40e_main.c
> > index 3749f32ef95a..a3c52aa6255b 100644
> > --- a/drivers/net/ethernet/intel/i40e/i40e_main.c
> > +++ b/drivers/net/ethernet/intel/i40e/i40e_main.c
> > @@ -13831,7 +13831,6 @@ static int i40e_config_netdev(struct i40e_vsi *vsi)
> >   	netdev->neigh_priv_len = sizeof(u32) * 4;
> >   	netdev->priv_flags |= IFF_UNICAST_FLT;
> > -	netdev->priv_flags |= IFF_SUPP_NOFCS;
> >   	/* Setup netdev TC information */
> >   	i40e_vsi_config_netdev_tc(vsi, vsi->tc_config.enabled_tc);
> 
> Nice catch.
> 
> 
> Kind regards,
> 
> Paul

RE: [Intel-wired-lan] [PATCH iwl-net v1] i40e: don't advertise IFF_SUPP_NOFCS

[Loktionov, Aleksandr]

> -----Original Message-----
> From: Intel-wired-lan <intel-wired-lan-bounces@osuosl.org> On Behalf
> Of Kohei Enju
> Sent: Wednesday, March 25, 2026 9:50 PM
> To: intel-wired-lan@lists.osuosl.org; netdev@vger.kernel.org
> Cc: Nguyen, Anthony L <anthony.l.nguyen@intel.com>; Kitszel,
> Przemyslaw <przemyslaw.kitszel@intel.com>; Andrew Lunn
> <andrew+netdev@lunn.ch>; David S. Miller <davem@davemloft.net>; Eric
> Dumazet <edumazet@google.com>; Jakub Kicinski <kuba@kernel.org>; Paolo
> Abeni <pabeni@redhat.com>; Shannon Nelson <sln@onemain.com>; Jesse
> Brandeburg <jesse.brandeburg@intel.com>; kohei.enju@gmail.com; Kohei
> Enju <kohei@enjuk.jp>
> Subject: [Intel-wired-lan] [PATCH iwl-net v1] i40e: don't advertise
> IFF_SUPP_NOFCS
> 
> i40e advertises IFF_SUPP_NOFCS, allowing users to use the SO_NOFCS
> socket option. However, this option is silently ignored, as the driver
> does not check skb->no_fcs, and always enables FCS insertion offload.
> 
> Fix this by removing the advertisement of IFF_SUPP_NOFCS.
> 
> This behavior can be reproduced with a simple AF_PACKET socket:
> 
>   import socket
>   s = socket.socket(socket.AF_PACKET, socket.SOCK_RAW)
>   s.setsockopt(socket.SOL_SOCKET, 43, 1) # SO_NOFCS
>   s.bind(("eth0", 0))
>   s.send(b'\xff' * 64)
> 
> Previously, send() succeeds but the driver ignores SO_NOFCS.
> With this change, send() fails with -EPROTONOSUPPORT, as expected.
> 
> Fixes: 41c445ff0f48 ("i40e: main driver core")
> Signed-off-by: Kohei Enju <kohei@enjuk.jp>
> ---
>  drivers/net/ethernet/intel/i40e/i40e_main.c | 1 -
>  1 file changed, 1 deletion(-)
> 
> diff --git a/drivers/net/ethernet/intel/i40e/i40e_main.c
> b/drivers/net/ethernet/intel/i40e/i40e_main.c
> index 3749f32ef95a..a3c52aa6255b 100644
> --- a/drivers/net/ethernet/intel/i40e/i40e_main.c
> +++ b/drivers/net/ethernet/intel/i40e/i40e_main.c
> @@ -13831,7 +13831,6 @@ static int i40e_config_netdev(struct i40e_vsi
> *vsi)
>  	netdev->neigh_priv_len = sizeof(u32) * 4;
> 
>  	netdev->priv_flags |= IFF_UNICAST_FLT;
> -	netdev->priv_flags |= IFF_SUPP_NOFCS;
>  	/* Setup netdev TC information */
>  	i40e_vsi_config_netdev_tc(vsi, vsi->tc_config.enabled_tc);
> 
> --
> 2.51.0

I'd recommend to add Cc: stable@vger.kernel.org

Reviewed-by: Aleksandr Loktionov <aleksandr.loktionov@intel.com>