Re: [PATCH] sfc: siena: handle NULL returned by xdp_convert_buff_to_frame()

[Simon Horman <horms@kernel.org>]

On Fri, Jul 25, 2025 at 10:34:01AM +0100, Simon Horman wrote:
> On Tue, Jul 22, 2025 at 07:32:30PM -0500, Chenyuan Yang wrote:
> > The xdp_convert_buff_to_frame() function can return NULL when there is
> > insufficient headroom in the buffer to store the xdp_frame structure
> > or when the driver didn't reserve enough tailroom for skb_shared_info.
> > 
> > Currently, the sfc siena driver does not check for this NULL return value
> > in the XDP_TX case within efx_do_xdp().
> > 
> > Fix by adding a proper NULL check in the XDP_TX case. If conversion
> > fails, free the RX buffer and increment the bad drops counter, following
> > the same pattern used for other XDP error conditions in this driver.
> > 
> > Signed-off-by: Chenyuan Yang <chenyuan0y@gmail.com>
> > Fixes: d48523cb88e0 ("sfc: Copy shared files needed for Siena (part 2)")
> > ---
> >  drivers/net/ethernet/sfc/siena/rx.c | 6 ++++++
> >  1 file changed, 6 insertions(+)
> > 
> > diff --git a/drivers/net/ethernet/sfc/siena/rx.c b/drivers/net/ethernet/sfc/siena/rx.c
> > index 98d3c0743c0f..65f6daad3168 100644
> > --- a/drivers/net/ethernet/sfc/siena/rx.c
> > +++ b/drivers/net/ethernet/sfc/siena/rx.c
> > @@ -310,6 +310,12 @@ static bool efx_do_xdp(struct efx_nic *efx, struct efx_channel *channel,
> >  	case XDP_TX:
> >  		/* Buffer ownership passes to tx on success. */
> >  		xdpf = xdp_convert_buff_to_frame(&xdp);
> > +		if (unlikely(!xdpf)) {
> > +			efx_siena_free_rx_buffers(rx_queue, rx_buf, 1);
> > +			channel->n_rx_xdp_bad_drops++;
> > +			break;
> > +		}
> 
> I see the incidence of this cleanup code multiplying in this function.
> I would be nice to follow-up by consolidating that - I'd suggest
> in the form of a goto label at the end of the function.
> 
> But as a fix for net this minimal patch seems appropriate to me.
> 
> Reviewed-by: Simon Horman <horms@kernel.org>

Oops, a few moments later I now see that Paolo felt otherwise
for a similar patch [1].

  "AFAIC the sfc driver reserves both enough headroom and tailroom, but
   this is after ebpf run, which in turn could consume enough headroom to
   cause a failure, so I think this makes sense."

Let's go with his advice and and for a review by Edward.

[1] Re: [PATCH] sfc: handle NULL returned by xdp_convert_buff_to_frame()
    https://lore.kernel.org/all/045d1ff5-bb20-481d-a067-0a42345ab83d@redhat.com/