Re: [PATCH net] bpf: xdp: fallback to SKB mode if DRV flag is absent.

[Daniel Borkmann <daniel@iogearbox.net>]

On 10/15/24 5:36 AM, Hangbin Liu wrote:
> After commit c8a36f1945b2 ("bpf: xdp: Fix XDP mode when no mode flags
> specified"), the mode is automatically set to XDP_MODE_DRV if the driver
> implements the .ndo_bpf function. However, for drivers like bonding, which
> only support native XDP for specific modes, this may result in an
> "unsupported" response.
> 
> In such cases, let's fall back to SKB mode if the user did not explicitly
> request DRV mode.
> 
> Fixes: c8a36f1945b2 ("bpf: xdp: Fix XDP mode when no mode flags specified")
> Reported-by: Liang Li <liali@redhat.com>
> Closes: https://issues.redhat.com/browse/RHEL-62339

nit: The link is not accessible to the public.

Also, this breaks BPF CI with regards to existing bonding selftest :

   https://github.com/kernel-patches/bpf/actions/runs/11340153361/job/31536275257

Given this issue is related to only bonding driver, could this be fixed
there instead?

> Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
> ---
>   net/core/dev.c | 12 +++++++++++-
>   1 file changed, 11 insertions(+), 1 deletion(-)
> 
> diff --git a/net/core/dev.c b/net/core/dev.c
> index ea5fbcd133ae..e32069d81cd7 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -9579,6 +9579,7 @@ static int dev_xdp_attach(struct net_device *dev, struct netlink_ext_ack *extack
>   
>   	/* don't call drivers if the effective program didn't change */
>   	if (new_prog != cur_prog) {
> +reinstall:
>   		bpf_op = dev_xdp_bpf_op(dev, mode);
>   		if (!bpf_op) {
>   			NL_SET_ERR_MSG(extack, "Underlying driver does not support XDP in native mode");
> @@ -9586,8 +9587,17 @@ static int dev_xdp_attach(struct net_device *dev, struct netlink_ext_ack *extack
>   		}
>   
>   		err = dev_xdp_install(dev, mode, bpf_op, extack, flags, new_prog);
> -		if (err)
> +		if (err) {
> +			/* The driver returns not supported even .ndo_bpf
> +			 * implemented, fall back to SKB mode.
> +			 */
> +			if (err == -EOPNOTSUPP && mode == XDP_MODE_DRV &&
> +			    !(flags & XDP_FLAGS_DRV_MODE)) {
> +				mode = XDP_MODE_SKB;
> +				goto reinstall;
> +			}
>   			return err;
> +		}
>   	}
>   
>   	if (link)