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

[Jesper Dangaard Brouer <hawk@kernel.org>]

On 25/07/2025 12.11, Edward Cree wrote:
> On 7/24/25 10:57, Paolo Abeni wrote:
>> On 7/23/25 2:32 AM, 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.
>>
>> 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.
> 
> Your reasoning seems plausible to me.

Hmm... have you actually tested that XDP/BPF can adjust headroom so much
that xdp_convert_buff_to_frame() function fails?

I really doubt this possible for BPF-progs to violate this.

The XDP BPF-prog can only adjust the headroom via the helpers
bpf_xdp_adjust_head() and bpf_xdp_adjust_meta().  These helpers reserve
room for sizeof(struct xdp_frame).

The tailroom can be adjusted via helper bpf_xdp_adjust_tail() and it
also reserve room for sizeof(struct skb_shared_info) such that BPF-progs
cannot get access to this area. See define for xdp_data_hard_end.

--Jesper