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SubjectRe: [PATCH net] veth: xdp: use head instead of hard_start
On 2020/3/31 7:35, Toshiaki Makita wrote:
> Hi Mao & Jesper
> (Resending with plain text...)
> On 2020/03/30 20:34, Jesper Dangaard Brouer wrote:
>> On Mon, 30 Mar 2020 18:26:31 +0800
>> Mao Wenan <> wrote:
>>> xdp.data_hard_start is mapped to the first
>>> address of xdp_frame, but the pointer hard_start
>>> is the offset(sizeof(struct xdp_frame)) of xdp_frame,
>>> it should use head instead of hard_start to
>>> set xdp.data_hard_start. Otherwise, if BPF program
>>> calls helper_function such as bpf_xdp_adjust_head, it
>>> will be confused for xdp_frame_end.
>> I have noticed this[1] and have a patch in my current patchset for
>> fixing this.  IMHO is is not so important fix right now, as the effect
>> is that you currently only lose 32 bytes of headroom.
I consider that it is needed because bpf_xdp_adjust_head() just a common helper function,
veth as one driver application should keep the same as 32 bytes of headroom as other driver.
And convert_to_xdp_frame set() also store info in top of packet, and set:
xdp_frame = xdp->data_hard_start;

>> [1]
> You are right, the subtraction is not necessary here.
I guess you mean that previous subtraction is not necessary ? this line : void *head = hard_start - sizeof(struct xdp_frame); ?
But in the veth_xdp_rcv_one,below line will use head pointer,
case XDP_TX:
orig_frame = *frame;
xdp.data_hard_start = head;

> Thank you for working on this.
> Toshiaki Makita
> .

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