Re: [PATCH bpf] bpf, test_run: fix &xdp_frame misplacement for LIVE_FRAMES

[Alexander Lobakin]

From: Alexander Lobakin <alexandr.lobakin@xxxxxxxxx>
Date: Thu, 9 Feb 2023 18:28:27 +0100

> &xdp_buff and &xdp_frame are bound in a way that
> 
> xdp_buff->data_hard_start == xdp_frame

[...]

> diff --git a/net/bpf/test_run.c b/net/bpf/test_run.c
> index 2723623429ac..c3cce7a8d47d 100644
> --- a/net/bpf/test_run.c
> +++ b/net/bpf/test_run.c
> @@ -97,8 +97,11 @@ static bool bpf_test_timer_continue(struct bpf_test_timer *t, int iterations,
>  struct xdp_page_head {
>  	struct xdp_buff orig_ctx;
>  	struct xdp_buff ctx;
> -	struct xdp_frame frm;
> -	u8 data[];
> +	union {
> +		/* ::data_hard_start starts here */
> +		DECLARE_FLEX_ARRAY(struct xdp_frame, frm);
> +		DECLARE_FLEX_ARRAY(u8, data);
> +	};

BTW, xdp_frame here starts at 112 byte offset, i.e. in 16 bytes a
cacheline boundary is hit, so xdp_frame gets sliced into halves: 16
bytes in CL1 + 24 bytes in CL2. Maybe we'd better align this union to
%NET_SKB_PAD / %SMP_CACHE_BYTES / ... to avoid this?

(but in bpf-next probably)

>  };
>  
>  struct xdp_test_data {
Thanks,
Olek