linux-kernel - Re: [PATCH bpf] bpf, test_run: fix &xdp_frame misplacement for LIVE

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Date:   Thu, 9 Feb 2023 21:04:31 +0100
From:   Alexander Lobakin <alexandr.lobakin@...el.com>
To:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>
CC:     Toke Høiland-Jørgensen <toke@...hat.com>,
        "Martin KaFai Lau" <martin.lau@...ux.dev>,
        Song Liu <song@...nel.org>,
        "Jesper Dangaard Brouer" <hawk@...nel.org>,
        Jakub Kicinski <kuba@...nel.org>, <bpf@...r.kernel.org>,
        <netdev@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH bpf] bpf, test_run: fix &xdp_frame misplacement for
 LIVE_FRAMES

From: Alexander Lobakin <alexandr.lobakin@...el.com>
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