netdev - Re: [PATCH v11 bpf-next 17/18] net: xdp: introduce bpf_xdp_adjust

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Date:   Wed, 18 Aug 2021 14:31:07 +0200
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     Lorenzo Bianconi <lorenzo@...nel.org>, bpf@...r.kernel.org,
        netdev@...r.kernel.org
Cc:     lorenzo.bianconi@...hat.com, davem@...emloft.net, kuba@...nel.org,
        ast@...nel.org, daniel@...earbox.net, shayagr@...zon.com,
        john.fastabend@...il.com, dsahern@...nel.org, brouer@...hat.com,
        echaudro@...hat.com, jasowang@...hat.com,
        alexander.duyck@...il.com, saeed@...nel.org,
        maciej.fijalkowski@...el.com, magnus.karlsson@...el.com,
        tirthendu.sarkar@...el.com
Subject: Re: [PATCH v11 bpf-next 17/18] net: xdp: introduce
 bpf_xdp_adjust_data helper

Lorenzo Bianconi <lorenzo@...nel.org> writes:

> For XDP frames split over multiple buffers, the xdp_md->data and
> xdp_md->data_end pointers will point to the start and end of the first
> fragment only. bpf_xdp_adjust_data can be used to access subsequent
> fragments by moving the data pointers. To use, an XDP program can call
> this helper with the byte offset of the packet payload that
> it wants to access; the helper will move xdp_md->data and xdp_md ->data_end
> so they point to the requested payload offset and to the end of the
> fragment containing this byte offset, and return the byte offset of the
> start of the fragment.
> To move back to the beginning of the packet, simply call the
> helper with an offset of '0'.
> Note also that the helpers that modify the packet boundaries
> (bpf_xdp_adjust_head(), bpf_xdp_adjust_tail() and
> bpf_xdp_adjust_meta()) will fail if the pointers have been
> moved; it is the responsibility of the BPF program to move them
> back before using these helpers.
>
> Suggested-by: John Fastabend <john.fastabend@...il.com>
> Signed-off-by: Lorenzo Bianconi <lorenzo@...nel.org>
> ---
>  include/net/xdp.h              |  8 +++++
>  include/uapi/linux/bpf.h       | 32 ++++++++++++++++++
>  net/bpf/test_run.c             |  8 +++++
>  net/core/filter.c              | 61 +++++++++++++++++++++++++++++++++-
>  tools/include/uapi/linux/bpf.h | 32 ++++++++++++++++++
>  5 files changed, 140 insertions(+), 1 deletion(-)
>
> diff --git a/include/net/xdp.h b/include/net/xdp.h
> index cdaecf8d4d61..ce4764c7cd40 100644
> --- a/include/net/xdp.h
> +++ b/include/net/xdp.h
> @@ -82,6 +82,11 @@ struct xdp_buff {
>  	struct xdp_txq_info *txq;
>  	u32 frame_sz; /* frame size to deduce data_hard_end/reserved tailroom*/
>  	u16 flags; /* supported values defined in xdp_flags */
> +	/* xdp multi-buff metadata used for frags iteration */
> +	struct {
> +		u16 headroom;	/* frame headroom: data - data_hard_start */
> +		u16 headlen;	/* first buffer length: data_end - data */
> +	} mb;
>  };
>  
>  static __always_inline bool xdp_buff_is_mb(struct xdp_buff *xdp)
> @@ -127,6 +132,9 @@ xdp_prepare_buff(struct xdp_buff *xdp, unsigned char *hard_start,
>  	xdp->data = data;
>  	xdp->data_end = data + data_len;
>  	xdp->data_meta = meta_valid ? data : data + 1;
> +	/* mb metadata for frags iteration */
> +	xdp->mb.headroom = headroom;
> +	xdp->mb.headlen = data_len;
>  }
>  
>  /* Reserve memory area at end-of data area.
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index ddbf9ccc2f74..c20a8b7c5c7c 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -4853,6 +4853,37 @@ union bpf_attr {
>   *		Get the total size of a given xdp buff (linear and paged area)
>   *	Return
>   *		The total size of a given xdp buffer.
> + *
> + * long bpf_xdp_adjust_data(struct xdp_buff *xdp_md, u32 offset)
> + *	Description
> + *		For XDP frames split over multiple buffers, the
> + *		*xdp_md*\ **->data** and*xdp_md *\ **->data_end** pointers
> + *		will point to the start and end of the first fragment only.
> + *		This helper can be used to access subsequent fragments by
> + *		moving the data pointers. To use, an XDP program can call
> + *		this helper with the byte offset of the packet payload that
> + *		it wants to access; the helper will move *xdp_md*\ **->data**
> + *		and *xdp_md *\ **->data_end** so they point to the requested
> + *		payload offset and to the end of the fragment containing this
> + *		byte offset, and return the byte offset of the start of the
> + *		fragment.

This comment is wrong now :)

-Toke