netdev - Re: [PATCH bpf-next] filter: add BPF_ADJ_ROOM_DATA mode to bpf_skb_adjust

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Message-ID: <20181109185131.7f7osxg2tfp2xapx@kafai-mbp.dhcp.thefacebook.com>
Date:   Fri, 9 Nov 2018 18:51:32 +0000
From:   Martin Lau <kafai@...com>
To:     Nicolas Dichtel <nicolas.dichtel@...nd.com>
CC:     "ast@...nel.org" <ast@...nel.org>,
        "daniel@...earbox.net" <daniel@...earbox.net>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH bpf-next] filter: add BPF_ADJ_ROOM_DATA mode to
 bpf_skb_adjust_room()

On Thu, Nov 08, 2018 at 04:11:37PM +0100, Nicolas Dichtel wrote:
> This new mode enables to add or remove an l2 header in a programmatic way
> with cls_bpf.
> For example, it enables to play with mpls headers.
> 
> Signed-off-by: Nicolas Dichtel <nicolas.dichtel@...nd.com>
> ---
>  include/uapi/linux/bpf.h       |  3 ++
>  net/core/filter.c              | 54 ++++++++++++++++++++++++++++++++++
>  tools/include/uapi/linux/bpf.h |  3 ++
>  3 files changed, 60 insertions(+)
> 
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index 852dc17ab47a..47407fd5162b 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -1467,6 +1467,8 @@ union bpf_attr {
>   *
>   * 		* **BPF_ADJ_ROOM_NET**: Adjust room at the network layer
>   * 		  (room space is added or removed below the layer 3 header).
> + * 		* **BPF_ADJ_ROOM_DATA**: Adjust room at the beginning of the
> + * 		  packet (room space is added or removed below skb->data).
>   *
>   * 		All values for *flags* are reserved for future usage, and must
>   * 		be left at zero.
> @@ -2408,6 +2410,7 @@ enum bpf_func_id {
>  /* Mode for BPF_FUNC_skb_adjust_room helper. */
>  enum bpf_adj_room_mode {
>  	BPF_ADJ_ROOM_NET,
> +	BPF_ADJ_ROOM_DATA,
>  };
>  
>  /* Mode for BPF_FUNC_skb_load_bytes_relative helper. */
> diff --git a/net/core/filter.c b/net/core/filter.c
> index e521c5ebc7d1..e699849b269d 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -2884,6 +2884,58 @@ static int bpf_skb_adjust_net(struct sk_buff *skb, s32 len_diff)
>  	return ret;
>  }
>  
> +static int bpf_skb_data_shrink(struct sk_buff *skb, u32 len)
> +{
> +	unsigned short hhlen = skb->dev->header_ops ?
> +			       skb->dev->hard_header_len : 0;
> +	int ret;
> +
> +	ret = skb_unclone(skb, GFP_ATOMIC);
> +	if (unlikely(ret < 0))
> +		return ret;
> +
> +	__skb_pull(skb, len);
> +	skb_reset_mac_header(skb);
> +	skb_reset_network_header(skb);
> +	skb->network_header += hhlen;
> +	skb_reset_transport_header(skb);
hmm...why transport_header does not need += hhlen here
while network_header does?

> +	return 0;
> +}
> +
> +static int bpf_skb_data_grow(struct sk_buff *skb, u32 len)
> +{
> +	unsigned short hhlen = skb->dev->header_ops ?
> +			       skb->dev->hard_header_len : 0;
> +	int ret;
> +
> +	ret = skb_cow(skb, len);
> +	if (unlikely(ret < 0))
> +		return ret;
> +
> +	skb_push(skb, len);
> +	skb_reset_mac_header(skb);
> +	return 0;
> +}
> +
> +static int bpf_skb_adjust_data(struct sk_buff *skb, s32 len_diff)
> +{
> +	u32 len_diff_abs = abs(len_diff);
> +	bool shrink = len_diff < 0;
> +	int ret;
> +
> +	if (unlikely(len_diff_abs > 0xfffU))
> +		return -EFAULT;
> +
> +	if (shrink && len_diff_abs >= skb_headlen(skb))
> +		return -EFAULT;
> +
> +	ret = shrink ? bpf_skb_data_shrink(skb, len_diff_abs) :
> +		       bpf_skb_data_grow(skb, len_diff_abs);
> +
> +	bpf_compute_data_pointers(skb);
> +	return ret;
> +}
> +
>  BPF_CALL_4(bpf_skb_adjust_room, struct sk_buff *, skb, s32, len_diff,
>  	   u32, mode, u64, flags)
>  {
> @@ -2891,6 +2943,8 @@ BPF_CALL_4(bpf_skb_adjust_room, struct sk_buff *, skb, s32, len_diff,
>  		return -EINVAL;
>  	if (likely(mode == BPF_ADJ_ROOM_NET))
>  		return bpf_skb_adjust_net(skb, len_diff);
> +	if (likely(mode == BPF_ADJ_ROOM_DATA))
> +		return bpf_skb_adjust_data(skb, len_diff);
>  
>  	return -ENOTSUPP;
>  }
> diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
> index 852dc17ab47a..47407fd5162b 100644
> --- a/tools/include/uapi/linux/bpf.h
> +++ b/tools/include/uapi/linux/bpf.h
> @@ -1467,6 +1467,8 @@ union bpf_attr {
>   *
>   * 		* **BPF_ADJ_ROOM_NET**: Adjust room at the network layer
>   * 		  (room space is added or removed below the layer 3 header).
> + * 		* **BPF_ADJ_ROOM_DATA**: Adjust room at the beginning of the
> + * 		  packet (room space is added or removed below skb->data).
>   *
>   * 		All values for *flags* are reserved for future usage, and must
>   * 		be left at zero.
> @@ -2408,6 +2410,7 @@ enum bpf_func_id {
>  /* Mode for BPF_FUNC_skb_adjust_room helper. */
>  enum bpf_adj_room_mode {
>  	BPF_ADJ_ROOM_NET,
> +	BPF_ADJ_ROOM_DATA,
>  };
>  
>  /* Mode for BPF_FUNC_skb_load_bytes_relative helper. */
> -- 
> 2.18.0
>