netdev - Re: [PATCH] Bpf Helper Function BPF_FUNC_skb_change

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Date:   Mon, 20 Dec 2021 19:18:42 -0800
From:   Yonghong Song <yhs@...com>
To:     Tyler Wear <quic_twear@...cinc.com>, <netdev@...r.kernel.org>,
        <bpf@...r.kernel.org>
Subject: Re: [PATCH] Bpf Helper Function BPF_FUNC_skb_change_dsfield



On 12/20/21 12:40 PM, Tyler Wear wrote:
> New bpf helper function BPF_FUNC_skb_change_dsfield
> "int bpf_skb_change_dsfield(struct sk_buff *skb, u8 mask, u8 value)".
> BPF_PROG_TYPE_CGROUP_SKB typed bpf_prog which currently can
> be attached to the ingress and egress path. The helper is needed
> because this type of bpf_prog cannot modify the skb directly.
> 
> Used by a bpf_prog to specify DS field values on egress or
> ingress.

Maybe you can expand a little bit here for your use case?
I know DS field might help but a description of your actual
use case will make adding this helper more compelling.

> 
> Signed-off-by: Tyler Wear <quic_twear@...cinc.com>
> ---
>   include/uapi/linux/bpf.h |  9 ++++++++
>   net/core/filter.c        | 46 ++++++++++++++++++++++++++++++++++++++++
>   2 files changed, 55 insertions(+)
> 
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index 556216dc9703..742cea7dcf8c 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -3742,6 +3742,14 @@ union bpf_attr {
>    * 	Return
>    * 		The helper returns **TC_ACT_REDIRECT** on success or
>    * 		**TC_ACT_SHOT** on error.
> + *
> + * long bpf_skb_change_dsfield(struct sk_buff *skb, u8 mask, u8 value)
> + *	Description
> + *		Set DS field of IP header to the specified *value*. The *value*
> + *		is masked with the provided *mask* when ds field is updated.

This description probably not precise. If I understand correctly, the 
*mask* is used to mask the current skb iph->tos value which is then 
'or'ed with *value*.

I am also debating the helper name, bpf_skb_change_dsfield vs.
bpf_skb_update_dsfield vs. bpf_skb_update_ds. Here, we are actually
doing an update instead of completely overwrite the original value.
Maybe "update" is better than "change". Maybe we can just do
"bpf_skb_update_ds"? We have an existing helper bpf_skb_ecn_set_ce
to update ecn. We don't have any helper with suffix "field".

> + *		Works with IPv6 and IPv4.
> + *	Return
> + *		1 if the DS field is set, 0 if it is not set.
>    */
>   #define __BPF_FUNC_MAPPER(FN)		\
>   	FN(unspec),			\
> @@ -3900,6 +3908,7 @@ union bpf_attr {
>   	FN(per_cpu_ptr),		\
>   	FN(this_cpu_ptr),		\
>   	FN(redirect_peer),		\
> +	FN(skb_change_dsfield),		\
>   	/* */
>   
>   /* integer value in 'imm' field of BPF_CALL instruction selects which helper
> diff --git a/net/core/filter.c b/net/core/filter.c
> index 035d66227ae2..71ea943c8059 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -6402,6 +6402,50 @@ BPF_CALL_1(bpf_skb_ecn_set_ce, struct sk_buff *, skb)
>   	return INET_ECN_set_ce(skb);
>   }
>   
> +BPF_CALL_3(bpf_skb_change_dsfield, struct sk_buff *, skb, u8, mask, u8, value)
> +{
> +	unsigned int iphdr_len;
> +
> +	switch (skb_protocol(skb, true)) {
> +	case cpu_to_be16(ETH_P_IP):
> +		iphdr_len = sizeof(struct iphdr);
> +		break;
> +	case cpu_to_be16(ETH_P_IPV6):
> +		iphdr_len = sizeof(struct ipv6hdr);
> +		break;
> +	default:
> +		return 0;
> +	}
> +
> +	if (skb_headlen(skb) < iphdr_len)
> +		return 0;
> +
> +	if (skb_cloned(skb) && !skb_clone_writable(skb, iphdr_len))
> +		return 0;
> +
> +	switch (skb_protocol(skb, true)) {
> +	case cpu_to_be16(ETH_P_IP):
> +		ipv4_change_dsfield(ipip_hdr(skb), mask, value);
> +		break;
> +	case cpu_to_be16(ETH_P_IPV6):
> +		ipv6_change_dsfield(ipv6_hdr(skb), mask, value);
> +		break;
> +	default:
> +		return 0;
> +	}

There are some repetition here. For example, in the above, 'default' is 
not possible at all. Is it possible to remove the second 'switch' 
statement with simple
	if (...)
		ipv4_change_dsfield(ipip_hdr(skb), mask, value);
	else
		ipv6_change_dsfield(ipv6_hdr(skb), mask, value);
?

> +
> +	return 1;
> +}
> +
> +static const struct bpf_func_proto bpf_skb_change_dsfield_proto = {
> +	.func           = bpf_skb_change_dsfield,
> +	.gpl_only       = false,
> +	.ret_type       = RET_INTEGER,
> +	.arg1_type      = ARG_PTR_TO_CTX,
> +	.arg2_type      = ARG_ANYTHING,
> +	.arg3_type      = ARG_ANYTHING,
> +};
> +
>   bool bpf_xdp_sock_is_valid_access(int off, int size, enum bpf_access_type type,
>   				  struct bpf_insn_access_aux *info)
>   {
> @@ -7057,6 +7101,8 @@ cg_skb_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
>   		return &bpf_get_listener_sock_proto;
>   	case BPF_FUNC_skb_ecn_set_ce:
>   		return &bpf_skb_ecn_set_ce_proto;
> +	case BPF_FUNC_skb_change_dsfield:
> +		return &bpf_skb_change_dsfield_proto;

We do need a self test to exercise this helper.

>   #endif
>   	default:
>   		return sk_filter_func_proto(func_id, prog);