RE: [Patch bpf-next v3 4/5] skmsg: use skb ext instead of TCP_SKB_CB

[John Fastabend <john.fastabend@gmail.com>]

Cong Wang wrote:
> From: Cong Wang <cong.wang@bytedance.com>
> 
> Currently TCP_SKB_CB() is hard-coded in skmsg code, it certainly
> does not work for any other non-TCP protocols. We can move them to
> skb ext instead of playing with skb cb, which is harder to make
> correct.
> 
> Cc: John Fastabend <john.fastabend@gmail.com>
> Cc: Daniel Borkmann <daniel@iogearbox.net>
> Cc: Jakub Sitnicki <jakub@cloudflare.com>
> Reviewed-by: Lorenz Bauer <lmb@cloudflare.com>
> Signed-off-by: Cong Wang <cong.wang@bytedance.com>
> ---

I'm not seeing the advantage of doing this at the moment. We can
continue to use cb[] here, which is simpler IMO and use the ext
if needed for the other use cases. This is adding a per packet
alloc cost that we don't have at the moment as I understand it.

[...]

> diff --git a/include/linux/skmsg.h b/include/linux/skmsg.h
> index e3bb712af257..d5c711ef6d4b 100644
> --- a/include/linux/skmsg.h
> +++ b/include/linux/skmsg.h
> @@ -459,4 +459,44 @@ static inline bool sk_psock_strp_enabled(struct sk_psock *psock)
>  		return false;
>  	return !!psock->saved_data_ready;
>  }
> +
> +struct skb_bpf_ext {
> +	__u32 flags;
> +	struct sock *sk_redir;
> +};
> +
> +#if IS_ENABLED(CONFIG_NET_SOCK_MSG)
> +static inline
> +bool skb_bpf_ext_ingress(const struct sk_buff *skb)
> +{
> +	struct skb_bpf_ext *ext = skb_ext_find(skb, SKB_EXT_BPF);
> +
> +	return ext->flags & BPF_F_INGRESS;
> +}
> +
> +static inline
> +void skb_bpf_ext_set_ingress(const struct sk_buff *skb)
> +{
> +	struct skb_bpf_ext *ext = skb_ext_find(skb, SKB_EXT_BPF);
> +
> +	ext->flags |= BPF_F_INGRESS;
> +}
> +
> +static inline
> +struct sock *skb_bpf_ext_redirect_fetch(struct sk_buff *skb)
> +{
> +	struct skb_bpf_ext *ext = skb_ext_find(skb, SKB_EXT_BPF);
> +
> +	return ext->sk_redir;
> +}
> +
> +static inline
> +void skb_bpf_ext_redirect_clear(struct sk_buff *skb)
> +{
> +	struct skb_bpf_ext *ext = skb_ext_find(skb, SKB_EXT_BPF);
> +
> +	ext->flags = 0;
> +	ext->sk_redir = NULL;
> +}
> +#endif /* CONFIG_NET_SOCK_MSG */

So we will have some slight duplication for cb[] variant and ext
variant above. I'm OK with that to avoid an allocation.

[...]

> @@ -1003,11 +1008,17 @@ static int sk_psock_verdict_recv(read_descriptor_t *desc, struct sk_buff *skb,
>  		goto out;
>  	}
>  	skb_set_owner_r(skb, sk);
> +	if (!skb_ext_add(skb, SKB_EXT_BPF)) {
> +		len = 0;
> +		kfree_skb(skb);
> +		goto out;
> +	}
> +

per packet cost here. Perhaps you can argue small alloc will usually not be 
noticable in such a large stack, but once we convert over it will be very
hard to go back. And I'm looking at optimizing this path now.

>  	prog = READ_ONCE(psock->progs.skb_verdict);
>  	if (likely(prog)) {
> -		tcp_skb_bpf_redirect_clear(skb);
> +		skb_bpf_ext_redirect_clear(skb);
>  		ret = sk_psock_bpf_run(psock, prog, skb);
> -		ret = sk_psock_map_verd(ret, tcp_skb_bpf_redirect_fetch(skb));
> +		ret = sk_psock_map_verd(ret, skb_bpf_ext_redirect_fetch(skb));
>  	}
>  	sk_psock_verdict_apply(psock, skb, ret);

Thanks for the series Cong. Drop this patch and resubmit carry ACKs forward
and then lets revisit this later.

Thanks,
John