Subject: Re: [RFC bpf-next v2 06/14] xdp: Carry over xdp metadata into skb context

From: Martin KaFai Lau <[email-masked]>
Date: Mon, 7 Nov 2022 14:01:53 -0800

On 11/3/22 8:25 PM, Stanislav Fomichev wrote:
> > diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> index 59c9fd55699d..dba857f212d7 100644
> --- a/include/linux/skbuff.h
> +++ b/include/linux/skbuff.h
> @@ -4217,9 +4217,13 @@ static inline bool skb_metadata_differs(const struct sk_buff *skb_a,
>   	       true : __skb_metadata_differs(skb_a, skb_b, len_a);
>   }
>   
> +void skb_metadata_import_from_xdp(struct sk_buff *skb, size_t len);
> +
>   static inline void skb_metadata_set(struct sk_buff *skb, u8 meta_len)
>   {
>   	skb_shinfo(skb)->meta_len = meta_len;
> +	if (meta_len)
> +		skb_metadata_import_from_xdp(skb, meta_len);
>   }
>
[ ... ]

> +struct xdp_to_skb_metadata {
> +	u32 magic; /* xdp_metadata_magic */
> +	u64 rx_timestamp;
> +} __randomize_layout;
> +
> +struct bpf_patch;
> +

[ ... ]

> +void skb_metadata_import_from_xdp(struct sk_buff *skb, size_t len)
> +{
> +	struct xdp_to_skb_metadata *meta = (void *)(skb_mac_header(skb) - len);
> +
> +	/* Optional SKB info, currently missing:
> +	 * - HW checksum info		(skb->ip_summed)
> +	 * - HW RX hash			(skb_set_hash)
> +	 * - RX ring dev queue index	(skb_record_rx_queue)
> +	 */
> +
> +	if (len != sizeof(struct xdp_to_skb_metadata))
> +		return;
> +
> +	if (meta->magic != xdp_metadata_magic)
> +		return;
> +
> +	if (meta->rx_timestamp) {
> +		*skb_hwtstamps(skb) = (struct skb_shared_hwtstamps){
> +			.hwtstamp = ns_to_ktime(meta->rx_timestamp),
> +		};
> +	}
> +}

Considering the metadata will affect the gro, should the meta be cleared after 
importing to the skb?

[ ... ]

> +/* Since we're not actually doing a call but instead rewriting
> + * in place, we can only afford to use R0-R5 scratch registers.
> + *
> + * We reserve R1 for bpf_xdp_metadata_export_to_skb and let individual
> + * metadata kfuncs use only R0,R4-R5.
> + *
> + * The above also means we _cannot_ easily call any other helper/kfunc
> + * because there is no place for us to preserve our R1 argument;
> + * existing R6-R9 belong to the callee.
> + */
> +void xdp_metadata_export_to_skb(const struct bpf_prog *prog, struct bpf_patch *patch)
> +{
> +	u32 func_id;
> +
> +	/*
> +	 * The code below generates the following:
> +	 *
> +	 * void bpf_xdp_metadata_export_to_skb(struct xdp_md *ctx)
> +	 * {
> +	 *	struct xdp_to_skb_metadata *meta;
> +	 *	int ret;
> +	 *
> +	 *	ret = bpf_xdp_adjust_meta(ctx, -sizeof(*meta));
> +	 *	if (!ret)
> +	 *		return;
> +	 *
> +	 *	meta = ctx->data_meta;
> +	 *	meta->magic = xdp_metadata_magic;
> +	 *	meta->rx_timestamp = bpf_xdp_metadata_rx_timestamp(ctx);
> +	 * }
> +	 *
> +	 */
> +
> +	bpf_patch_append(patch,
> +		/* r2 = ((struct xdp_buff *)r1)->data_meta; */
> +		BPF_LDX_MEM(BPF_DW, BPF_REG_2, BPF_REG_1,
> +			    offsetof(struct xdp_buff, data_meta)),
> +		/* r3 = ((struct xdp_buff *)r1)->data; */
> +		BPF_LDX_MEM(BPF_DW, BPF_REG_3, BPF_REG_1,
> +			    offsetof(struct xdp_buff, data)),
> +		/* if (data_meta != data) return;
> +		 *
> +		 *	data_meta > data: xdp_data_meta_unsupported()
> +		 *	data_meta < data: already used, no need to touch
> +		 */
> +		BPF_JMP_REG(BPF_JNE, BPF_REG_2, BPF_REG_3, S16_MAX),
> +
> +		/* r2 -= sizeof(struct xdp_to_skb_metadata); */
> +		BPF_ALU64_IMM(BPF_SUB, BPF_REG_2,
> +			      sizeof(struct xdp_to_skb_metadata)),
> +		/* r3 = ((struct xdp_buff *)r1)->data_hard_start; */
> +		BPF_LDX_MEM(BPF_DW, BPF_REG_3, BPF_REG_1,
> +			    offsetof(struct xdp_buff, data_hard_start)),
> +		/* r3 += sizeof(struct xdp_frame) */
> +		BPF_ALU64_IMM(BPF_ADD, BPF_REG_3,
> +			      sizeof(struct xdp_frame)),
> +		/* if (data-sizeof(struct xdp_to_skb_metadata) < data_hard_start+sizeof(struct xdp_frame)) return; */
> +		BPF_JMP_REG(BPF_JLT, BPF_REG_2, BPF_REG_3, S16_MAX),
> +
> +		/* ((struct xdp_buff *)r1)->data_meta = r2; */
> +		BPF_STX_MEM(BPF_DW, BPF_REG_1, BPF_REG_2,
> +			    offsetof(struct xdp_buff, data_meta)),
> +
> +		/* *((struct xdp_to_skb_metadata *)r2)->magic = xdp_metadata_magic; */
> +		BPF_ST_MEM(BPF_W, BPF_REG_2,
> +			   offsetof(struct xdp_to_skb_metadata, magic),
> +			   xdp_metadata_magic),
> +	);
> +
> +	/*	r0 = bpf_xdp_metadata_rx_timestamp(ctx); */
> +	func_id = xdp_metadata_kfunc_id(XDP_METADATA_KFUNC_RX_TIMESTAMP);
> +	prog->aux->xdp_kfunc_ndo->ndo_unroll_kfunc(prog, func_id, patch);
> +
> +	bpf_patch_append(patch,
> +		/* r2 = ((struct xdp_buff *)r1)->data_meta; */
> +		BPF_LDX_MEM(BPF_DW, BPF_REG_2, BPF_REG_1,
> +			    offsetof(struct xdp_buff, data_meta)),
> +		/* *((struct xdp_to_skb_metadata *)r2)->rx_timestamp = r0; */
> +		BPF_STX_MEM(BPF_DW, BPF_REG_2, BPF_REG_0,
> +			    offsetof(struct xdp_to_skb_metadata, rx_timestamp)),

Can the xdp prog still change the metadata through xdp->data_meta? tbh, I am not 
sure it is solid enough by asking the xdp prog not to use the same random number 
in its own metadata + not to change the metadata through xdp->data_meta after 
calling bpf_xdp_metadata_export_to_skb().

Does xdp_to_skb_metadata have a use case for XDP_PASS (like patch 7) or the 
xdp_to_skb_metadata can be limited to XDP_REDIRECT only?

> +	);
> +
> +	bpf_patch_resolve_jmp(patch);
> +}
> +
>   static int __init xdp_metadata_init(void)
>   {
> +	xdp_metadata_magic = get_random_u32() | 1;
>   	return register_btf_kfunc_id_set(BPF_PROG_TYPE_XDP, &xdp_metadata_kfunc_set);
>   }
>   late_initcall(xdp_metadata_init);