veth: Rework veth_xdp_rcv_skb in order to accept non-linear skb

Introduce veth_convert_skb_to_xdp_buff routine in order to
convert a non-linear skb into a xdp buffer. If the received skb
is cloned or shared, veth_convert_skb_to_xdp_buff will copy it
in a new skb composed by order-0 pages for the linear and the
fragmented area. Moreover veth_convert_skb_to_xdp_buff guarantees
we have enough headroom for xdp.
This is a preliminary patch to allow attaching xdp programs with frags
support on veth devices.

Signed-off-by: Lorenzo Bianconi <[e-mail]>
Signed-off-by: Daniel Borkmann <[e-mail]>
Acked-by: Toke Høiland-Jørgensen <[e-mail]>
Acked-by: John Fastabend <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

drivers/net/veth.c

···
 {
 }
 
-static struct sk_buff *veth_build_skb(void *head, int headroom, int len,
-				      int buflen)
-{
-	struct sk_buff *skb;
-
-	skb = build_skb(head, buflen);
-	if (!skb)
-		return NULL;
-
-	skb_reserve(skb, headroom);
-	skb_put(skb, len);
-
-	return skb;
-}
-
 static int veth_select_rxq(struct net_device *dev)
 {
 	return smp_processor_id() % dev->real_num_rx_queues;
···
 	}
 }
 
+static void veth_xdp_get(struct xdp_buff *xdp)
+{
+	struct skb_shared_info *sinfo = xdp_get_shared_info_from_buff(xdp);
+	int i;
+
+	get_page(virt_to_page(xdp->data));
+	if (likely(!xdp_buff_has_frags(xdp)))
+		return;
+
+	for (i = 0; i < sinfo->nr_frags; i++)
+		__skb_frag_ref(&sinfo->frags[i]);
+}
+
+static int veth_convert_skb_to_xdp_buff(struct veth_rq *rq,
+					struct xdp_buff *xdp,
+					struct sk_buff **pskb)
+{
+	struct sk_buff *skb = *pskb;
+	u32 frame_sz;
+
+	if (skb_shared(skb) || skb_head_is_locked(skb) ||
+	    skb_shinfo(skb)->nr_frags) {
+		u32 size, len, max_head_size, off;
+		struct sk_buff *nskb;
+		struct page *page;
+		int i, head_off;
+
+		/* We need a private copy of the skb and data buffers since
+		 * the ebpf program can modify it. We segment the original skb
+		 * into order-0 pages without linearize it.
+		 *
+		 * Make sure we have enough space for linear and paged area
+		 */
+		max_head_size = SKB_WITH_OVERHEAD(PAGE_SIZE -
+						  VETH_XDP_HEADROOM);
+		if (skb->len > PAGE_SIZE * MAX_SKB_FRAGS + max_head_size)
+			goto drop;
+
+		/* Allocate skb head */
+		page = alloc_page(GFP_ATOMIC | __GFP_NOWARN);
+		if (!page)
+			goto drop;
+
+		nskb = build_skb(page_address(page), PAGE_SIZE);
+		if (!nskb) {
+			put_page(page);
+			goto drop;
+		}
+
+		skb_reserve(nskb, VETH_XDP_HEADROOM);
+		size = min_t(u32, skb->len, max_head_size);
+		if (skb_copy_bits(skb, 0, nskb->data, size)) {
+			consume_skb(nskb);
+			goto drop;
+		}
+		skb_put(nskb, size);
+
+		skb_copy_header(nskb, skb);
+		head_off = skb_headroom(nskb) - skb_headroom(skb);
+		skb_headers_offset_update(nskb, head_off);
+
+		/* Allocate paged area of new skb */
+		off = size;
+		len = skb->len - off;
+
+		for (i = 0; i < MAX_SKB_FRAGS && off < skb->len; i++) {
+			page = alloc_page(GFP_ATOMIC | __GFP_NOWARN);
+			if (!page) {
+				consume_skb(nskb);
+				goto drop;
+			}
+
+			size = min_t(u32, len, PAGE_SIZE);
+			skb_add_rx_frag(nskb, i, page, 0, size, PAGE_SIZE);
+			if (skb_copy_bits(skb, off, page_address(page),
+					  size)) {
+				consume_skb(nskb);
+				goto drop;
+			}
+
+			len -= size;
+			off += size;
+		}
+
+		consume_skb(skb);
+		skb = nskb;
+	} else if (skb_headroom(skb) < XDP_PACKET_HEADROOM &&
+		   pskb_expand_head(skb, VETH_XDP_HEADROOM, 0, GFP_ATOMIC)) {
+		goto drop;
+	}
+
+	/* SKB "head" area always have tailroom for skb_shared_info */
+	frame_sz = skb_end_pointer(skb) - skb->head;
+	frame_sz += SKB_DATA_ALIGN(sizeof(struct skb_shared_info));
+	xdp_init_buff(xdp, frame_sz, &rq->xdp_rxq);
+	xdp_prepare_buff(xdp, skb->head, skb_headroom(skb),
+			 skb_headlen(skb), true);
+
+	if (skb_is_nonlinear(skb)) {
+		skb_shinfo(skb)->xdp_frags_size = skb->data_len;
+		xdp_buff_set_frags_flag(xdp);
+	} else {
+		xdp_buff_clear_frags_flag(xdp);
+	}
+	*pskb = skb;
+
+	return 0;
+drop:
+	consume_skb(skb);
+	*pskb = NULL;
+
+	return -ENOMEM;
+}
+
 static struct sk_buff *veth_xdp_rcv_skb(struct veth_rq *rq,
 					struct sk_buff *skb,
 					struct veth_xdp_tx_bq *bq,
 					struct veth_stats *stats)
 {
-	u32 pktlen, headroom, act, metalen, frame_sz;
 	void *orig_data, *orig_data_end;
 	struct bpf_prog *xdp_prog;
-	int mac_len, delta, off;
 	struct xdp_buff xdp;
+	u32 act, metalen;
+	int off;
 
 	skb_prepare_for_gro(skb);
 
···
 		goto out;
 	}
 
-	mac_len = skb->data - skb_mac_header(skb);
-	pktlen = skb->len + mac_len;
-	headroom = skb_headroom(skb) - mac_len;
-
-	if (skb_shared(skb) || skb_head_is_locked(skb) ||
-	    skb_is_nonlinear(skb) || headroom < XDP_PACKET_HEADROOM) {
-		struct sk_buff *nskb;
-		int size, head_off;
-		void *head, *start;
-		struct page *page;
-
-		size = SKB_DATA_ALIGN(VETH_XDP_HEADROOM + pktlen) +
-		       SKB_DATA_ALIGN(sizeof(struct skb_shared_info));
-		if (size > PAGE_SIZE)
-			goto drop;
-
-		page = alloc_page(GFP_ATOMIC | __GFP_NOWARN);
-		if (!page)
-			goto drop;
-
-		head = page_address(page);
-		start = head + VETH_XDP_HEADROOM;
-		if (skb_copy_bits(skb, -mac_len, start, pktlen)) {
-			page_frag_free(head);
-			goto drop;
-		}
-
-		nskb = veth_build_skb(head, VETH_XDP_HEADROOM + mac_len,
-				      skb->len, PAGE_SIZE);
-		if (!nskb) {
-			page_frag_free(head);
-			goto drop;
-		}
-
-		skb_copy_header(nskb, skb);
-		head_off = skb_headroom(nskb) - skb_headroom(skb);
-		skb_headers_offset_update(nskb, head_off);
-		consume_skb(skb);
-		skb = nskb;
-	}
-
-	/* SKB "head" area always have tailroom for skb_shared_info */
-	frame_sz = skb_end_pointer(skb) - skb->head;
-	frame_sz += SKB_DATA_ALIGN(sizeof(struct skb_shared_info));
-	xdp_init_buff(&xdp, frame_sz, &rq->xdp_rxq);
-	xdp_prepare_buff(&xdp, skb->head, skb->mac_header, pktlen, true);
+	__skb_push(skb, skb->data - skb_mac_header(skb));
+	if (veth_convert_skb_to_xdp_buff(rq, &xdp, &skb))
+		goto drop;
 
 	orig_data = xdp.data;
 	orig_data_end = xdp.data_end;
···
 	case XDP_PASS:
 		break;
 	case XDP_TX:
-		get_page(virt_to_page(xdp.data));
+		veth_xdp_get(&xdp);
 		consume_skb(skb);
 		xdp.rxq->mem = rq->xdp_mem;
 		if (unlikely(veth_xdp_tx(rq, &xdp, bq) < 0)) {
···
 		rcu_read_unlock();
 		goto xdp_xmit;
 	case XDP_REDIRECT:
-		get_page(virt_to_page(xdp.data));
+		veth_xdp_get(&xdp);
 		consume_skb(skb);
 		xdp.rxq->mem = rq->xdp_mem;
 		if (xdp_do_redirect(rq->dev, &xdp, xdp_prog)) {
···
 	rcu_read_unlock();
 
 	/* check if bpf_xdp_adjust_head was used */
-	delta = orig_data - xdp.data;
-	off = mac_len + delta;
+	off = orig_data - xdp.data;
 	if (off > 0)
 		__skb_push(skb, off);
 	else if (off < 0)
 		__skb_pull(skb, -off);
-	skb->mac_header -= delta;
+
+	skb_reset_mac_header(skb);
 
 	/* check if bpf_xdp_adjust_tail was used */
 	off = xdp.data_end - orig_data_end;
 	if (off != 0)
 		__skb_put(skb, off); /* positive on grow, negative on shrink */
+
+	/* XDP frag metadata (e.g. nr_frags) are updated in eBPF helpers
+	 * (e.g. bpf_xdp_adjust_tail), we need to update data_len here.
+	 */
+	if (xdp_buff_has_frags(&xdp))
+		skb->data_len = skb_shinfo(skb)->xdp_frags_size;
+	else
+		skb->data_len = 0;
+
 	skb->protocol = eth_type_trans(skb, rq->dev);
 
 	metalen = xdp.data - xdp.data_meta;
···
 	return NULL;
 err_xdp:
 	rcu_read_unlock();
-	page_frag_free(xdp.data);
+	xdp_return_buff(&xdp);
 xdp_xmit:
 	return NULL;
 }

net/core/xdp.c

···
 out:
 	__xdp_return(xdp->data, &xdp->rxq->mem, true, xdp);
 }
+EXPORT_SYMBOL_GPL(xdp_return_buff);
 
 /* Only called for MEM_TYPE_PAGE_POOL see xdp.h */
 void __xdp_release_frame(void *data, struct xdp_mem_info *mem)