netdev - [PATCH RFC v1 10/15] net: XDP-generic determining XDP frame size

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Date:   Tue, 17 Mar 2020 18:29:58 +0100
From:   Jesper Dangaard Brouer <brouer@...hat.com>
To:     sameehj@...zon.com
Cc:     Jesper Dangaard Brouer <brouer@...hat.com>, netdev@...r.kernel.org,
        bpf@...r.kernel.org, zorik@...zon.com, akiyano@...zon.com,
        gtzalik@...zon.com,
        Toke Høiland-Jørgensen <toke@...e.dk>,
        Daniel Borkmann <borkmann@...earbox.net>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        John Fastabend <john.fastabend@...il.com>,
        Alexander Duyck <alexander.duyck@...il.com>,
        Jeff Kirsher <jeffrey.t.kirsher@...el.com>,
        David Ahern <dsahern@...il.com>,
        Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        Ilias Apalodimas <ilias.apalodimas@...aro.org>,
        Lorenzo Bianconi <lorenzo@...nel.org>
Subject: [PATCH RFC v1 10/15] net: XDP-generic determining XDP frame size

The SKB "head" pointer points to the data area that contains
skb_shared_info, that can be found via skb_end_pointer(). Given
xdp->data_hard_start have been established (basically pointing to
skb->head), frame size is between skb_end_pointer() and data_hard_start,
plus the size reserved to skb_shared_info.

Change the bpf_xdp_adjust_tail fix up of skb->len, to be a positive
offset number on grow, and negative number on shrink.  As this seems more
natural when reading the code

Signed-off-by: Jesper Dangaard Brouer <brouer@...hat.com>
---
 net/core/dev.c |   14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/net/core/dev.c b/net/core/dev.c
index 9c941cd38b13..645e5c48a52f 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -4546,6 +4546,11 @@ static u32 netif_receive_generic_xdp(struct sk_buff *skb,
 	xdp->data_meta = xdp->data;
 	xdp->data_end = xdp->data + hlen;
 	xdp->data_hard_start = skb->data - skb_headroom(skb);
+
+	/* SKB "head" area always have tailroom for skb_shared_info */
+	xdp->frame_sz  = (void *)skb_end_pointer(skb) - xdp->data_hard_start;
+	xdp->frame_sz += SKB_DATA_ALIGN(sizeof(struct skb_shared_info));
+
 	orig_data_end = xdp->data_end;
 	orig_data = xdp->data;
 	eth = (struct ethhdr *)xdp->data;
@@ -4569,14 +4574,11 @@ static u32 netif_receive_generic_xdp(struct sk_buff *skb,
 		skb_reset_network_header(skb);
 	}
 
-	/* check if bpf_xdp_adjust_tail was used. it can only "shrink"
-	 * pckt.
-	 */
-	off = orig_data_end - xdp->data_end;
+	/* check if bpf_xdp_adjust_tail was used */
+	off = xdp->data_end - orig_data_end;
 	if (off != 0) {
 		skb_set_tail_pointer(skb, xdp->data_end - xdp->data);
-		skb->len -= off;
-
+		skb->len += off; /* positive on grow, negative on shrink */
 	}
 
 	/* check if XDP changed eth hdr such SKB needs update */