[PATCH net-next v2 06/33] net: XDP-generic determining XDP frame size

Jesper Dangaard Brouer
2020-04-30T11:21:07Z

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 offset adjust 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@redhat.com>
Acked-by: Toke Høiland-Jørgensen <toke@redhat.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 afff16849c26..b364e6f3a37a 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -4549,6 +4549,11 @@ 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;
@@ -4572,14 +4577,11 @@ 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 */