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 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>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Toke Høiland-Jørgensen <toke@redhat.com>
Link: https://lore.kernel.org/bpf/158945336804.97035.7164852191163722056.stgit@firesoul

diff --git a/net/core/dev.c b/net/core/dev.c
index 4c91de39890a98a57097adf9826eeab52d57d791..f937a3ff668d8d7f786660007b4d2c212b8bd548 100644 (file)
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -4617,6 +4617,11 @@ 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;
@@ -4640,14 +4645,11 @@ 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 */