xdp: Allow bpf_xdp_adjust_tail() to grow packet size

Finally, after all drivers have a frame size, allow BPF-helper
bpf_xdp_adjust_tail() to grow or extend packet size at frame tail.

Remember that helper/macro xdp_data_hard_end have reserved some
tailroom.  Thus, this helper makes sure that the BPF-prog don't have
access to this tailroom area.

V2: Remove one chicken check and use WARN_ONCE for other
Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/158945348530.97035.12577148209134239291.stgit@firesoul

include/uapi/linux/bpf.h

@@ -2015,8 +2015,8 @@ union bpf_attr {
  * int bpf_xdp_adjust_tail(struct xdp_buff *xdp_md, int delta)
  * 	Description
  * 		Adjust (move) *xdp_md*\ **->data_end** by *delta* bytes. It is
- * 		only possible to shrink the packet as of this writing,
- * 		therefore *delta* must be a negative integer.
+ * 		possible to both shrink and grow the packet tail.
+ * 		Shrink done via *delta* being a negative integer.
  *
  * 		A call to this helper is susceptible to change the underlying
  * 		packet buffer. Therefore, at load time, all checks on pointers

net/core/filter.c

@@ -3411,12 +3411,19 @@ static const struct bpf_func_proto bpf_xdp_adjust_head_proto = {
 
 BPF_CALL_2(bpf_xdp_adjust_tail, struct xdp_buff *, xdp, int, offset)
 {
+	void *data_hard_end = xdp_data_hard_end(xdp); /* use xdp->frame_sz */
 	void *data_end = xdp->data_end + offset;
 
-	/* only shrinking is allowed for now. */
-	if (unlikely(offset >= 0))
+	/* Notice that xdp_data_hard_end have reserved some tailroom */
+	if (unlikely(data_end > data_hard_end))
 		return -EINVAL;
 
+	/* ALL drivers MUST init xdp->frame_sz, chicken check below */
+	if (unlikely(xdp->frame_sz > PAGE_SIZE)) {
+		WARN_ONCE(1, "Too BIG xdp->frame_sz = %d\n", xdp->frame_sz);
+		return -EINVAL;
+	}
+
 	if (unlikely(data_end < xdp->data + ETH_HLEN))
 		return -EINVAL;