[PATCH RFC v2 29/33] xdp: allow bpf_xdp_adjust_tail() to grow packet size

[Jesper Dangaard Brouer <brouer@redhat.com>]

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.

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
---
 include/uapi/linux/bpf.h |    4 ++--
 net/core/filter.c        |   18 ++++++++++++++++--
 2 files changed, 18 insertions(+), 4 deletions(-)

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index 2e29a671d67e..0e5abe991ca3 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -1969,8 +1969,8 @@ 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
diff --git a/net/core/filter.c b/net/core/filter.c
index 7628b947dbc3..4d58a147eed0 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -3422,12 +3422,26 @@ 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);
 	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;
 
+	/* DANGER: ALL drivers MUST be converted to init xdp->frame_sz
+	 * - Adding some chicken checks below
+	 * - Will (likely) not be for upstream
+	 */
+	if (unlikely(xdp->frame_sz < (xdp->data_end - xdp->data_hard_start))) {
+		WARN(1, "Too small xdp->frame_sz = %d\n", xdp->frame_sz);
+		return -EINVAL;
+	}
+	if (unlikely(xdp->frame_sz > PAGE_SIZE)) {
+		WARN(1, "Too BIG xdp->frame_sz = %d\n", xdp->frame_sz);
+		return -EINVAL;
+	}
+
 	if (unlikely(data_end < xdp->data + ETH_HLEN))
 		return -EINVAL;