[PATCH net-next v4 29/33] xdp: allow bpf_xdp_adjust_tail() to grow packet size

[Jesper Dangaard Brouer]

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 <bro...@redhat.com>
---
 include/uapi/linux/bpf.h |    4 ++--
 net/core/filter.c        |   11 +++++++++--
 2 files changed, 11 insertions(+), 4 deletions(-)

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index 101b0c8a3784..3e44853388d8 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -2004,8 +2004,8 @@ 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 dfaf5df13722..ec3ab2e2d800 100644
--- a/net/core/filter.c
+++ b/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;