Similar to the skb path issue explained in the previous change, ENETC
could end up transmitting short frames coming from XDP.

The way in which this could happen is a bit contrived, but it involves
XDP_REDIRECT from a veth interface pair.

Introduce a xdp_frame_pad() generic helper and call it from enetc's
ndo_xdp_xmit() implementation. This should be safe, because
ndo_xdp_xmit() is the hand-off function where the XDP frames become the
responsibility of the driver, so modifying them is fine. AFAIU, struct
xdp_frame doesn't have multiple copies.

Fixes: 9d2b68cc108d ("net: enetc: add support for XDP_REDIRECT")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
---
 drivers/net/ethernet/freescale/enetc/enetc.c |  3 +++
 include/net/xdp.h                            | 17 +++++++++++++++++
 2 files changed, 20 insertions(+)

diff --git a/drivers/net/ethernet/freescale/enetc/enetc.c b/drivers/net/ethernet/freescale/enetc/enetc.c
index 4b87fbfde0d6..3ceb9dfd2316 100644
--- a/drivers/net/ethernet/freescale/enetc/enetc.c
+++ b/drivers/net/ethernet/freescale/enetc/enetc.c
@@ -1801,6 +1801,9 @@ int enetc_xdp_xmit(struct net_device *ndev, int num_frames,
 	prefetchw(ENETC_TXBD(*tx_ring, tx_ring->next_to_use));
 
 	for (k = 0; k < num_frames; k++) {
+		if (unlikely(xdp_frame_pad(frames[k])))
+			break;
+
 		xdp_tx_bd_cnt = enetc_xdp_frame_to_xdp_tx_swbd(tx_ring,
 							       xdp_redirect_arr,
 							       frames[k]);
diff --git a/include/net/xdp.h b/include/net/xdp.h
index aa742f413c35..0cdeb23c6bd7 100644
--- a/include/net/xdp.h
+++ b/include/net/xdp.h
@@ -477,6 +477,23 @@ xdp_get_frame_len(const struct xdp_frame *xdpf)
 	return len;
 }
 
+static inline int xdp_frame_pad(struct xdp_frame *xdpf)
+{
+	void *sinfo;
+
+	if (likely(xdpf->len >= ETH_ZLEN))
+		return 0;
+
+	sinfo = xdp_get_shared_info_from_frame(xdpf);
+	if (unlikely(xdpf->data + ETH_ZLEN > sinfo))
+		return -ENOMEM;
+
+	memset(xdpf->data + xdpf->len, 0, ETH_ZLEN - xdpf->len);
+	xdpf->len = ETH_ZLEN;
+
+	return 0;
+}
+
 int __xdp_rxq_info_reg(struct xdp_rxq_info *xdp_rxq,
 		       struct net_device *dev, u32 queue_index,
 		       unsigned int napi_id, u32 frag_size);
-- 
2.43.0