xsk: move xdp_buff's data length check to xsk_rcv_check

If the data in xdp_buff exceeds the xsk frame length, the packet needs
to be dropped. This check is currently being done in __xsk_rcv(). Move
the described logic to xsk_rcv_check() so that such a xdp_buff will
only be dropped if the application does not support multi-buffer
(absence of XDP_USE_SG bind flag). This is applicable for all cases:
copy mode, zero copy mode as well as skb mode.

Signed-off-by: Tirthendu Sarkar <tirthendu.sarkar@intel.com>
Link: https://lore.kernel.org/r/20230719132421.584801-5-maciej.fijalkowski@intel.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

net/xdp/xsk.c

@@ -177,18 +177,11 @@ static void xsk_copy_xdp(struct xdp_buff *to, struct xdp_buff *from, u32 len)
 	memcpy(to_buf, from_buf, len + metalen);
 }
 
-static int __xsk_rcv(struct xdp_sock *xs, struct xdp_buff *xdp)
+static int __xsk_rcv(struct xdp_sock *xs, struct xdp_buff *xdp, u32 len)
 {
 	struct xdp_buff_xsk *xskb;
 	struct xdp_buff *xsk_xdp;
 	int err;
-	u32 len;
-
-	len = xdp->data_end - xdp->data;
-	if (len > xsk_pool_get_rx_frame_size(xs->pool)) {
-		xs->rx_dropped++;
-		return -ENOSPC;
-	}
 
 	xsk_xdp = xsk_buff_alloc(xs->pool);
 	if (!xsk_xdp) {
@@ -224,14 +217,19 @@ static bool xsk_is_bound(struct xdp_sock *xs)
 	return false;
 }
 
-static int xsk_rcv_check(struct xdp_sock *xs, struct xdp_buff *xdp)
+static int xsk_rcv_check(struct xdp_sock *xs, struct xdp_buff *xdp, u32 len)
 {
 	if (!xsk_is_bound(xs))
 		return -ENXIO;
 
 	if (xs->dev != xdp->rxq->dev || xs->queue_id != xdp->rxq->queue_index)
 		return -EINVAL;
 
+	if (len > xsk_pool_get_rx_frame_size(xs->pool)) {
+		xs->rx_dropped++;
+		return -ENOSPC;
+	}
+
 	sk_mark_napi_id_once_xdp(&xs->sk, xdp);
 	return 0;
 }
@@ -245,12 +243,13 @@ static void xsk_flush(struct xdp_sock *xs)
 
 int xsk_generic_rcv(struct xdp_sock *xs, struct xdp_buff *xdp)
 {
+	u32 len = xdp_get_buff_len(xdp);
 	int err;
 
 	spin_lock_bh(&xs->rx_lock);
-	err = xsk_rcv_check(xs, xdp);
+	err = xsk_rcv_check(xs, xdp, len);
 	if (!err) {
-		err = __xsk_rcv(xs, xdp);
+		err = __xsk_rcv(xs, xdp, len);
 		xsk_flush(xs);
 	}
 	spin_unlock_bh(&xs->rx_lock);
@@ -259,10 +258,10 @@ int xsk_generic_rcv(struct xdp_sock *xs, struct xdp_buff *xdp)
 
 static int xsk_rcv(struct xdp_sock *xs, struct xdp_buff *xdp)
 {
+	u32 len = xdp_get_buff_len(xdp);
 	int err;
-	u32 len;
 
-	err = xsk_rcv_check(xs, xdp);
+	err = xsk_rcv_check(xs, xdp, len);
 	if (err)
 		return err;
 
@@ -271,7 +270,7 @@ static int xsk_rcv(struct xdp_sock *xs, struct xdp_buff *xdp)
 		return xsk_rcv_zc(xs, xdp, len);
 	}
 
-	err = __xsk_rcv(xs, xdp);
+	err = __xsk_rcv(xs, xdp, len);
 	if (!err)
 		xdp_return_buff(xdp);
 	return err;