net: systemport: check harder for out of memory conditions

There is a potential case where we might be failing to refill a
control block, leaving it with both a NULL skb pointer *and* a NULL
dma_unmap_addr.

The way we process incoming packets, by first calling
dma_unmap_single(), and then only checking for a potential NULL skb can
lead to situations where do pass a NULL dma_unmap_addr() to
dma_unmap_single(), resulting in an oops.

Fix this my moving the NULL skb check earlier, since no backing skb
also means no corresponding DMA mapping for this packet.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/broadcom/bcmsysport.c

@@ -534,6 +534,25 @@ static unsigned int bcm_sysport_desc_rx(struct bcm_sysport_priv *priv,
 	while ((processed < to_process) && (processed < budget)) {
 		cb = &priv->rx_cbs[priv->rx_read_ptr];
 		skb = cb->skb;
+
+		processed++;
+		priv->rx_read_ptr++;
+
+		if (priv->rx_read_ptr == priv->num_rx_bds)
+			priv->rx_read_ptr = 0;
+
+		/* We do not have a backing SKB, so we do not a corresponding
+		 * DMA mapping for this incoming packet since
+		 * bcm_sysport_rx_refill always either has both skb and mapping
+		 * or none.
+		 */
+		if (unlikely(!skb)) {
+			netif_err(priv, rx_err, ndev, "out of memory!\n");
+			ndev->stats.rx_dropped++;
+			ndev->stats.rx_errors++;
+			goto refill;
+		}
+
 		dma_unmap_single(kdev, dma_unmap_addr(cb, dma_addr),
 				 RX_BUF_LENGTH, DMA_FROM_DEVICE);
 
@@ -543,23 +562,11 @@ static unsigned int bcm_sysport_desc_rx(struct bcm_sysport_priv *priv,
 		status = (rsb->rx_status_len >> DESC_STATUS_SHIFT) &
 			  DESC_STATUS_MASK;
 
-		processed++;
-		priv->rx_read_ptr++;
-		if (priv->rx_read_ptr == priv->num_rx_bds)
-			priv->rx_read_ptr = 0;
-
 		netif_dbg(priv, rx_status, ndev,
 			  "p=%d, c=%d, rd_ptr=%d, len=%d, flag=0x%04x\n",
 			  p_index, priv->rx_c_index, priv->rx_read_ptr,
 			  len, status);
 
-		if (unlikely(!skb)) {
-			netif_err(priv, rx_err, ndev, "out of memory!\n");
-			ndev->stats.rx_dropped++;
-			ndev->stats.rx_errors++;
-			goto refill;
-		}
-
 		if (unlikely(!(status & DESC_EOP) || !(status & DESC_SOP))) {
 			netif_err(priv, rx_status, ndev, "fragmented packet!\n");
 			ndev->stats.rx_dropped++;