[PATCH net 2/3] net: enetc: reserve VLAN extension BDs for software TSO

[Vladimir Oltean <vladimir.oltean@nxp.com>]

enetc_start_xmit() uses tso_count_descs() to check whether the TX ring
has enough free BDs for a software TSO frame. That estimates one header
BD and one data BD per segment, plus one BD per frag. However, when the
skb carries a VLAN tag, enetc_map_tx_tso_hdr() also adds an extension
BD to every segment, which is not accounted for.

With a highly segmented VLAN TSO skb and a nearly full ring, the driver
can then write past next_to_clean and overwrite BDs still owned by the
hardware.

Add enetc_tso_count_descs() which accounts for the extension BD of each
segment when VLAN insertion is needed.

Fixes: fb8629e2cbfc ("net: enetc: add support for software TSO")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
---
This comes from the previous patch set:
https://lore.kernel.org/netdev/20261021145314.1435049-2-vladimir.oltean@nxp.com/

v5->v1: also patch enetc_features_check()
v4->v5: patch is new
---
 drivers/net/ethernet/freescale/enetc/enetc.c | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/drivers/net/ethernet/freescale/enetc/enetc.c b/drivers/net/ethernet/freescale/enetc/enetc.c
index 1606ac57016f..7f089a84d9a6 100644
--- a/drivers/net/ethernet/freescale/enetc/enetc.c
+++ b/drivers/net/ethernet/freescale/enetc/enetc.c
@@ -674,6 +674,17 @@ static void enetc_tso_complete_csum(struct enetc_bdr *tx_ring, struct tso_t *tso
 	}
 }
 
+static int enetc_tso_count_descs(const struct sk_buff *skb)
+{
+	int count = tso_count_descs(skb);
+
+	/* Each segment also needs an extension BD for VLAN insertion */
+	if (skb_vlan_tag_present(skb))
+		count += skb_shinfo(skb)->gso_segs;
+
+	return count;
+}
+
 static int enetc_lso_count_descs(const struct sk_buff *skb)
 {
 	/* 4 BDs: 1 BD for LSO header + 1 BD for extended BD + 1 BD
@@ -1034,7 +1045,7 @@ static netdev_tx_t enetc_start_xmit(struct sk_buff *skb,
 
 			count = enetc_lso_hw_offload(tx_ring, skb);
 		} else {
-			if (enetc_bd_unused(tx_ring) < tso_count_descs(skb)) {
+			if (enetc_bd_unused(tx_ring) < enetc_tso_count_descs(skb)) {
 				netif_stop_subqueue(ndev, tx_ring->index);
 				return NETDEV_TX_BUSY;
 			}
@@ -1114,7 +1125,7 @@ netdev_features_t enetc_features_check(struct sk_buff *skb,
 	if (!skb_is_gso(skb) || enetc_skb_uses_lso(priv, skb))
 		return features;
 
-	if (tso_count_descs(skb) >= priv->tx_bd_count)
+	if (enetc_tso_count_descs(skb) >= priv->tx_bd_count)
 		features &= ~NETIF_F_GSO_MASK;
 
 	return features;
-- 
2.53.0