net: dsa: factor skb freeing on xmit

The taggers are currently responsible to free the original SKB if they
made a copy of it, or in case of error.

This patch simplifies this by freeing the original SKB in the
dsa_slave_xmit caller, but only if an error (NULL) is returned.

It is still the responsibility of the tagger to free the original SKB if
it returned a copy of it.

At the same time, fix the checkpatch NULL comparison check:

        CHECK: Comparison to NULL could be written "!nskb"
    torvalds#208: FILE: net/dsa/tag_trailer.c:35:
    +	if (nskb == NULL)

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>

net/dsa/slave.c

@@ -357,10 +357,14 @@ static netdev_tx_t dsa_slave_xmit(struct sk_buff *skb, struct net_device *dev)
 	dev->stats.tx_packets++;
 	dev->stats.tx_bytes += skb->len;
 
-	/* Transmit function may have to reallocate the original SKB */
+	/* Transmit function may have to reallocate the original SKB,
+	 * in which case it must have freed it. Only free it here on error.
+	 */
 	nskb = p->xmit(skb, dev);
-	if (!nskb)
+	if (!nskb) {
+		kfree_skb(skb);
 		return NETDEV_TX_OK;
+	}
 
 	/* SKB for netpoll still need to be mangled with the protocol-specific
 	 * tag to be successfully transmitted

net/dsa/tag_brcm.c

@@ -65,7 +65,7 @@ static struct sk_buff *brcm_tag_xmit(struct sk_buff *skb, struct net_device *dev
 	u8 *brcm_tag;
 
 	if (skb_cow_head(skb, BRCM_TAG_LEN) < 0)
-		goto out_free;
+		return NULL;
 
 	skb_push(skb, BRCM_TAG_LEN);
 
@@ -86,10 +86,6 @@ static struct sk_buff *brcm_tag_xmit(struct sk_buff *skb, struct net_device *dev
 	brcm_tag[3] = (1 << p->dp->index) & BRCM_IG_DSTMAP1_MASK;
 
 	return skb;
-
-out_free:
-	kfree_skb(skb);
-	return NULL;
 }
 
 static struct sk_buff *brcm_tag_rcv(struct sk_buff *skb, struct net_device *dev)

net/dsa/tag_dsa.c

@@ -28,7 +28,7 @@ static struct sk_buff *dsa_xmit(struct sk_buff *skb, struct net_device *dev)
 	 */
 	if (skb->protocol == htons(ETH_P_8021Q)) {
 		if (skb_cow_head(skb, 0) < 0)
-			goto out_free;
+			return NULL;
 
 		/*
 		 * Construct tagged FROM_CPU DSA tag from 802.1q tag.
@@ -46,7 +46,7 @@ static struct sk_buff *dsa_xmit(struct sk_buff *skb, struct net_device *dev)
 		}
 	} else {
 		if (skb_cow_head(skb, DSA_HLEN) < 0)
-			goto out_free;
+			return NULL;
 		skb_push(skb, DSA_HLEN);
 
 		memmove(skb->data, skb->data + DSA_HLEN, 2 * ETH_ALEN);
@@ -62,10 +62,6 @@ static struct sk_buff *dsa_xmit(struct sk_buff *skb, struct net_device *dev)
 	}
 
 	return skb;
-
-out_free:
-	kfree_skb(skb);
-	return NULL;
 }
 
 static struct sk_buff *dsa_rcv(struct sk_buff *skb, struct net_device *dev)

net/dsa/tag_edsa.c

@@ -30,7 +30,7 @@ static struct sk_buff *edsa_xmit(struct sk_buff *skb, struct net_device *dev)
 	 */
 	if (skb->protocol == htons(ETH_P_8021Q)) {
 		if (skb_cow_head(skb, DSA_HLEN) < 0)
-			goto out_free;
+			return NULL;
 		skb_push(skb, DSA_HLEN);
 
 		memmove(skb->data, skb->data + DSA_HLEN, 2 * ETH_ALEN);
@@ -55,7 +55,7 @@ static struct sk_buff *edsa_xmit(struct sk_buff *skb, struct net_device *dev)
 		}
 	} else {
 		if (skb_cow_head(skb, EDSA_HLEN) < 0)
-			goto out_free;
+			return NULL;
 		skb_push(skb, EDSA_HLEN);
 
 		memmove(skb->data, skb->data + EDSA_HLEN, 2 * ETH_ALEN);
@@ -75,10 +75,6 @@ static struct sk_buff *edsa_xmit(struct sk_buff *skb, struct net_device *dev)
 	}
 
 	return skb;
-
-out_free:
-	kfree_skb(skb);
-	return NULL;
 }
 
 static struct sk_buff *edsa_rcv(struct sk_buff *skb, struct net_device *dev)

net/dsa/tag_lan9303.c

@@ -52,7 +52,7 @@ static struct sk_buff *lan9303_xmit(struct sk_buff *skb, struct net_device *dev)
 	if (skb_cow_head(skb, LAN9303_TAG_LEN) < 0) {
 		dev_dbg(&dev->dev,
 			"Cannot make room for the special tag. Dropping packet\n");
-		goto out_free;
+		return NULL;
 	}
 
 	/* provide 'LAN9303_TAG_LEN' bytes additional space */
@@ -66,9 +66,6 @@ static struct sk_buff *lan9303_xmit(struct sk_buff *skb, struct net_device *dev)
 	lan9303_tag[1] = htons(p->dp->index | BIT(4));
 
 	return skb;
-out_free:
-	kfree_skb(skb);
-	return NULL;
 }
 
 static struct sk_buff *lan9303_rcv(struct sk_buff *skb, struct net_device *dev)

net/dsa/tag_mtk.c

@@ -27,7 +27,7 @@ static struct sk_buff *mtk_tag_xmit(struct sk_buff *skb,
 	u8 *mtk_tag;
 
 	if (skb_cow_head(skb, MTK_HDR_LEN) < 0)
-		goto out_free;
+		return NULL;
 
 	skb_push(skb, MTK_HDR_LEN);
 
@@ -41,10 +41,6 @@ static struct sk_buff *mtk_tag_xmit(struct sk_buff *skb,
 	mtk_tag[3] = 0;
 
 	return skb;
-
-out_free:
-	kfree_skb(skb);
-	return NULL;
 }
 
 static struct sk_buff *mtk_tag_rcv(struct sk_buff *skb, struct net_device *dev)

net/dsa/tag_qca.c

@@ -45,7 +45,7 @@ static struct sk_buff *qca_tag_xmit(struct sk_buff *skb, struct net_device *dev)
 	dev->stats.tx_bytes += skb->len;
 
 	if (skb_cow_head(skb, 0) < 0)
-		goto out_free;
+		return NULL;
 
 	skb_push(skb, QCA_HDR_LEN);
 
@@ -60,10 +60,6 @@ static struct sk_buff *qca_tag_xmit(struct sk_buff *skb, struct net_device *dev)
 	*phdr = htons(hdr);
 
 	return skb;
-
-out_free:
-	kfree_skb(skb);
-	return NULL;
 }
 
 static struct sk_buff *qca_tag_rcv(struct sk_buff *skb, struct net_device *dev)

net/dsa/tag_trailer.c

@@ -32,10 +32,8 @@ static struct sk_buff *trailer_xmit(struct sk_buff *skb, struct net_device *dev)
 		padlen = 60 - skb->len;
 
 	nskb = alloc_skb(NET_IP_ALIGN + skb->len + padlen + 4, GFP_ATOMIC);
-	if (nskb == NULL) {
-		kfree_skb(skb);
+	if (!nskb)
 		return NULL;
-	}
 	skb_reserve(nskb, NET_IP_ALIGN);
 
 	skb_reset_mac_header(nskb);