xsk: remove @first_frag from xsk_build_skb()

Instead of using auxiliary boolean that tracks if we are at first frag
when gathering all elements of skb, same functionality can be achieved
with checking if skb_shared_info::nr_frags is 0.

Remove @first_frag but be careful around xsk_build_skb_zerocopy() and
NULL the skb pointer when it failed so that common error path does not
incorrectly interpret it during decision whether to call kfree_skb().

Signed-off-by: Maciej Fijalkowski <[email protected]>
Acked-by: Stanislav Fomichev <[email protected]>

Author: mfijalko

net/xdp/xsk.c

@@ -730,13 +730,13 @@ static struct sk_buff *xsk_build_skb(struct xdp_sock *xs,
 	struct xsk_tx_metadata *meta = NULL;
 	struct net_device *dev = xs->dev;
 	struct sk_buff *skb = xs->skb;
-	bool first_frag = false;
 	int err;
 
 	if (dev->priv_flags & IFF_TX_SKB_NO_LINEAR) {
 		skb = xsk_build_skb_zerocopy(xs, desc);
 		if (IS_ERR(skb)) {
 			err = PTR_ERR(skb);
+			skb = NULL;
 			goto free_err;
 		}
 	} else {
@@ -747,8 +747,6 @@ static struct sk_buff *xsk_build_skb(struct xdp_sock *xs,
 		len = desc->len;
 
 		if (!skb) {
-			first_frag = true;
-
 			hr = max(NET_SKB_PAD, L1_CACHE_ALIGN(dev->needed_headroom));
 			tr = dev->needed_tailroom;
 			skb = sock_alloc_send_skb(&xs->sk, hr + len + tr, 1, &err);
@@ -798,7 +796,7 @@ static struct sk_buff *xsk_build_skb(struct xdp_sock *xs,
 			list_add_tail(&xsk_addr->addr_node, &XSKCB(skb)->addrs_list);
 		}
 
-		if (first_frag && desc->options & XDP_TX_METADATA) {
+		if (!skb_shinfo(skb)->nr_frags && desc->options & XDP_TX_METADATA) {
 			if (unlikely(xs->pool->tx_metadata_len == 0)) {
 				err = -EINVAL;
 				goto free_err;
@@ -840,7 +838,7 @@ static struct sk_buff *xsk_build_skb(struct xdp_sock *xs,
 	return skb;
 
 free_err:
-	if (first_frag && skb)
+	if (skb && !skb_shinfo(skb)->nr_frags)
 		kfree_skb(skb);
 
 	if (err == -EOVERFLOW) {