[PATCH NET 2/7] ipv6: use pskb_realloc_headroom in ip6_finish_output2

[Vasily Averin]

Unlike skb_realloc_headroom, new helper pskb_realloc_headroom
does not allocate a new skb if possible.

Additionally this patch replaces commonly used dereferencing with variables.
---
NB: this patch depends on
"ipv6: allocate enough headroom in ip6_finish_output2()"
https://lkml.org/lkml/2021/7/12/732

Signed-off-by: Vasily Averin <vvs@xxxxxxxxxxxxx>
---
 net/ipv6/ip6_output.c | 50 ++++++++++++++++----------------------------------
 1 file changed, 16 insertions(+), 34 deletions(-)

diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
index 0efcb9b..2054da3 100644
--- a/net/ipv6/ip6_output.c
+++ b/net/ipv6/ip6_output.c
@@ -60,46 +60,30 @@ static int ip6_finish_output2(struct net *net, struct sock *sk, struct sk_buff *
 {
 	struct dst_entry *dst = skb_dst(skb);
 	struct net_device *dev = dst->dev;
+	struct inet6_dev *idev = ip6_dst_idev(dst);
 	unsigned int hh_len = LL_RESERVED_SPACE(dev);
-	int delta = hh_len - skb_headroom(skb);
-	const struct in6_addr *nexthop;
+	const struct in6_addr *daddr, *nexthop;
+	struct ipv6hdr *hdr;
 	struct neighbour *neigh;
 	int ret;
 
 	/* Be paranoid, rather than too clever. */
-	if (unlikely(delta > 0) && dev->header_ops) {
-		/* pskb_expand_head() might crash, if skb is shared */
-		if (skb_shared(skb)) {
-			struct sk_buff *nskb = skb_clone(skb, GFP_ATOMIC);
+	if (unlikely(hh_len > skb_headroom(skb)) && dev->header_ops) {
+		skb = pskb_realloc_headroom(skb, hh_len);
 
-			if (likely(nskb)) {
-				if (skb->sk)
-					skb_set_owner_w(skb, skb->sk);
-				consume_skb(skb);
-			} else {
-				kfree_skb(skb);
-			}
-			skb = nskb;
-		}
-		if (skb &&
-		    pskb_expand_head(skb, SKB_DATA_ALIGN(delta), 0, GFP_ATOMIC)) {
-			kfree_skb(skb);
-			skb = NULL;
-		}
 		if (!skb) {
-			IP6_INC_STATS(net, ip6_dst_idev(dst), IPSTATS_MIB_OUTDISCARDS);
+			IP6_INC_STATS(net, idev, IPSTATS_MIB_OUTDISCARDS);
 			return -ENOMEM;
 		}
 	}
 
-	if (ipv6_addr_is_multicast(&ipv6_hdr(skb)->daddr)) {
-		struct inet6_dev *idev = ip6_dst_idev(skb_dst(skb));
-
+	hdr = ipv6_hdr(skb);
+	daddr = &hdr->daddr;
+	if (ipv6_addr_is_multicast(daddr)) {
 		if (!(dev->flags & IFF_LOOPBACK) && sk_mc_loop(sk) &&
 		    ((mroute6_is_socket(net, skb) &&
 		     !(IP6CB(skb)->flags & IP6SKB_FORWARDED)) ||
-		     ipv6_chk_mcast_addr(dev, &ipv6_hdr(skb)->daddr,
-					 &ipv6_hdr(skb)->saddr))) {
+		     ipv6_chk_mcast_addr(dev, daddr, &hdr->saddr))) {
 			struct sk_buff *newskb = skb_clone(skb, GFP_ATOMIC);
 
 			/* Do not check for IFF_ALLMULTI; multicast routing
@@ -110,7 +94,7 @@ static int ip6_finish_output2(struct net *net, struct sock *sk, struct sk_buff *
 					net, sk, newskb, NULL, newskb->dev,
 					dev_loopback_xmit);
 
-			if (ipv6_hdr(skb)->hop_limit == 0) {
+			if (hdr->hop_limit == 0) {
 				IP6_INC_STATS(net, idev,
 					      IPSTATS_MIB_OUTDISCARDS);
 				kfree_skb(skb);
@@ -119,9 +103,7 @@ static int ip6_finish_output2(struct net *net, struct sock *sk, struct sk_buff *
 		}
 
 		IP6_UPD_PO_STATS(net, idev, IPSTATS_MIB_OUTMCAST, skb->len);
-
-		if (IPV6_ADDR_MC_SCOPE(&ipv6_hdr(skb)->daddr) <=
-		    IPV6_ADDR_SCOPE_NODELOCAL &&
+		if (IPV6_ADDR_MC_SCOPE(daddr) <= IPV6_ADDR_SCOPE_NODELOCAL &&
 		    !(dev->flags & IFF_LOOPBACK)) {
 			kfree_skb(skb);
 			return 0;
@@ -136,10 +118,10 @@ static int ip6_finish_output2(struct net *net, struct sock *sk, struct sk_buff *
 	}
 
 	rcu_read_lock_bh();
-	nexthop = rt6_nexthop((struct rt6_info *)dst, &ipv6_hdr(skb)->daddr);
-	neigh = __ipv6_neigh_lookup_noref(dst->dev, nexthop);
+	nexthop = rt6_nexthop((struct rt6_info *)dst, daddr);
+	neigh = __ipv6_neigh_lookup_noref(dev, nexthop);
 	if (unlikely(!neigh))
-		neigh = __neigh_create(&nd_tbl, nexthop, dst->dev, false);
+		neigh = __neigh_create(&nd_tbl, nexthop, dev, false);
 	if (!IS_ERR(neigh)) {
 		sock_confirm_neigh(skb, neigh);
 		ret = neigh_output(neigh, skb, false);
@@ -148,7 +130,7 @@ static int ip6_finish_output2(struct net *net, struct sock *sk, struct sk_buff *
 	}
 	rcu_read_unlock_bh();
 
-	IP6_INC_STATS(net, ip6_dst_idev(dst), IPSTATS_MIB_OUTNOROUTES);
+	IP6_INC_STATS(net, idev, IPSTATS_MIB_OUTNOROUTES);
 	kfree_skb(skb);
 	return -EINVAL;
 }
-- 
1.8.3.1