ipv6: pass proto by value to ipv6_push_nfrag_opts() and ipv6_push_frag_opts()

With CONFIG_STACKPROTECTOR_STRONG=y, it is better to avoid passing
a pointer to an automatic variable.

Change these exported functions to return 'u8 proto'
instead of void.

- ipv6_push_nfrag_opts()
- ipv6_push_frag_opts()

For instance, replace
	ipv6_push_frag_opts(skb, opt, &proto);
with:
	proto = ipv6_push_frag_opts(skb, opt, proto);

Note that even after this change, ip6_xmit() has to use a stack canary
because of @first_hop variable.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20260130210303.3888261-2-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

4 files changed, 46 insertions, 40 deletions

diff --git a/include/net/ipv6.h b/include/net/ipv6.h
index cf2203ff2bfd..a35f0a8114c0 100644
--- a/include/net/ipv6.h
+++ b/include/net/ipv6.h
@@ -1151,11 +1151,11 @@ int ip6_local_out(struct net *net, struct sock *sk, struct sk_buff *skb);
  *	Extension header (options) processing
  */
 
-void ipv6_push_nfrag_opts(struct sk_buff *skb, struct ipv6_txoptions *opt,
-			  u8 *proto, struct in6_addr **daddr_p,
-			  struct in6_addr *saddr);
-void ipv6_push_frag_opts(struct sk_buff *skb, struct ipv6_txoptions *opt,
-			 u8 *proto);
+u8 ipv6_push_nfrag_opts(struct sk_buff *skb, struct ipv6_txoptions *opt,
+			u8 proto, struct in6_addr **daddr_p,
+			struct in6_addr *saddr);
+u8 ipv6_push_frag_opts(struct sk_buff *skb, struct ipv6_txoptions *opt,
+		       u8 proto);
 
 int ipv6_skip_exthdr(const struct sk_buff *, int start, u8 *nexthdrp,
 		     __be16 *frag_offp);
diff --git a/net/ipv6/exthdrs.c b/net/ipv6/exthdrs.c
index cd318ff0dcd8..209fdf1b1aa9 100644
--- a/net/ipv6/exthdrs.c
+++ b/net/ipv6/exthdrs.c
@@ -1074,9 +1074,9 @@ fail_and_free:
  *	for headers.
  */
 
-static void ipv6_push_rthdr0(struct sk_buff *skb, u8 *proto,
-			     struct ipv6_rt_hdr *opt,
-			     struct in6_addr **addr_p, struct in6_addr *saddr)
+static u8 ipv6_push_rthdr0(struct sk_buff *skb, u8 proto,
+			   struct ipv6_rt_hdr *opt,
+			   struct in6_addr **addr_p, struct in6_addr *saddr)
 {
 	struct rt0_hdr *phdr, *ihdr;
 	int hops;
@@ -1095,13 +1095,13 @@ static void ipv6_push_rthdr0(struct sk_buff *skb, u8 *proto,
 	phdr->addr[hops - 1] = **addr_p;
 	*addr_p = ihdr->addr;
 
-	phdr->rt_hdr.nexthdr = *proto;
-	*proto = NEXTHDR_ROUTING;
+	phdr->rt_hdr.nexthdr = proto;
+	return NEXTHDR_ROUTING;
 }
 
-static void ipv6_push_rthdr4(struct sk_buff *skb, u8 *proto,
-			     struct ipv6_rt_hdr *opt,
-			     struct in6_addr **addr_p, struct in6_addr *saddr)
+static u8 ipv6_push_rthdr4(struct sk_buff *skb, u8 proto,
+			   struct ipv6_rt_hdr *opt,
+			   struct in6_addr **addr_p, struct in6_addr *saddr)
 {
 	struct ipv6_sr_hdr *sr_phdr, *sr_ihdr;
 	int plen, hops;
@@ -1144,58 +1144,61 @@ static void ipv6_push_rthdr4(struct sk_buff *skb, u8 *proto,
 	}
 #endif
 
-	sr_phdr->nexthdr = *proto;
-	*proto = NEXTHDR_ROUTING;
+	sr_phdr->nexthdr = proto;
+	return NEXTHDR_ROUTING;
 }
 
-static void ipv6_push_rthdr(struct sk_buff *skb, u8 *proto,
-			    struct ipv6_rt_hdr *opt,
-			    struct in6_addr **addr_p, struct in6_addr *saddr)
+static u8 ipv6_push_rthdr(struct sk_buff *skb, u8 proto,
+			  struct ipv6_rt_hdr *opt,
+			  struct in6_addr **addr_p, struct in6_addr *saddr)
 {
 	switch (opt->type) {
 	case IPV6_SRCRT_TYPE_0:
 	case IPV6_SRCRT_STRICT:
 	case IPV6_SRCRT_TYPE_2:
-		ipv6_push_rthdr0(skb, proto, opt, addr_p, saddr);
+		proto = ipv6_push_rthdr0(skb, proto, opt, addr_p, saddr);
 		break;
 	case IPV6_SRCRT_TYPE_4:
-		ipv6_push_rthdr4(skb, proto, opt, addr_p, saddr);
+		proto = ipv6_push_rthdr4(skb, proto, opt, addr_p, saddr);
 		break;
 	default:
 		break;
 	}
+	return proto;
 }
 
-static void ipv6_push_exthdr(struct sk_buff *skb, u8 *proto, u8 type, struct ipv6_opt_hdr *opt)
+static u8 ipv6_push_exthdr(struct sk_buff *skb, u8 proto, u8 type, struct ipv6_opt_hdr *opt)
 {
 	struct ipv6_opt_hdr *h = skb_push(skb, ipv6_optlen(opt));
 
 	memcpy(h, opt, ipv6_optlen(opt));
-	h->nexthdr = *proto;
-	*proto = type;
+	h->nexthdr = proto;
+	return type;
 }
 
-void ipv6_push_nfrag_opts(struct sk_buff *skb, struct ipv6_txoptions *opt,
-			  u8 *proto,
-			  struct in6_addr **daddr, struct in6_addr *saddr)
+u8 ipv6_push_nfrag_opts(struct sk_buff *skb, struct ipv6_txoptions *opt,
+			u8 proto,
+			struct in6_addr **daddr, struct in6_addr *saddr)
 {
 	if (opt->srcrt) {
-		ipv6_push_rthdr(skb, proto, opt->srcrt, daddr, saddr);
+		proto = ipv6_push_rthdr(skb, proto, opt->srcrt, daddr, saddr);
 		/*
 		 * IPV6_RTHDRDSTOPTS is ignored
 		 * unless IPV6_RTHDR is set (RFC3542).
 		 */
 		if (opt->dst0opt)
-			ipv6_push_exthdr(skb, proto, NEXTHDR_DEST, opt->dst0opt);
+			proto = ipv6_push_exthdr(skb, proto, NEXTHDR_DEST, opt->dst0opt);
 	}
 	if (opt->hopopt)
-		ipv6_push_exthdr(skb, proto, NEXTHDR_HOP, opt->hopopt);
+		proto = ipv6_push_exthdr(skb, proto, NEXTHDR_HOP, opt->hopopt);
+	return proto;
 }
 
-void ipv6_push_frag_opts(struct sk_buff *skb, struct ipv6_txoptions *opt, u8 *proto)
+u8 ipv6_push_frag_opts(struct sk_buff *skb, struct ipv6_txoptions *opt, u8 proto)
 {
 	if (opt->dst1opt)
-		ipv6_push_exthdr(skb, proto, NEXTHDR_DEST, opt->dst1opt);
+		proto = ipv6_push_exthdr(skb, proto, NEXTHDR_DEST, opt->dst1opt);
+	return proto;
 }
 EXPORT_SYMBOL(ipv6_push_frag_opts);
 
diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
index f904739e99b9..e4bed95ae5bb 100644
--- a/net/ipv6/ip6_output.c
+++ b/net/ipv6/ip6_output.c
@@ -305,11 +305,12 @@ int ip6_xmit(const struct sock *sk, struct sk_buff *skb, struct flowi6 *fl6,
 		seg_len += opt->opt_nflen + opt->opt_flen;
 
 		if (opt->opt_flen)
-			ipv6_push_frag_opts(skb, opt, &proto);
+			proto = ipv6_push_frag_opts(skb, opt, proto);
 
 		if (opt->opt_nflen)
-			ipv6_push_nfrag_opts(skb, opt, &proto, &first_hop,
-					     &fl6->saddr);
+			proto = ipv6_push_nfrag_opts(skb, opt, proto,
+						     &first_hop,
+						     &fl6->saddr);
 	}
 
 	if (unlikely(seg_len > IPV6_MAXPLEN)) {
@@ -1940,11 +1941,13 @@ struct sk_buff *__ip6_make_skb(struct sock *sk,
 	__skb_pull(skb, skb_network_header_len(skb));
 
 	final_dst = &fl6->daddr;
-	if (opt && opt->opt_flen)
-		ipv6_push_frag_opts(skb, opt, &proto);
-	if (opt && opt->opt_nflen)
-		ipv6_push_nfrag_opts(skb, opt, &proto, &final_dst, &fl6->saddr);
-
+	if (opt) {
+		if (opt->opt_flen)
+			proto = ipv6_push_frag_opts(skb, opt, proto);
+		if (opt->opt_nflen)
+			proto = ipv6_push_nfrag_opts(skb, opt, proto,
+						     &final_dst, &fl6->saddr);
+	}
 	skb_push(skb, sizeof(struct ipv6hdr));
 	skb_reset_network_header(skb);
 	hdr = ipv6_hdr(skb);
diff --git a/net/ipv6/ip6_tunnel.c b/net/ipv6/ip6_tunnel.c
index f68f6f110a3e..6fca0868b5a2 100644
--- a/net/ipv6/ip6_tunnel.c
+++ b/net/ipv6/ip6_tunnel.c
@@ -1265,7 +1265,7 @@ route_lookup:
 
 	if (encap_limit >= 0) {
 		init_tel_txopt(&opt, encap_limit);
-		ipv6_push_frag_opts(skb, &opt.ops, &proto);
+		proto = ipv6_push_frag_opts(skb, &opt.ops, proto);
 	}
 
 	skb_push(skb, sizeof(struct ipv6hdr));