net: ip6_gre: Distribute switch variables for initialization

Variables declared in a switch statement before any case statements
cannot be automatically initialized with compiler instrumentation (as
they are not part of any execution flow). With GCC's proposed automatic
stack variable initialization feature, this triggers a warning (and they
don't get initialized). Clang's automatic stack variable initialization
(via CONFIG_INIT_STACK_ALL=y) doesn't throw a warning, but it also
doesn't initialize such variables[1]. Note that these warnings (or silent
skipping) happen before the dead-store elimination optimization phase,
so even when the automatic initializations are later elided in favor of
direct initializations, the warnings remain.

To avoid these problems, move such variables into the "case" where
they're used or lift them up into the main function body.

net/ipv6/ip6_gre.c: In function ‘ip6gre_err’:
net/ipv6/ip6_gre.c:440:32: warning: statement will never be executed [-Wswitch-unreachable]
  440 |   struct ipv6_tlv_tnl_enc_lim *tel;
      |                                ^~~

net/ipv6/ip6_tunnel.c: In function ‘ip6_tnl_err’:
net/ipv6/ip6_tunnel.c:520:32: warning: statement will never be executed [-Wswitch-unreachable]
  520 |   struct ipv6_tlv_tnl_enc_lim *tel;
      |                                ^~~

[1] https://bugs.llvm.org/show_bug.cgi?id=44916

Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/ip6_gre.c

@@ -437,8 +437,6 @@ static int ip6gre_err(struct sk_buff *skb, struct inet6_skb_parm *opt,
 		return -ENOENT;
 
 	switch (type) {
-		struct ipv6_tlv_tnl_enc_lim *tel;
-		__u32 teli;
 	case ICMPV6_DEST_UNREACH:
 		net_dbg_ratelimited("%s: Path to destination invalid or inactive!\n",
 				    t->parms.name);
@@ -452,7 +450,10 @@ static int ip6gre_err(struct sk_buff *skb, struct inet6_skb_parm *opt,
 			break;
 		}
 		return 0;
-	case ICMPV6_PARAMPROB:
+	case ICMPV6_PARAMPROB: {
+		struct ipv6_tlv_tnl_enc_lim *tel;
+		__u32 teli;
+
 		teli = 0;
 		if (code == ICMPV6_HDR_FIELD)
 			teli = ip6_tnl_parse_tlv_enc_lim(skb, skb->data);
@@ -468,6 +469,7 @@ static int ip6gre_err(struct sk_buff *skb, struct inet6_skb_parm *opt,
 					    t->parms.name);
 		}
 		return 0;
+	}
 	case ICMPV6_PKT_TOOBIG:
 		ip6_update_pmtu(skb, net, info, 0, 0, sock_net_uid(net, NULL));
 		return 0;

net/ipv6/ip6_tunnel.c

@@ -517,8 +517,6 @@ ip6_tnl_err(struct sk_buff *skb, __u8 ipproto, struct inet6_skb_parm *opt,
 	err = 0;
 
 	switch (*type) {
-		struct ipv6_tlv_tnl_enc_lim *tel;
-		__u32 mtu, teli;
 	case ICMPV6_DEST_UNREACH:
 		net_dbg_ratelimited("%s: Path to destination invalid or inactive!\n",
 				    t->parms.name);
@@ -531,7 +529,10 @@ ip6_tnl_err(struct sk_buff *skb, __u8 ipproto, struct inet6_skb_parm *opt,
 			rel_msg = 1;
 		}
 		break;
-	case ICMPV6_PARAMPROB:
+	case ICMPV6_PARAMPROB: {
+		struct ipv6_tlv_tnl_enc_lim *tel;
+		__u32 teli;
+
 		teli = 0;
 		if ((*code) == ICMPV6_HDR_FIELD)
 			teli = ip6_tnl_parse_tlv_enc_lim(skb, skb->data);
@@ -548,7 +549,10 @@ ip6_tnl_err(struct sk_buff *skb, __u8 ipproto, struct inet6_skb_parm *opt,
 					    t->parms.name);
 		}
 		break;
-	case ICMPV6_PKT_TOOBIG:
+	}
+	case ICMPV6_PKT_TOOBIG: {
+		__u32 mtu;
+
 		ip6_update_pmtu(skb, net, htonl(*info), 0, 0,
 				sock_net_uid(net, NULL));
 		mtu = *info - offset;
@@ -562,6 +566,7 @@ ip6_tnl_err(struct sk_buff *skb, __u8 ipproto, struct inet6_skb_parm *opt,
 			rel_msg = 1;
 		}
 		break;
+	}
 	case NDISC_REDIRECT:
 		ip6_redirect(skb, net, skb->dev->ifindex, 0,
 			     sock_net_uid(net, NULL));