if CONFIG_NET_IPGRE is enabled, but CONFIG_IPV6 is disabled:

    net/ipv4/ip_gre.c: In function ‘ipgre_err’:
    net/ipv4/ip_gre.c:144:22: error: variable ‘data_len’ set but not used [-Werror=unused-but-set-variable]
      144 |         unsigned int data_len = 0;
	  |                      ^~~~~~~~

Fix this by moving all data_len processing inside the IPV6-only section
that uses its result.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202501121007.2GofXmh5-lkp@intel.com/
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
---
 net/ipv4/ip_gre.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/net/ipv4/ip_gre.c b/net/ipv4/ip_gre.c
index f1f31ebfc7934467..6f871af7bb5772b7 100644
--- a/net/ipv4/ip_gre.c
+++ b/net/ipv4/ip_gre.c
@@ -141,7 +141,6 @@ static int ipgre_err(struct sk_buff *skb, u32 info,
 	const struct iphdr *iph;
 	const int type = icmp_hdr(skb)->type;
 	const int code = icmp_hdr(skb)->code;
-	unsigned int data_len = 0;
 	struct ip_tunnel *t;
 
 	if (tpi->proto == htons(ETH_P_TEB))
@@ -182,7 +181,6 @@ static int ipgre_err(struct sk_buff *skb, u32 info,
 	case ICMP_TIME_EXCEEDED:
 		if (code != ICMP_EXC_TTL)
 			return 0;
-		data_len = icmp_hdr(skb)->un.reserved[1] * 4; /* RFC 4884 4.1 */
 		break;
 
 	case ICMP_REDIRECT:
@@ -191,8 +189,9 @@ static int ipgre_err(struct sk_buff *skb, u32 info,
 
 #if IS_ENABLED(CONFIG_IPV6)
 	if (tpi->proto == htons(ETH_P_IPV6) &&
-	    !ip6_err_gen_icmpv6_unreach(skb, iph->ihl * 4 + tpi->hdr_len,
-					type, data_len))
+	    !ip6_err_gen_icmpv6_unreach(skb, iph->ihl * 4 + tpi->hdr_len, type,
+		type == ICMP_TIME_EXCEEDED ?
+		icmp_hdr(skb)->un.reserved[1] * 4 /* RFC 4884 4.1 */ : 0))
 		return 0;
 #endif
 
-- 
2.43.0

From: Geert Uytterhoeven <geert@linux-m68k.org>
Date: Sun, 12 Jan 2025 10:05:10 +0100
> @@ -191,8 +189,9 @@ static int ipgre_err(struct sk_buff *skb, u32 info,
>  
>  #if IS_ENABLED(CONFIG_IPV6)
>  	if (tpi->proto == htons(ETH_P_IPV6) &&
> -	    !ip6_err_gen_icmpv6_unreach(skb, iph->ihl * 4 + tpi->hdr_len,
> -					type, data_len))
> +	    !ip6_err_gen_icmpv6_unreach(skb, iph->ihl * 4 + tpi->hdr_len, type,
> +		type == ICMP_TIME_EXCEEDED ?
> +		icmp_hdr(skb)->un.reserved[1] * 4 /* RFC 4884 4.1 */ : 0))
>  		return 0;
>  #endif

No need to pack everything in a single line.

---8<---
#if IS_ENABLED(CONFIG_IPV6)
	if (tpi->proto == htons(ETH_P_IPV6)) {
		unsigned int data_len = 0;

		if (type == ICMP_TIME_EXCEEDED)
			data_len = icmp_hdr(skb)->un.reserved[1] * 4; /* RFC 4884 4.1 */

		if (!ip6_err_gen_icmpv6_unreach(skb, iph->ihl * 4 + tpi->hdr_len,
						type, data_len)
			return 0;
	}
#endif
---8<---


For the future submission, please specify the target tree in Subject.

  [PATCH net-next v2] ...

See: Documentation/process/maintainer-netdev.rst

Thanks