netlink: prepare for removal of genl_family_attrbuf in 5.5

src/compat/compat.h

@@ -843,6 +843,24 @@ static inline void skb_mark_not_on_list(struct sk_buff *skb)
 #define cpu_have_named_feature(name) (elf_hwcap & (HWCAP_ ## name))
 #endif
 
+#if LINUX_VERSION_CODE < KERNEL_VERSION(4, 1, 0)
+#include <linux/stddef.h>
+#ifndef offsetofend
+#define offsetofend(TYPE, MEMBER) (offsetof(TYPE, MEMBER) + sizeof(((TYPE *)0)->MEMBER))
+#endif
+#endif
+
+#if LINUX_VERSION_CODE < KERNEL_VERSION(5, 5, 0)
+#define genl_dumpit_info(cb) ({ \
+	struct { struct nlattr **attrs; } *a = (void *)((u8 *)cb->args + offsetofend(struct dump_ctx, next_allowedip)); \
+	BUILD_BUG_ON(sizeof(cb->args) < offsetofend(struct dump_ctx, next_allowedip) + sizeof(*a)); \
+	a->attrs = genl_family_attrbuf(&genl_family); \
+	if (nlmsg_parse(cb->nlh, GENL_HDRLEN + genl_family.hdrsize, a->attrs, genl_family.maxattr, device_policy, NULL) < 0) \
+		memset(a->attrs, 0, (genl_family.maxattr + 1) * sizeof(struct nlattr *)); \
+	a; \
+})
+#endif
+
 #if defined(ISUBUNTU1604)
 #include <linux/siphash.h>
 #ifndef _WG_LINUX_SIPHASH_H

src/netlink.c

@@ -94,8 +94,8 @@ static int get_allowedips(struct sk_buff *skb, const u8 *ip, u8 cidr,
 struct dump_ctx {
 	struct wg_device *wg;
 	struct wg_peer *next_peer;
-	struct allowedips_node *next_allowedip;
 	u64 allowedips_seq;
+	struct allowedips_node *next_allowedip;
 };
 
 #define DUMP_CTX(cb) ((struct dump_ctx *)(cb)->args)
@@ -196,15 +196,9 @@ get_peer(struct wg_peer *peer, struct sk_buff *skb, struct dump_ctx *ctx)
 
 static int wg_get_device_start(struct netlink_callback *cb)
 {
-	struct nlattr **attrs = genl_family_attrbuf(&genl_family);
 	struct wg_device *wg;
-	int ret;
 
-	ret = nlmsg_parse(cb->nlh, GENL_HDRLEN + genl_family.hdrsize, attrs,
-			  genl_family.maxattr, device_policy, NULL);
-	if (ret < 0)
-		return ret;
-	wg = lookup_interface(attrs, cb->skb);
+	wg = lookup_interface(genl_dumpit_info(cb)->attrs, cb->skb);
 	if (IS_ERR(wg))
 		return PTR_ERR(wg);
 	DUMP_CTX(cb)->wg = wg;