Document and enforce assumptions about struct (in6_)ifreq.

- The two types must be type-punnable for shared members of ifr_ifru.
  This allows compatibility accessors to be shared.

- There must be no padding gap between ifr_name and ifr_ifru.  This is
  assumed in tcpdump's use of SIOCGIFFLAGS output which attempts to be
  broadly portable.  This is true for all current architectures, but very
  large (256-bit) fat-pointers could violate this invariant.

Reviewed by:	kib
Obtained from:	CheriBSD
Sponsored by:	DARPA, AFRL
Differential Revision:	https://reviews.freebsd.org/D14910

sys/net/if.c

@@ -97,6 +97,13 @@
 
 #include <security/mac/mac_framework.h>
 
+/*
+ * Consumers of struct ifreq such as tcpdump assume no pad between ifr_name
+ * and ifr_ifru when it is used in SIOCGIFCONF.
+ */
+_Static_assert(sizeof(((struct ifreq *)0)->ifr_name) ==
+    offsetof(struct ifreq, ifr_ifru), "gap between ifr_name and ifr_ifru");
+
 #ifdef COMPAT_FREEBSD32
 #include <sys/mount.h>
 #include <compat/freebsd32/freebsd32.h>

sys/netinet6/in6.c

@@ -112,6 +112,14 @@ __FBSDID("$FreeBSD$");
 #include <netinet6/in6_fib.h>
 #include <netinet6/in6_pcb.h>
 
+/*
+ * struct in6_ifreq and struct ifreq must be type punnable for common members
+ * of ifr_ifru to allow accessors to be shared.
+ */
+_Static_assert(offsetof(struct in6_ifreq, ifr_ifru) ==
+    offsetof(struct ifreq, ifr_ifru),
+    "struct in6_ifreq and struct ifreq are not type punnable");
+
 VNET_DECLARE(int, icmp6_nodeinfo_oldmcprefix);
 #define V_icmp6_nodeinfo_oldmcprefix	VNET(icmp6_nodeinfo_oldmcprefix)