[SLOF] [PATCH] libnet: Fix compiler warnings with GCC 9

[Thomas Huth]

When compiling the libnet code with GCC 9, there are some new
compiler warnings popping up now:

ipv6.c: In function ‘handle_ipv6’:
ipv6.c:152:21: warning: taking address of packed member of ‘struct ip6hdr’
 may result in an unaligned pointer value [-Waddress-of-packed-member]
  152 |  if (! find_ip6addr(&(ip6->dst)))
      |                     ^~~~~~~~~~~
ipv6.c: In function ‘ip6_checksum’:
ipv6.c:455:2: warning: converting a packed ‘struct ip6hdr’ pointer
 (alignment 1) to a ‘short unsigned int’ pointer (alignment 2) may result
 in an unaligned pointer value [-Waddress-of-packed-member]
  455 |  pip6h = (unsigned short *) &pseudo_ip6h;
      |  ^~~~~
In file included from ipv6.c:21:
ipv6.h:86:8: note: defined here
   86 | struct ip6hdr {
      |        ^~~~~~
ipv6.c:522:8: warning: converting a packed ‘struct icmp6hdr’ pointer
 (alignment 1) to a ‘short unsigned int’ pointer (alignment 2) may result
 in an unaligned pointer value [-Waddress-of-packed-member]
  522 |        ip6h->pl >> 1);
      |        ^~~~
In file included from ipv6.c:22:
icmpv6.h:123:8: note: defined here
  123 | struct icmp6hdr {
      |        ^~~~~~~~
etc.

The entries in struct ip6hdr are naturally aligned, so we can simply
drop the __attribute__ ((packed)) here and use a _Static_assert() for
the correct size instead.
icmp6hdr is a more complex struct since it contains a union of
packed structs, but the entries before the union are naturally aligned,
too, so we can silence the compiler warning by dropping the "packed"
attribute from the struct icmp6hdr and just asserting that the union
is at the correct offset.

Signed-off-by: Thomas Huth <thuth at redhat.com>
---
 lib/libnet/icmpv6.h | 4 +++-
 lib/libnet/ipv6.h   | 3 ++-
 2 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/lib/libnet/icmpv6.h b/lib/libnet/icmpv6.h
index 41b0c70..ba51524 100644
--- a/lib/libnet/icmpv6.h
+++ b/lib/libnet/icmpv6.h
@@ -130,6 +130,8 @@ struct icmp6hdr {
 		struct router_solicitation router_solicit;
 		struct router_advertisement ra;
 	} icmp6body;
-} __attribute((packed));
+};
+_Static_assert((long)&(((struct icmp6hdr *)NULL)->icmp6body) == 4,
+               "unexpected padding in icmp6hdr");
 
 #endif
diff --git a/lib/libnet/ipv6.h b/lib/libnet/ipv6.h
index 7b71b50..5fb718e 100644
--- a/lib/libnet/ipv6.h
+++ b/lib/libnet/ipv6.h
@@ -90,7 +90,8 @@ struct ip6hdr {
 	uint8_t  hl;		/**< Hop limit				*/
 	ip6_addr_t src;		/**< IPv6 source address		*/
 	ip6_addr_t dst;		/**< IPv6 destination address		*/
-} __attribute((packed));
+};
+_Static_assert(sizeof(struct ip6hdr) == 40, "unexpected padding in ip6hdr");
 
 /** \struct packeth
  * Struct with pointers to headers within a packet
-- 
2.18.1