socket: Use may_alias on sockaddr structs (bug 19622)

This supports common coding patterns.  The GCC C front end before
version 7 rejects the may_alias attribute on a struct definition
if it was not present in a previous forward declaration, so this
attribute can only be conditionally applied.

This implements the spirit of the change in Austin Group issue 1641.

Suggested-by: Marek Polacek <polacek@redhat.com>
Suggested-by: Jakub Jelinek <jakub@redhat.com>
Reviewed-by: Sam James <sam@gentoo.org>
Reviewed-by: Carlos O'Donell <carlos@redhat.com>
(cherry picked from commit 8d7b6b4cb27d4dec1dd5f7960298c1699275f962)
(cherry picked from commit 26e7005728f0eea2972474e6be2905c467661237)

10 files changed, 25 insertions, 14 deletions

diff --git a/bits/socket.h b/bits/socket.h
index 13de124bac..772074d52a 100644
--- a/bits/socket.h
+++ b/bits/socket.h
@@ -149,7 +149,7 @@ enum __socket_type
 #include <bits/sockaddr.h>
 
 /* Structure describing a generic socket address.  */
-struct sockaddr
+struct __attribute_struct_may_alias__ sockaddr
   {
     __SOCKADDR_COMMON (sa_);	/* Common data: address family and length.  */
     char sa_data[14];		/* Address data.  */
@@ -166,7 +166,7 @@ struct sockaddr
 #define _SS_PADSIZE \
   (_SS_SIZE - __SOCKADDR_COMMON_SIZE - sizeof (__ss_aligntype))
 
-struct sockaddr_storage
+struct __attribute_struct_may_alias__ sockaddr_storage
   {
     __SOCKADDR_COMMON (ss_);	/* Address family, etc.  */
     char __ss_padding[_SS_PADSIZE];
diff --git a/inet/netinet/in.h b/inet/netinet/in.h
index fa57b61079..f684be5beb 100644
--- a/inet/netinet/in.h
+++ b/inet/netinet/in.h
@@ -244,7 +244,7 @@ extern const struct in6_addr in6addr_loopback;   /* ::1 */
 
 
 /* Structure describing an Internet socket address.  */
-struct sockaddr_in
+struct __attribute_struct_may_alias__ sockaddr_in
   {
     __SOCKADDR_COMMON (sin_);
     in_port_t sin_port;			/* Port number.  */
@@ -257,9 +257,11 @@ struct sockaddr_in
 			   - sizeof (struct in_addr)];
   };
 
-#if !__USE_KERNEL_IPV6_DEFS
+#if __USE_KERNEL_IPV6_DEFS
+struct __attribute_struct_may_alias__ sockaddr_in6;
+#else
 /* Ditto, for IPv6.  */
-struct sockaddr_in6
+struct __attribute_struct_may_alias__ sockaddr_in6
   {
     __SOCKADDR_COMMON (sin6_);
     in_port_t sin6_port;	/* Transport layer port # */
diff --git a/misc/sys/cdefs.h b/misc/sys/cdefs.h
index 520231dbea..399ee7d24c 100644
--- a/misc/sys/cdefs.h
+++ b/misc/sys/cdefs.h
@@ -720,4 +720,13 @@ _Static_assert (0, "IEEE 128-bits long double requires redirection on this platf
 # define __attribute_returns_twice__ /* Ignore.  */
 #endif
 
+/* Mark struct types as aliasable.  Restricted to compilers that
+   support forward declarations of structs in the presence of the
+   attribute.  */
+#if __GNUC_PREREQ (7, 1) || defined __clang__
+# define __attribute_struct_may_alias__ __attribute__ ((__may_alias__))
+#else
+# define __attribute_struct_may_alias__
+#endif
+
 #endif	 /* sys/cdefs.h */
diff --git a/socket/sys/un.h b/socket/sys/un.h
index bf03b7d6ce..ff9cbd6efa 100644
--- a/socket/sys/un.h
+++ b/socket/sys/un.h
@@ -26,7 +26,7 @@
 __BEGIN_DECLS
 
 /* Structure describing the address of an AF_LOCAL (aka AF_UNIX) socket.  */
-struct sockaddr_un
+struct __attribute_struct_may_alias__ sockaddr_un
   {
     __SOCKADDR_COMMON (sun_);
     char sun_path[108];		/* Path name.  */
diff --git a/sysdeps/mach/hurd/bits/socket.h b/sysdeps/mach/hurd/bits/socket.h
index 3e72f9fa93..b5eeac3731 100644
--- a/sysdeps/mach/hurd/bits/socket.h
+++ b/sysdeps/mach/hurd/bits/socket.h
@@ -153,7 +153,7 @@ enum __socket_type
 #include <bits/sockaddr.h>
 
 /* Structure describing a generic socket address.  */
-struct sockaddr
+struct __attribute_struct_may_alias__ sockaddr
   {
     __SOCKADDR_COMMON (sa_);	/* Common data: address family and length.  */
     char sa_data[14];		/* Address data.  */
@@ -170,7 +170,7 @@ struct sockaddr
 #define _SS_PADSIZE \
   (_SS_SIZE - __SOCKADDR_COMMON_SIZE - sizeof (__ss_aligntype))
 
-struct sockaddr_storage
+struct __attribute_struct_may_alias__ sockaddr_storage
   {
     __SOCKADDR_COMMON (ss_);	/* Address family, etc.  */
     char __ss_padding[_SS_PADSIZE];
diff --git a/sysdeps/unix/sysv/linux/bits/socket.h b/sysdeps/unix/sysv/linux/bits/socket.h
index 0d86feb4ca..6dab283a2e 100644
--- a/sysdeps/unix/sysv/linux/bits/socket.h
+++ b/sysdeps/unix/sysv/linux/bits/socket.h
@@ -180,7 +180,7 @@ typedef __socklen_t socklen_t;
 #include <bits/sockaddr.h>
 
 /* Structure describing a generic socket address.  */
-struct sockaddr
+struct __attribute_struct_may_alias__ sockaddr
   {
     __SOCKADDR_COMMON (sa_);	/* Common data: address family and length.  */
     char sa_data[14];		/* Address data.  */
@@ -193,7 +193,7 @@ struct sockaddr
 #define _SS_PADSIZE \
   (_SS_SIZE - __SOCKADDR_COMMON_SIZE - sizeof (__ss_aligntype))
 
-struct sockaddr_storage
+struct __attribute_struct_may_alias__ sockaddr_storage
   {
     __SOCKADDR_COMMON (ss_);	/* Address family, etc.  */
     char __ss_padding[_SS_PADSIZE];
diff --git a/sysdeps/unix/sysv/linux/net/if_packet.h b/sysdeps/unix/sysv/linux/net/if_packet.h
index 9ffb69b508..c17e1c23c5 100644
--- a/sysdeps/unix/sysv/linux/net/if_packet.h
+++ b/sysdeps/unix/sysv/linux/net/if_packet.h
@@ -26,7 +26,7 @@
    From Linux 2.1 the AF_PACKET interface is preferred and you should
    consider using it in place of this one.  */
 
-struct sockaddr_pkt
+struct __attribute_struct_may_alias__ sockaddr_pkt
   {
     __SOCKADDR_COMMON (spkt_);
     unsigned char spkt_device[14];
diff --git a/sysdeps/unix/sysv/linux/netash/ash.h b/sysdeps/unix/sysv/linux/netash/ash.h
index 7d885d17cc..7a6ff50b17 100644
--- a/sysdeps/unix/sysv/linux/netash/ash.h
+++ b/sysdeps/unix/sysv/linux/netash/ash.h
@@ -22,7 +22,7 @@
 #include <features.h>
 #include <bits/sockaddr.h>
 
-struct sockaddr_ash
+struct __attribute_struct_may_alias__ sockaddr_ash
   {
     __SOCKADDR_COMMON (sash_);		/* Common data: address family etc.  */
     int sash_ifindex;			/* Interface to use.  */
diff --git a/sysdeps/unix/sysv/linux/neteconet/ec.h b/sysdeps/unix/sysv/linux/neteconet/ec.h
index b07a107961..f3132f06ff 100644
--- a/sysdeps/unix/sysv/linux/neteconet/ec.h
+++ b/sysdeps/unix/sysv/linux/neteconet/ec.h
@@ -28,7 +28,7 @@ struct ec_addr
     unsigned char net;			/* Network number.  */
   };
 
-struct sockaddr_ec
+struct __attribute_struct_may_alias__ sockaddr_ec
   {
     __SOCKADDR_COMMON (sec_);
     unsigned char port;			/* Port number.  */
diff --git a/sysdeps/unix/sysv/linux/netiucv/iucv.h b/sysdeps/unix/sysv/linux/netiucv/iucv.h
index f5fad81751..27151e8bbe 100644
--- a/sysdeps/unix/sysv/linux/netiucv/iucv.h
+++ b/sysdeps/unix/sysv/linux/netiucv/iucv.h
@@ -23,7 +23,7 @@
 
 __BEGIN_DECLS
 
-struct sockaddr_iucv
+struct __attribute_struct_may_alias__ sockaddr_iucv
   {
     __SOCKADDR_COMMON (siucv_);
     unsigned short	siucv_port;		/* Reserved */