=========

- Synchronize linux's `include/uapi/linux/in6.h' 
  with glibc's `inet/netinet/in.h'.
- Synchronize glibc's `inet/netinet/in.h with linux's
  `include/uapi/linux/in6.h'.
- Allow including the headers in either other.
- First header included defines the structures and macros.

Details:
========

The kernel promises not to break the UAPI ABI so I don't 
see why we can't just have the two userspace headers
coordinate?

If you include the kernel headers first you get those,
and if you include the glibc headers first you get those,
and the following patch arranges a coordination and
synchronization between the two.

Let's handle `include/uapi/linux/in6.h' from linux, 
and `inet/netinet/in.h' from glibc and ensure they compile
in any order and preserve the required ABI.

These two patches pass the following compile tests:

cat >> test1.c <<EOF
#include <netinet/in.h>
#include <linux/in6.h>
int main (void) {
  return 0;
}
EOF
gcc -c test1.c

cat >> test2.c <<EOF
#include <linux/in6.h>
#include <netinet/in.h>
int main (void) {
  return 0;
}
EOF
gcc -c test2.c

One wrinkle is that the kernel has a different name for one of
the members in ipv6_mreq. In the kernel patch we create a macro
to cover the uses of the old name, and while that's not entirely
clean it's one of the best solutions (aside from an anonymous
union which has other issues).

I've reviewed the code and it looks to me like the ABI is
assured and everything matches on both sides.

Notes:
- You want netinet/in.h to include bits/in.h as early as possible,
  but it needs in_addr so define in_addr early.
- You want bits/in.h included as early as possible so you can use
  the linux specific code to define __USE_KERNEL_DEFS based on
  the _UAPI_* macro definition and use those to cull in.h.
- glibc was missing IPPROTO_MH, added here.

Compile tested and inspected.

Reported-by: Thomas Backlund <tmb@mageia.org>
Cc: Thomas Backlund <tmb@mageia.org>
Cc: libc-alpha@sourceware.org
Cc: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Cc: David S. Miller <davem@davemloft.net>
Tested-by: Cong Wang <amwang@redhat.com>
Signed-off-by: Carlos O'Donell <carlos@redhat.com>
Signed-off-by: Cong Wang <amwang@redhat.com>

---
diff --git a/include/uapi/linux/Kbuild b/include/uapi/linux/Kbuild
index 997f9f2..e7c94ee 100644
--- a/include/uapi/linux/Kbuild
+++ b/include/uapi/linux/Kbuild
@@ -227,6 +227,7 @@ header-y += kvm_para.h
 endif
 
 header-y += l2tp.h
+header-y += libc-compat.h
 header-y += limits.h
 header-y += llc.h
 header-y += loop.h
diff --git a/include/uapi/linux/in.h b/include/uapi/linux/in.h
index 9edb441..16d95d6 100644
--- a/include/uapi/linux/in.h
+++ b/include/uapi/linux/in.h
@@ -23,31 +23,54 @@
 
 /* Standard well-defined IP protocols.  */
 enum {
-  IPPROTO_IP = 0,		/* Dummy protocol for TCP		*/
-  IPPROTO_ICMP = 1,		/* Internet Control Message Protocol	*/
-  IPPROTO_IGMP = 2,		/* Internet Group Management Protocol	*/
-  IPPROTO_IPIP = 4,		/* IPIP tunnels (older KA9Q tunnels use 94) */
-  IPPROTO_TCP = 6,		/* Transmission Control Protocol	*/
-  IPPROTO_EGP = 8,		/* Exterior Gateway Protocol		*/
-  IPPROTO_PUP = 12,		/* PUP protocol				*/
-  IPPROTO_UDP = 17,		/* User Datagram Protocol		*/
-  IPPROTO_IDP = 22,		/* XNS IDP protocol			*/
-  IPPROTO_DCCP = 33,		/* Datagram Congestion Control Protocol */
-  IPPROTO_RSVP = 46,		/* RSVP protocol			*/
-  IPPROTO_GRE = 47,		/* Cisco GRE tunnels (rfc 1701,1702)	*/
-
-  IPPROTO_IPV6	 = 41,		/* IPv6-in-IPv4 tunnelling		*/
-
-  IPPROTO_ESP = 50,            /* Encapsulation Security Payload protocol */
-  IPPROTO_AH = 51,             /* Authentication Header protocol       */
-  IPPROTO_BEETPH = 94,	       /* IP option pseudo header for BEET */
-  IPPROTO_PIM    = 103,		/* Protocol Independent Multicast	*/
-
-  IPPROTO_COMP   = 108,                /* Compression Header protocol */
-  IPPROTO_SCTP   = 132,		/* Stream Control Transport Protocol	*/
-  IPPROTO_UDPLITE = 136,	/* UDP-Lite (RFC 3828)			*/
-
-  IPPROTO_RAW	 = 255,		/* Raw IP packets			*/
+  IPPROTO_IP = 0,		/* Dummy protocol for TCP. */
+#define IPPROTO_IP		IPPROTO_IP
+  IPPROTO_ICMP = 1,		/* Internet Control Message Protocol. */
+#define IPPROTO_ICMP		IPPROTO_ICMP
+  IPPROTO_IGMP = 2,		/* Internet Group Management Protocol. */
+#define IPPROTO_IGMP		IPPROTO_IGMP
+  IPPROTO_IPIP = 4,		/* IPIP tunnels (older KA9Q tunnels use 94). */
+#define IPPROTO_IPIP		IPPROTO_IPIP
+  IPPROTO_TCP = 6,		/* Transmission Control Protocol. */
+#define IPPROTO_TCP		IPPROTO_TCP
+  IPPROTO_EGP = 8,		/* Exterior Gateway Protocol. */
+#define IPPROTO_EGP		IPPROTO_EGP
+  IPPROTO_PUP = 12,		/* PUP protocol. */
+#define IPPROTO_PUP		IPPROTO_PUP
+  IPPROTO_UDP = 17,		/* User Datagram Protocol. */
+#define IPPROTO_UDP		IPPROTO_UDP
+  IPPROTO_IDP = 22,		/* XNS IDP protocol. */
+#define IPPROTO_IDP		IPPROTO_IDP
+  IPPROTO_TP = 29,		/* SO Transport Protocol Class 4. */
+#define IPPROTO_TP		IPPROTO_TP
+  IPPROTO_DCCP = 33,		/* Datagram Congestion Control Protocol. */
+#define IPPROTO_DCCP		IPPROTO_DCCP
+  IPPROTO_IPV6 = 41,		/* IPv6 header. */
+#define IPPROTO_IPV6		IPPROTO_IPV6
+  IPPROTO_RSVP = 46,		/* Reservation Protocol. */
+#define IPPROTO_RSVP		IPPROTO_RSVP
+  IPPROTO_GRE = 47,		/* General Routing Encapsulation. */
+#define IPPROTO_GRE		IPPROTO_GRE
+  IPPROTO_ESP = 50,		/* encapsulating security payload. */
+#define IPPROTO_ESP		IPPROTO_ESP
+  IPPROTO_AH = 51,		/* authentication header. */
+#define IPPROTO_AH		IPPROTO_AH
+  IPPROTO_MTP = 92,		/* Multicast Transport Protocol. */
+#define IPPROTO_MTP		IPPROTO_MTP
+  IPPROTO_BEETPH = 94,		/* IP option pseudo header for BEET. */
+#define IPPROTO_BEETPH		IPPROTO_BEETPH
+  IPPROTO_ENCAP = 98,		/* Encapsulation Header. */
+#define IPPROTO_ENCAP		IPPROTO_ENCAP
+  IPPROTO_PIM = 103,		/* Protocol Independent Multicast. */
+#define IPPROTO_PIM		IPPROTO_PIM
+  IPPROTO_COMP = 108,		/* Compression Header Protocol. */
+#define IPPROTO_COMP		IPPROTO_COMP
+  IPPROTO_SCTP = 132,		/* Stream Control Transmission Protocol. */
+#define IPPROTO_SCTP		IPPROTO_SCTP
+  IPPROTO_UDPLITE = 136,	/* UDP-Lite protocol. */
+#define IPPROTO_UDPLITE		IPPROTO_UDPLITE
+  IPPROTO_RAW = 255,		/* Raw IP packets. */
+#define IPPROTO_RAW		IPPROTO_RAW
   IPPROTO_MAX
 };
 
diff --git a/include/uapi/linux/in6.h b/include/uapi/linux/in6.h
index 53b1d56..d7ab44c 100644
--- a/include/uapi/linux/in6.h
+++ b/include/uapi/linux/in6.h
@@ -22,22 +22,29 @@
 #define _UAPI_LINUX_IN6_H
 
 #include <linux/types.h>
+#include <linux/libc-compat.h>
 
 /*
  *	IPv6 address structure
  */
-
+#if __UAPI_DEF_IN6_ADDR
 struct in6_addr {
 	union {
 		__u8		u6_addr8[16];
+#if __UAPI_DEF_IN6_ADDR
 		__be16		u6_addr16[8];
 		__be32		u6_addr32[4];
+#endif
 	} in6_u;
 #define s6_addr			in6_u.u6_addr8
+#if __UAPI_DEF_IN6_ADDR_ALT
 #define s6_addr16		in6_u.u6_addr16
 #define s6_addr32		in6_u.u6_addr32
+#endif
 };
+#endif /* __UAPI_DEF_IN6_ADDR */
 
+#if __UAPI_DEF_SOCKADDR_IN6
 struct sockaddr_in6 {
 	unsigned short int	sin6_family;    /* AF_INET6 */
 	__be16			sin6_port;      /* Transport layer port # */
@@ -45,7 +52,9 @@ struct sockaddr_in6 {
 	struct in6_addr		sin6_addr;      /* IPv6 address */
 	__u32			sin6_scope_id;  /* scope id (new in RFC2553) */
 };
+#endif /* __UAPI_DEF_SOCKADDR_IN6 */
 
+#if __UAPI_DEF_IPV6_MREQ
 struct ipv6_mreq {
 	/* IPv6 multicast address of group */
 	struct in6_addr ipv6mr_multiaddr;
@@ -53,6 +62,7 @@ struct ipv6_mreq {
 	/* local IPv6 address of interface */
 	int		ipv6mr_ifindex;
 };
+#endif /* __UAPI_DEF_IVP6_MREQ */
 
 #define ipv6mr_acaddr	ipv6mr_multiaddr
 
@@ -114,13 +124,24 @@ struct in6_flowlabel_req {
 /*
  *	IPV6 extension headers
  */
-#define IPPROTO_HOPOPTS		0	/* IPv6 hop-by-hop options	*/
-#define IPPROTO_ROUTING		43	/* IPv6 routing header		*/
-#define IPPROTO_FRAGMENT	44	/* IPv6 fragmentation header	*/
-#define IPPROTO_ICMPV6		58	/* ICMPv6			*/
-#define IPPROTO_NONE		59	/* IPv6 no next header		*/
-#define IPPROTO_DSTOPTS		60	/* IPv6 destination options	*/
-#define IPPROTO_MH		135	/* IPv6 mobility header		*/
+#if __UAPI_DEF_IPPROTO_V6
+enum {
+  IPPROTO_HOPOPTS = 0,		/* IPv6 hop-by-hop options      */
+#define IPPROTO_HOPOPTS		IPPROTO_HOPOPTS
+  IPPROTO_ROUTING = 43,		/* IPv6 routing header          */
+#define IPPROTO_ROUTING		IPPROTO_ROUTING
+  IPPROTO_FRAGMENT = 44,	/* IPv6 fragmentation header    */
+#define IPPROTO_FRAGMENT	IPPROTO_FRAGMENT
+  IPPROTO_ICMPV6 = 58,		/* ICMPv6                       */
+#define IPPROTO_ICMPV6		IPPROTO_ICMPV6
+  IPPROTO_NONE = 59,		/* IPv6 no next header          */
+#define IPPROTO_NONE		IPPROTO_NONE
+  IPPROTO_DSTOPTS = 60,		/* IPv6 destination options     */
+#define IPPROTO_DSTOPTS		IPPROTO_DSTOPTS
+  IPPROTO_MH = 135,		/* IPv6 mobility header         */
+#define IPPROTO_MH		IPPROTO_MH
+};
+#endif /* __UAPI_DEF_IPPROTO_V6 */
 
 /*
  *	IPv6 TLV options.
diff --git a/include/uapi/linux/libc-compat.h b/include/uapi/linux/libc-compat.h
new file mode 100644
index 0000000..335e8a7
--- /dev/null
+++ b/include/uapi/linux/libc-compat.h
@@ -0,0 +1,103 @@
+/*
+ * Compatibility interface for userspace libc header coordination:
+ *
+ * Define compatibility macros that are used to control the inclusion or
+ * exclusion of UAPI structures and definitions in coordination with another
+ * userspace C library.
+ *
+ * This header is intended to solve the problem of UAPI definitions that
+ * conflict with userspace definitions. If a UAPI header has such conflicting
+ * definitions then the solution is as follows:
+ *
+ * * Synchronize the UAPI header and the libc headers so either one can be
+ *   used and such that the ABI is preserved. If this is not possible then
+ *   no simple compatibility interface exists (you need to write translating
+ *   wrappers and rename things) and you can't use this interface.
+ *
+ * Then follow this process:
+ *
+ * (a) Include libc-compat.h in the UAPI header.
+ *      e.g. #include <linux/libc-compat.h>
+ *     This include must be as early as possible.
+ *
+ * (b) In libc-compat.h add enough code to detect that the comflicting
+ *     userspace libc header has been included first.
+ *
+ * (c) If the userspace libc header has been included first define a set of
+ *     guard macros of the form __UAPI_DEF_FOO and set their values to 1, else
+ *     set their values to 0.
+ *
+ * (d) Back in the UAPI header with the conflicting definitions, guard the
+ *     definitions with:
+ *     #if __UAPI_DEF_FOO
+ *       ...
+ *     #endif
+ *
+ * This fixes the situation where the linux headers are included *after* the
+ * libc headers. To fix the problem with the inclusion in the other order the
+ * userspace libc headers must be fixed like this:
+ *
+ * * For all definitions that conflict with kernel definitions wrap those
+ *   defines in the following:
+ *   #if !__UAPI_DEF_FOO
+ *     ...
+ *   #endif
+ *
+ * This prevents the redefinition of a construct already defined by the kernel.
+ */
+#ifndef _UAPI_LIBC_COMPAT_H
+#define _UAPI_LIBC_COMPAT_H
+
+/* We have included glibc headers... */
+#if defined(__GLIBC__)
+
+/* Coordinate with glibc netinet/in.h header. */
+#if defined(_NETINET_IN_H)
+
+/* GLIBC headers included first so don't define anything
+ * that would already be defined. */
+#define __UAPI_DEF_IN6_ADDR		0
+/* The exception is the in6_addr macros which must be defined
+ * if the glibc code didn't define them. This guard matches
+ * the guard in glibc/inet/netinet/in.h which defines the
+ * additional in6_addr macros e.g. s6_addr16, and s6_addr32. */
+#if defined(__USE_MISC) || defined (__USE_GNU)
+#define __UAPI_DEF_IN6_ADDR_ALT		0
+#else
+#define __UAPI_DEF_IN6_ADDR_ALT		1
+#endif
+#define __UAPI_DEF_SOCKADDR_IN6		0
+#define __UAPI_DEF_IPV6_MREQ		0
+#define __UAPI_DEF_IPPROTO_V6		0
+
+#else
+
+/* Linux headers included first, and we must define everything
+ * we need. The expectation is that glibc will check the
+ * __UAPI_DEF_* defines and adjust appropriately. */
+#define __UAPI_DEF_IN6_ADDR		1
+/* We unconditionally define the in6_addr macros and glibc must
+ * coordinate. */
+#define __UAPI_DEF_IN6_ADDR_ALT		1
+#define __UAPI_DEF_SOCKADDR_IN6		1
+#define __UAPI_DEF_IPV6_MREQ		1
+#define __UAPI_DEF_IPPROTO_V6		1
+
+#endif /* _NETINET_IN_H */
+
+
+/* If we did not see any headers from any supported C libraries,
+ * or we are being included in the kernel, then define everything
+ * that we need. */
+#else /* !defined(__GLIBC__) */
+
+/* Definitions for in6.h */
+#define __UAPI_DEF_IN6_ADDR		1
+#define __UAPI_DEF_IN6_ADDR_ALT		1
+#define __UAPI_DEF_SOCKADDR_IN6		1
+#define __UAPI_DEF_IPV6_MREQ		1
+#define __UAPI_DEF_IPPROTO_V6		1
+
+#endif /* __GLIBC__ */
+
+#endif /* _UAPI_LIBC_COMPAT_H */
