uapi glibc compat: fix compile errors when glibc net/if.h included before linux/if.h

glibc's net/if.h contains copies of definitions from linux/if.h and these
conflict and cause build failures if both files are included by application
source code. Changes in uapi headers, which fixed header file dependencies to
include linux/if.h when it was needed, e.g. commit 1ffad83d, made the
net/if.h and linux/if.h incompatibilities visible as build failures for
userspace applications like iproute2 and xtables-addons.

This patch fixes compile errors when glibc net/if.h is included before
linux/if.h:

./linux/if.h:99:21: error: redeclaration of enumerator ‘IFF_NOARP’
./linux/if.h:98:23: error: redeclaration of enumerator ‘IFF_RUNNING’
./linux/if.h:97:26: error: redeclaration of enumerator ‘IFF_NOTRAILERS’
./linux/if.h:96:27: error: redeclaration of enumerator ‘IFF_POINTOPOINT’
./linux/if.h:95:24: error: redeclaration of enumerator ‘IFF_LOOPBACK’
./linux/if.h:94:21: error: redeclaration of enumerator ‘IFF_DEBUG’
./linux/if.h:93:25: error: redeclaration of enumerator ‘IFF_BROADCAST’
./linux/if.h:92:19: error: redeclaration of enumerator ‘IFF_UP’
./linux/if.h:252:8: error: redefinition of ‘struct ifconf’
./linux/if.h:203:8: error: redefinition of ‘struct ifreq’
./linux/if.h:169:8: error: redefinition of ‘struct ifmap’
./linux/if.h:107:23: error: redeclaration of enumerator ‘IFF_DYNAMIC’
./linux/if.h:106:25: error: redeclaration of enumerator ‘IFF_AUTOMEDIA’
./linux/if.h:105:23: error: redeclaration of enumerator ‘IFF_PORTSEL’
./linux/if.h:104:25: error: redeclaration of enumerator ‘IFF_MULTICAST’
./linux/if.h:103:21: error: redeclaration of enumerator ‘IFF_SLAVE’
./linux/if.h:102:22: error: redeclaration of enumerator ‘IFF_MASTER’
./linux/if.h:101:24: error: redeclaration of enumerator ‘IFF_ALLMULTI’
./linux/if.h💯23: error: redeclaration of enumerator ‘IFF_PROMISC’

The cases where linux/if.h is included before net/if.h need a similar fix in
the glibc side, or the order of include files can be changed userspace
code as a workaround.

This change was tested in x86 userspace on Debian unstable with
scripts/headers_compile_test.sh:

$ make headers_install && \
  cd usr/include && ../../scripts/headers_compile_test.sh -l -k
...
cc -Wall -c -nostdinc -I /usr/lib/gcc/i586-linux-gnu/5/include -I /usr/lib/gcc/i586-linux-gnu/5/include-fixed -I . -I /home/mcfrisk/src/linux-2.6/usr/headers_compile_test_include.2uX2zH -I /home/mcfrisk/src/linux-2.6/usr/headers_compile_test_include.2uX2zH/i586-linux-gnu -o /dev/null ./linux/if.h_libc_before_kernel.h
PASSED libc before kernel test: ./linux/if.h

Reported-by: Jan Engelhardt <jengelh@inai.de>
Reported-by: Josh Boyer <jwboyer@fedoraproject.org>
Reported-by: Stephen Hemminger <shemming@brocade.com>
Reported-by: Waldemar Brodkorb <mail@waldemar-brodkorb.de>
Cc: Gabriel Laskar <gabriel@lse.epita.fr>
Signed-off-by: Mikko Rapeli <mikko.rapeli@iki.fi>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/uapi/linux/libc-compat.h

@@ -51,6 +51,40 @@
 /* We have included glibc headers... */
 #if defined(__GLIBC__)
 
+/* Coordinate with glibc net/if.h header. */
+#if defined(_NET_IF_H)
+
+/* GLIBC headers included first so don't define anything
+ * that would already be defined. */
+
+#define __UAPI_DEF_IF_IFCONF 0
+#define __UAPI_DEF_IF_IFMAP 0
+#define __UAPI_DEF_IF_IFNAMSIZ 0
+#define __UAPI_DEF_IF_IFREQ 0
+/* Everything up to IFF_DYNAMIC, matches net/if.h until glibc 2.23 */
+#define __UAPI_DEF_IF_NET_DEVICE_FLAGS 0
+/* For the future if glibc adds IFF_LOWER_UP, IFF_DORMANT and IFF_ECHO */
+#ifndef __UAPI_DEF_IF_NET_DEVICE_FLAGS_LOWER_UP_DORMANT_ECHO
+#define __UAPI_DEF_IF_NET_DEVICE_FLAGS_LOWER_UP_DORMANT_ECHO 1
+#endif /* __UAPI_DEF_IF_NET_DEVICE_FLAGS_LOWER_UP_DORMANT_ECHO */
+
+#else /* _NET_IF_H */
+
+/* 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_IF_IFCONF 1
+#define __UAPI_DEF_IF_IFMAP 1
+#define __UAPI_DEF_IF_IFNAMSIZ 1
+#define __UAPI_DEF_IF_IFREQ 1
+/* Everything up to IFF_DYNAMIC, matches net/if.h until glibc 2.23 */
+#define __UAPI_DEF_IF_NET_DEVICE_FLAGS 1
+/* For the future if glibc adds IFF_LOWER_UP, IFF_DORMANT and IFF_ECHO */
+#define __UAPI_DEF_IF_NET_DEVICE_FLAGS_LOWER_UP_DORMANT_ECHO 1
+
+#endif /* _NET_IF_H */
+
 /* Coordinate with glibc netinet/in.h header. */
 #if defined(_NETINET_IN_H)
 
@@ -117,6 +151,16 @@
  * that we need. */
 #else /* !defined(__GLIBC__) */
 
+/* Definitions for if.h */
+#define __UAPI_DEF_IF_IFCONF 1
+#define __UAPI_DEF_IF_IFMAP 1
+#define __UAPI_DEF_IF_IFNAMSIZ 1
+#define __UAPI_DEF_IF_IFREQ 1
+/* Everything up to IFF_DYNAMIC, matches net/if.h until glibc 2.23 */
+#define __UAPI_DEF_IF_NET_DEVICE_FLAGS 1
+/* For the future if glibc adds IFF_LOWER_UP, IFF_DORMANT and IFF_ECHO */
+#define __UAPI_DEF_IF_NET_DEVICE_FLAGS_LOWER_UP_DORMANT_ECHO 1
+
 /* Definitions for in.h */
 #define __UAPI_DEF_IN_ADDR		1
 #define __UAPI_DEF_IN_IPPROTO		1