all: add src/nm-compat-headers for patching included system headers

We already have src/linux-headers, where we have complete copies of linux
user space headers. Of course that exists, because we want to use certain
features and don't depend on the installed kernel headers. Which works
well, because kernel user space API is stable, and we anyway want to
support compiling against a newer kernel and run against an older (e.g.
in a container). So having our copy of newer kernel headers is merely
as if we compiled against as newer kernel.

Add "src/nm-compat-headers" which has a similar purpose, but a different
approach. Instead of replacing the included header entirely, include
the system header and patch it with #define.

Use this for "linux/if_addr.h". Of course, the approach here is that we
no longer include <linux/if_addr.h> directly, but instead include
"nm-compat-headers/linux/if_addr.h".

CONTRIBUTING.md

@@ -317,7 +317,10 @@ The second include is the header that belongs to the C source file. This
 is so that header files are self-contained (aside what default dependencies that
 they get and everybody can rely on).
 
-The next includes are system headers with `<>`.
+The next includes are system headers with `<>`. Exceptions are headers like
+"libnm-std-aux/nm-linux-compat.h" and "nm-compat-headers/\*" which are our small
+wrappers around system headers. These are also to be included together with system
+headers.
 
 Finally, all other headers from our source tree. Note that all build targets
 have `-I. -I./src/` in their build arguments. So to include a header like

Makefile.am

@@ -596,6 +596,8 @@ src_libnm_platform_libnm_platform_la_SOURCES = \
 	src/linux-headers/nl80211-vnd-intel.h \
 	src/linux-headers/nl802154.h \
 	\
+	src/nm-compat-headers/linux/if_addr.h \
+	\
 	src/libnm-platform/nm-linux-platform.c \
 	src/libnm-platform/nm-linux-platform.h \
 	src/libnm-platform/nm-netlink.c \

src/core/devices/nm-device.c

@@ -18,7 +18,7 @@
 #include <netinet/in.h>
 #include <netinet/if_ether.h>
 #include <linux/if.h>
-#include <linux/if_addr.h>
+#include "nm-compat-headers/linux/if_addr.h"
 #include <linux/rtnetlink.h>
 #include <linux/if_ether.h>
 #include <linux/if_infiniband.h>

src/core/ndisc/nm-ndisc.h

@@ -8,7 +8,7 @@
 
 #include <stdlib.h>
 #include <netinet/in.h>
-#include <linux/if_addr.h>
+#include "nm-compat-headers/linux/if_addr.h"
 
 #include "nm-setting-ip6-config.h"
 #include "NetworkManagerUtils.h"

src/core/nm-l3-config-data.c

@@ -5,7 +5,7 @@
 #include "nm-l3-config-data.h"
 
 #include <linux/if.h>
-#include <linux/if_addr.h>
+#include "nm-compat-headers/linux/if_addr.h"
 #include <linux/rtnetlink.h>
 
 #include "libnm-core-intern/nm-core-internal.h"

src/core/nm-l3-ipv6ll.c

@@ -4,7 +4,7 @@
 
 #include "nm-l3-ipv6ll.h"
 
-#include <linux/if_addr.h>
+#include "nm-compat-headers/linux/if_addr.h"
 
 #include "nm-core-utils.h"
 

src/core/nm-l3cfg.c

@@ -7,7 +7,7 @@
 #include "libnm-std-aux/nm-linux-compat.h"
 
 #include <net/if.h>
-#include <linux/if_addr.h>
+#include "nm-compat-headers/linux/if_addr.h"
 #include <linux/if_ether.h>
 #include <linux/rtnetlink.h>
 

src/core/tests/test-l3cfg.c

@@ -2,7 +2,7 @@
 
 #include "src/core/nm-default-daemon.h"
 
-#include <linux/if_addr.h>
+#include "nm-compat-headers/linux/if_addr.h"
 
 #include "nm-l3cfg.h"
 #include "nm-l3-ipv4ll.h"

src/linux-headers/README.md

@@ -8,3 +8,8 @@ certain version of kernel API.
 
 These headers should be taken without modification
 from Linux.
+
+Don't include any of these these headers directly, instead
+include "libnm-std-aux/nm-linux-compat.h" which drags these
+headers in. This ensures that we include at all places our own
+patched variant, instead of the system headers.

src/nm-compat-headers/README.md

@@ -0,0 +1,10 @@
+nm-compat-headers
+=================
+
+When we build against older system headers, we sometimes
+want to use newer features. This directory contains compat
+headers that patch the included sources with what we need.
+
+The goal is similar to linux-headers directory, but the approach
+is different. The former completely replaces system headers
+while this uses system headers and extends them.

src/nm-compat-headers/linux/if_addr.h

@@ -0,0 +1,16 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+
+#ifndef __NM_COMPAT_HEADER_LINUX_IF_ADDR_H__
+#define __NM_COMPAT_HEADER_LINUX_IF_ADDR_H__
+
+#include <linux/if_addr.h>
+
+#ifndef IFA_F_MANAGETEMPADDR
+#define IFA_F_MANAGETEMPADDR 0x100
+#endif
+
+#ifndef IFA_F_NOPREFIXROUTE
+#define IFA_F_NOPREFIXROUTE 0x200
+#endif
+
+#endif /* __NM_COMPAT_HEADER_LINUX_IF_ADDR_H__ */