selinux: do not include <linux/*.h> headers from host programs

The header, security/selinux/include/classmap.h, is included not only
from kernel space but also from host programs.

It includes <linux/capability.h> and <linux/socket.h>, which pull in
more <linux/*.h> headers. This makes the host programs less portable,
specifically causing build errors on macOS.

Those headers are included for the following purposes:

- <linux/capability.h> for checking CAP_LAST_CAP
- <linux/socket.h> for checking PF_MAX

These checks can be guarded by __KERNEL__ so they are skipped when
building host programs. Testing them when building the kernel should
be sufficient.

The header, security/selinux/include/initial_sid_to_string.h, includes
<linux/stddef.h> for the NULL definition, but this is not portable
either. Instead, <stddef.h> should be included for host programs.

Reported-by: Daniel Gomez <[e-mail]>
Closes: https://lore.kernel.org/lkml/[e-mail]/
Closes: https://lore.kernel.org/lkml/[e-mail]/
Signed-off-by: Masahiro Yamada <[e-mail]>
Signed-off-by: Paul Moore <[e-mail]>

scripts/selinux/genheaders/Makefile

···
 # SPDX-License-Identifier: GPL-2.0
 hostprogs-always-y += genheaders
-HOST_EXTRACFLAGS += \
-	-I$(srctree)/include/uapi -I$(srctree)/include \
-	-I$(srctree)/security/selinux/include
+HOST_EXTRACFLAGS += -I$(srctree)/security/selinux/include

scripts/selinux/genheaders/genheaders.c

···
 // SPDX-License-Identifier: GPL-2.0
 
-/* NOTE: we really do want to use the kernel headers here */
-#define __EXPORTED_HEADERS__
-
 #include <stdio.h>
 #include <stdlib.h>
 #include <unistd.h>

scripts/selinux/mdp/Makefile

···
 # SPDX-License-Identifier: GPL-2.0
 hostprogs-always-y += mdp
 HOST_EXTRACFLAGS += \
-	-I$(srctree)/include/uapi -I$(srctree)/include \
+	-I$(srctree)/include \
 	-I$(srctree)/security/selinux/include -I$(objtree)/include
 
 clean-files	:= policy.* file_contexts

scripts/selinux/mdp/mdp.c

···
  * Authors: Serge E. Hallyn <[e-mail]>
  */
 
-
-/* NOTE: we really do want to use the kernel headers here */
-#define __EXPORTED_HEADERS__
-
 #include <stdio.h>
 #include <stdlib.h>
 #include <unistd.h>

security/selinux/include/classmap.h

···
 /* SPDX-License-Identifier: GPL-2.0 */
 
-#include <linux/capability.h>
-#include <linux/socket.h>
-
 #define COMMON_FILE_SOCK_PERMS                                            \
 	"ioctl", "read", "write", "create", "getattr", "setattr", "lock", \
 		"relabelfrom", "relabelto", "append", "map"
···
 	"mac_override", "mac_admin", "syslog", "wake_alarm", "block_suspend", \
 		"audit_read", "perfmon", "bpf", "checkpoint_restore"
 
+#ifdef __KERNEL__ /* avoid this check when building host programs */
+#include <linux/capability.h>
+
 #if CAP_LAST_CAP > CAP_CHECKPOINT_RESTORE
 #error New capability defined, please update COMMON_CAP2_PERMS.
+#endif
 #endif
 
 /*
···
 	{ NULL }
 };
 
+#ifdef __KERNEL__ /* avoid this check when building host programs */
+#include <linux/socket.h>
+
 #if PF_MAX > 46
 #error New address family defined, please update secclass_map.
+#endif
 #endif

security/selinux/include/initial_sid_to_string.h

···
 /* SPDX-License-Identifier: GPL-2.0 */
 
+#ifdef __KERNEL__
 #include <linux/stddef.h>
+#else
+#include <stddef.h>
+#endif
 
 static const char *const initial_sid_to_string[] = {
 	NULL, /* zero placeholder, not used */