Build: Fix Linux Landlock feature test in Autotools and CMake builds.

The previous Linux Landlock feature test assumed that having the
linux/landlock.h header file was enough. The new feature tests also
requires that prctl() and the required Landlock system calls are
supported.

CMakeLists.txt

@@ -901,10 +901,29 @@ endif()
 
 # Sandboxing: Landlock
 if(NOT SANDBOX_FOUND AND ENABLE_SANDBOX MATCHES "^ON$|^landlock$")
-    check_include_file(linux/landlock.h HAVE_LINUX_LANDLOCK_H)
+    # A compile check is done here because some systems have
+    # linux/landlock.h, but do not have the syscalls defined
+    # in order to actually use Linux Landlock.
+    check_c_source_compiles("
+        #include <linux/landlock.h>
+        #include <sys/syscall.h>
+        #include <sys/prctl.h>
+.
+        void my_sandbox(void)
+        {
+            (void)prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0);
+            (void)SYS_landlock_create_ruleset;
+            (void)SYS_landlock_restrict_self;
+            (void)LANDLOCK_CREATE_RULESET_VERSION;
+            return;
+        }
 
-    if(HAVE_LINUX_LANDLOCK_H)
-        set(SANDBOX_COMPILE_DEFINITION "HAVE_LINUX_LANDLOCK_H")
+        int main(void) { return 0; }
+        "
+    HAVE_LINUX_LANDLOCK)
+
+    if(HAVE_LINUX_LANDLOCK)
+        set(SANDBOX_COMPILE_DEFINITION "HAVE_LINUX_LANDLOCK")
         set(SANDBOX_FOUND ON)
 
         # Of our three sandbox methods, only Landlock is incompatible

configure.ac

@@ -1177,12 +1177,37 @@ AS_CASE([$enable_sandbox],
 )
 AS_CASE([$enable_sandbox],
 	[auto | landlock], [
-		AC_CHECK_HEADERS([linux/landlock.h], [
+		AC_MSG_CHECKING([if Linux Landlock is usable])
+
+		# A compile check is done here because some systems have
+		# linux/landlock.h, but do not have the syscalls defined
+		# in order to actually use Linux Landlock.
+		AC_COMPILE_IFELSE([AC_LANG_SOURCE([[
+			#include <linux/landlock.h>
+			#include <sys/syscall.h>
+			#include <sys/prctl.h>
+
+			void my_sandbox(void)
+			{
+				(void)prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0);
+				(void)SYS_landlock_create_ruleset;
+				(void)SYS_landlock_restrict_self;
+				(void)LANDLOCK_CREATE_RULESET_VERSION;
+				return;
+			}
+		]])], [
 			enable_sandbox=found
 
 			AS_CASE([$CFLAGS], [*-fsanitize=*], [AC_MSG_ERROR([
     CFLAGS contains '-fsanitize=' which is incompatible with the Landlock
     sandboxing. Use --disable-sandbox when using '-fsanitize'.])])
+
+			AC_DEFINE([HAVE_LINUX_LANDLOCK], [1],
+				[Define to 1 if Linux Landlock is supported.
+				See configure.ac for details.])
+			AC_MSG_RESULT([yes])
+		], [
+			AC_MSG_RESULT([no])
 		])
 	]
 )

src/xz/sandbox.c

@@ -109,7 +109,7 @@ sandbox_enable_strict_if_allowed(int src_fd lzma_attribute((__unused__)),
 }
 
 
-#elif defined(HAVE_LINUX_LANDLOCK_H)
+#elif defined(HAVE_LINUX_LANDLOCK)
 
 //////////////
 // Landlock //

src/xz/sandbox.h

@@ -9,7 +9,7 @@
 //
 ///////////////////////////////////////////////////////////////////////////////
 
-#if defined(HAVE_PLEDGE) || defined(HAVE_LINUX_LANDLOCK_H) \
+#if defined(HAVE_PLEDGE) || defined(HAVE_LINUX_LANDLOCK) \
 		|| defined(HAVE_CAP_RIGHTS_LIMIT)
 #	define ENABLE_SANDBOX 1
 #endif

src/xzdec/xzdec.c

@@ -24,14 +24,14 @@
 #	include <sys/capsicum.h>
 #endif
 
-#ifdef HAVE_LINUX_LANDLOCK_H
+#ifdef HAVE_LINUX_LANDLOCK
 #	include <linux/landlock.h>
 #	include <sys/prctl.h>
 #	include <sys/syscall.h>
 #endif
 
 #if defined(HAVE_CAP_RIGHTS_LIMIT) || defined(HAVE_PLEDGE) \
-		|| defined(HAVE_LINUX_LANDLOCK_H)
+		|| defined(HAVE_LINUX_LANDLOCK)
 #	define ENABLE_SANDBOX 1
 #endif
 
@@ -325,7 +325,7 @@ sandbox_enter(int src_fd)
 		goto error;
 
 	(void)src_fd;
-#elif defined(HAVE_LINUX_LANDLOCK_H)
+#elif defined(HAVE_LINUX_LANDLOCK)
 	int landlock_abi = syscall(SYS_landlock_create_ruleset,
 			(void *)NULL, 0, LANDLOCK_CREATE_RULESET_VERSION);
 
@@ -389,7 +389,7 @@ main(int argc, char **argv)
 	}
 #endif
 
-#ifdef HAVE_LINUX_LANDLOCK_H
+#ifdef HAVE_LINUX_LANDLOCK
 	// Prevent the process from gaining new privileges. The return
 	// is ignored to keep compatibility with old kernels.
 	(void)prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0);