Linux: work around use of GPL-only symbol kasan_flag_enabled

We may not be able to avoid our code referencing the symbol, but we can
ensure that a symbol of that name is available to the linker during
build, and so not require linking the GPL-exported version.

Sponsored-by: https://despairlabs.com/sponsor/
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Closes #18009
Closes #18040

config/kernel-kasan-enabled.m4

@@ -0,0 +1,23 @@
+dnl #
+dnl # 6.18: some architectures and config option causes the kasan_ inline
+dnl #       functions to reference the GPL-only symbol 'kasan_flag_enabled',
+dnl #       breaking the build. Detect this and work
+dnl #       around it.
+AC_DEFUN([ZFS_AC_KERNEL_SRC_KASAN_ENABLED], [
+	ZFS_LINUX_TEST_SRC([kasan_enabled], [
+		#include <linux/kasan.h>
+	], [
+		kasan_enabled();
+	], [], [ZFS_META_LICENSE])
+])
+AC_DEFUN([ZFS_AC_KERNEL_KASAN_ENABLED], [
+	AC_MSG_CHECKING([whether kasan_enabled() is GPL-only])
+	ZFS_LINUX_TEST_RESULT([kasan_enabled_license], [
+		AC_MSG_RESULT(no)
+	], [
+		AC_MSG_RESULT(yes)
+		AC_DEFINE(HAVE_KASAN_ENABLED_GPL_ONLY, 1,
+		    [kasan_enabled() is GPL-only])
+	])
+])
+

config/kernel.m4

@@ -138,6 +138,7 @@ AC_DEFUN([ZFS_AC_KERNEL_TEST_SRC], [
 	ZFS_AC_KERNEL_SRC_SOPS_FREE_INODE
 	ZFS_AC_KERNEL_SRC_NAMESPACE
 	ZFS_AC_KERNEL_SRC_INODE_GENERIC_DROP
+	ZFS_AC_KERNEL_SRC_KASAN_ENABLED
 	case "$host_cpu" in
 		powerpc*)
 			ZFS_AC_KERNEL_SRC_CPU_HAS_FEATURE
@@ -260,6 +261,7 @@ AC_DEFUN([ZFS_AC_KERNEL_TEST_RESULT], [
 	ZFS_AC_KERNEL_SOPS_FREE_INODE
 	ZFS_AC_KERNEL_NAMESPACE
 	ZFS_AC_KERNEL_INODE_GENERIC_DROP
+	ZFS_AC_KERNEL_KASAN_ENABLED
 	case "$host_cpu" in
 		powerpc*)
 			ZFS_AC_KERNEL_CPU_HAS_FEATURE