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

module/Kbuild.in

@@ -433,6 +433,7 @@ ZFS_OBJS := \
 ZFS_OBJS_OS := \
 	abd_os.o \
 	arc_os.o \
+	kasan_compat.o \
 	mmp_os.o \
 	policy.o \
 	qat.o \

module/os/linux/zfs/kasan_compat.c

@@ -0,0 +1,48 @@
+// SPDX-License-Identifier: CDDL-1.0
+/*
+ * CDDL HEADER START
+ *
+ * The contents of this file are subject to the terms of the
+ * Common Development and Distribution License (the "License").
+ * You may not use this file except in compliance with the License.
+ *
+ * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
+ * or https://opensource.org/licenses/CDDL-1.0.
+ * See the License for the specific language governing permissions
+ * and limitations under the License.
+ *
+ * When distributing Covered Code, include this CDDL HEADER in each
+ * file and include the License file at usr/src/OPENSOLARIS.LICENSE.
+ * If applicable, add the following below this CDDL HEADER, with the
+ * fields enclosed by brackets "[]" replaced with your own identifying
+ * information: Portions Copyright [yyyy] [name of copyright owner]
+ *
+ * CDDL HEADER END
+ */
+
+/*
+ * Copyright (c) 2025, Rob Norris <robn@despairlabs.com>
+ */
+
+#ifndef _ZFS_LINUX_KASAN_ENABLED_H
+#define	_ZFS_LINUX_KASAN_ENABLED_H
+
+#ifdef HAVE_KASAN_ENABLED_GPL_ONLY
+/*
+ * The kernel supports a runtime setting to enable/disable KASAN. The control
+ * flag kasan_flag_enabled is a GPL-only symbol, which prevents us from
+ * accessing it. Unfortunately, this is called by the header function
+ * kasan_enabled(), which in turn is used to call or skip instrumentation
+ * functions in various header-based kernel facilities. If we inadvertently
+ * call one, the build breaks.
+ *
+ * To work around this, we define our own `kasan_flag_enabled` set to "false",
+ * disabling use of KASAN inside our code. The linker will resolve this symbol
+ * at build time, and so never need to reach out to the off-limits kernel
+ * symbol.
+ */
+#include <linux/static_key.h>
+struct static_key_false kasan_flag_enabled = STATIC_KEY_FALSE_INIT;
+#endif
+
+#endif

module/os/linux/zfs/zfs_vnops_os.c

@@ -31,7 +31,6 @@
 /* Portions Copyright 2007 Jeremy Teo */
 /* Portions Copyright 2010 Robert Milkowski */
 
-
 #include <sys/types.h>
 #include <sys/param.h>
 #include <sys/time.h>