libspl: implement ZFS_MODULE_PARAM for userspace

For each tunable declaration, we create a zfs_tunable_t with its
details, and then a pointer to it in the 'zfs_tunables' ELF section,
that we can access later with a little support from the linker.

Sponsored-by: https://despairlabs.com/sponsor/
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Closes #17537

lib/libspl/Makefile.am

@@ -20,6 +20,7 @@ libspl_la_SOURCES = \
 	%D%/strlcat.c \
 	%D%/strlcpy.c \
 	%D%/timestamp.c \
+	%D%/tunables.c \
 	%D%/include/sys/list.h \
 	%D%/include/sys/list_impl.h
 

lib/libspl/include/Makefile.am

@@ -59,6 +59,7 @@ libspl_sys_HEADERS = \
 	%D%/sys/time.h \
 	%D%/sys/trace_spl.h \
 	%D%/sys/trace_zfs.h \
+	%D%/sys/tunables.h \
 	%D%/sys/types.h \
 	%D%/sys/types32.h \
 	%D%/sys/uio.h \

lib/libspl/include/sys/mod.h

@@ -24,12 +24,29 @@
  * Copyright 2011 Nexenta Systems, Inc.  All rights reserved.
  * Copyright (c) 2012, 2018 by Delphix. All rights reserved.
  * Copyright (c) 2012, Joyent, Inc. All rights reserved.
+ * Copyright (c) 2025, Rob Norris <robn@despairlabs.com>
  */
 
 #ifndef _SYS_MOD_H
 #define	_SYS_MOD_H
 
-#define	ZFS_MODULE_PARAM(scope, prefix, name, type, perm, desc)
+#include <sys/tunables.h>
+
+#define	ZFS_MODULE_PARAM(scope, prefix, name, type, perm, desc)		\
+	static const zfs_tunable_t _zfs_tunable_##prefix##name = {	\
+		.zt_name = #prefix#name,				\
+		.zt_varp = &prefix##name,				\
+		.zt_varsz = sizeof (prefix##name),			\
+		.zt_type = ZFS_TUNABLE_TYPE_##type,			\
+		.zt_perm = ZFS_TUNABLE_PERM_##perm,			\
+		.zt_desc = desc						\
+	};								\
+	static const zfs_tunable_t *					\
+	__zfs_tunable_##prefix##name					\
+	__attribute__((__section__("zfs_tunables")))			\
+	__attribute__((__used__))					\
+	= &_zfs_tunable_##prefix##name;
+
 #define	ZFS_MODULE_PARAM_ARGS void
 #define	ZFS_MODULE_PARAM_CALL(scope_prefix, name_prefix, name, setfunc, \
 	getfunc, perm, desc)

lib/libspl/include/sys/tunables.h

@@ -0,0 +1,52 @@
+// 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 _SYS_TUNABLES_H
+#define	_SYS_TUNABLES_H
+
+typedef enum {
+	ZFS_TUNABLE_TYPE_INT,
+	ZFS_TUNABLE_TYPE_UINT,
+	ZFS_TUNABLE_TYPE_ULONG,
+	ZFS_TUNABLE_TYPE_U64,
+	ZFS_TUNABLE_TYPE_STRING,
+} zfs_tunable_type_t;
+
+typedef enum {
+	ZFS_TUNABLE_PERM_ZMOD_RW,
+	ZFS_TUNABLE_PERM_ZMOD_RD,
+} zfs_tunable_perm_t;
+
+typedef struct zfs_tunable {
+	const char		*zt_name;
+	void			*zt_varp;
+	size_t			zt_varsz;
+	zfs_tunable_type_t	zt_type;
+	zfs_tunable_perm_t	zt_perm;
+	const char		*zt_desc;
+} zfs_tunable_t;
+
+#endif

lib/libspl/tunables.c

@@ -0,0 +1,72 @@
+// 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>
+ */
+
+#include <stddef.h>
+#include <sys/tunables.h>
+
+/*
+ * Userspace tunables.
+ *
+ * Tunables are external pointers to global variables that are wired up to the
+ * host environment in some way that allows the operator to directly change
+ * their values "under the hood".
+ *
+ * In userspace, the "host environment" is the program using libzpool.so. So
+ * that it can manipulate tunables if it wants, we provide an API to access
+ * them.
+ *
+ * Tunables are declared through the ZFS_MODULE_PARAM* macros, which associate
+ * a global variable with some metadata we can use to describe and access the
+ * tunable. This is done by creating a uniquely-named zfs_tunable_t.
+ *
+ * At runtime, we need a way to discover these zfs_tunable_t items. Since they
+ * are declared globally, all over the codebase, there's no central place to
+ * record or list them. So, we take advantage of the compiler's "linker set"
+ * feature.
+ *
+ * In the ZFS_MODULE_PARAM macro, after we create the zfs_tunable_t, we also
+ * create a zfs_tunable_t* pointing to it. That pointer is forced into the
+ * "zfs_tunables" ELF section in compiled object. At link time, the linker will
+ * collect all these pointers into one single big "zfs_tunable" section, and
+ * will generate two new symbols in the final object: __start_zfs_tunable and
+ * __stop_zfs_tunable. These point to the first and last item in that section,
+ * which allows us to access the pointers in that section like an array, and
+ * through those pointers access the tunable metadata, and from there the
+ * actual C variable that the tunable describes.
+ */
+
+extern const zfs_tunable_t *__start_zfs_tunables;
+extern const zfs_tunable_t *__stop_zfs_tunables;
+
+/*
+ * Because there are no tunables in libspl itself, the above symbols will not
+ * be generated, which will stop libspl being linked at all. To work around
+ * that, we force a symbol into that section, and then when iterating, skip
+ * any NULL pointers.
+ */
+static void *__zfs_tunable__placeholder
+	__attribute__((__section__("zfs_tunables")))
+	__attribute__((__used__)) = NULL;