Linux 7.0: add shims for the fs_context-based mount API

The traditional mount API has been removed, so detect when its not
available and instead use a small adapter to allow our existing mount
functions to keep working.

Sponsored-by: TrueNAS
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <rob.norris@truenas.com>
Closes #18216

config/kernel-fst-mount.m4

@@ -4,6 +4,10 @@ dnl # 2.6.38 API change
 dnl # The .get_sb callback has been replaced by a .mount callback
 dnl # in the file_system_type structure.
 dnl #
+dnl # 7.0 API change
+dnl # The .mount callback has been removed, requiring all mount work
+dnl # to be done through the "new" mount API introduced in 5.2.
+dnl #
 AC_DEFUN([ZFS_AC_KERNEL_SRC_FST_MOUNT], [
         ZFS_LINUX_TEST_SRC([file_system_type_mount], [
                 #include <linux/fs.h>
@@ -25,7 +29,8 @@ AC_DEFUN([ZFS_AC_KERNEL_FST_MOUNT], [
         AC_MSG_CHECKING([whether fst->mount() exists])
         ZFS_LINUX_TEST_RESULT([file_system_type_mount], [
                 AC_MSG_RESULT(yes)
+		AC_DEFINE(HAVE_FST_MOUNT, 1, [fst->mount() exists])
         ],[
-		ZFS_LINUX_TEST_ERROR([fst->mount()])
+		AC_MSG_RESULT(no)
         ])
 ])

module/os/linux/zfs/zpl_super.c

@@ -24,6 +24,7 @@
  * Copyright (c) 2023, Datto Inc. All rights reserved.
  * Copyright (c) 2025, Klara, Inc.
  * Copyright (c) 2025, Rob Norris <robn@despairlabs.com>
+ * Copyright (c) 2026, TrueNAS.
  */
 
 
@@ -36,6 +37,10 @@
 #include <linux/version.h>
 #include <linux/vfs_compat.h>
 
+#ifndef HAVE_FST_MOUNT
+#include <linux/fs_context.h>
+#endif
+
 /*
  * What to do when the last reference to an inode is released. If 0, the kernel
  * will cache it on the superblock. If 1, the inode will be freed immediately.
@@ -504,6 +509,61 @@ zpl_prune_sb(uint64_t nr_to_scan, void *arg)
 #endif
 }
 
+#ifndef HAVE_FST_MOUNT
+/*
+ * In kernel 7.0, the file_system_type->mount() and
+ * super_operations->remount_fs() callbacks have been removed, requiring all
+ * users to convert to the "new" fs_context-based mount API introduced in 5.2.
+ *
+ * This is the simplest compatibility shim possible to adapt the fs_context
+ * interface to the old-style calls. Although this interface exists in almost
+ * all versions of Linux currently supported by OpenZFS, we only use it when
+ * the kernel-provided shims are unavailable, to avoid bugs in these new shims
+ * affecting all OpenZFS deployments.
+ */
+static int
+zpl_parse_monolithic(struct fs_context *fc, void *data)
+{
+	/*
+	 * We do options parsing in zfs_domount(); just stash the options blob
+	 * in the fs_context so we can pass it down later.
+	 */
+	fc->fs_private = data;
+	return (0);
+}
+
+static int
+zpl_get_tree(struct fs_context *fc)
+{
+	struct dentry *root =
+	    zpl_mount(fc->fs_type, fc->sb_flags, fc->source, fc->fs_private);
+	if (IS_ERR(root))
+		return (PTR_ERR(root));
+
+	fc->root = root;
+	return (0);
+}
+
+static int
+zpl_reconfigure(struct fs_context *fc)
+{
+	return (zpl_remount_fs(fc->root->d_sb, &fc->sb_flags, fc->fs_private));
+}
+
+const struct fs_context_operations zpl_fs_context_operations = {
+	.parse_monolithic	= zpl_parse_monolithic,
+	.get_tree		= zpl_get_tree,
+	.reconfigure		= zpl_reconfigure,
+};
+
+static int
+zpl_init_fs_context(struct fs_context *fc)
+{
+	fc->ops = &zpl_fs_context_operations;
+	return (0);
+}
+#endif
+
 const struct super_operations zpl_super_operations = {
 	.alloc_inode		= zpl_inode_alloc,
 #ifdef HAVE_SOPS_FREE_INODE
@@ -517,7 +577,9 @@ const struct super_operations zpl_super_operations = {
 	.put_super		= zpl_put_super,
 	.sync_fs		= zpl_sync_fs,
 	.statfs			= zpl_statfs,
+#ifdef HAVE_FST_MOUNT
 	.remount_fs		= zpl_remount_fs,
+#endif
 	.show_devname		= zpl_show_devname,
 	.show_options		= zpl_show_options,
 	.show_stats		= NULL,
@@ -560,7 +622,11 @@ struct file_system_type zpl_fs_type = {
 #else
 	.fs_flags		= FS_USERNS_MOUNT,
 #endif
+#ifdef HAVE_FST_MOUNT
 	.mount			= zpl_mount,
+#else
+	.init_fs_context	= zpl_init_fs_context,
+#endif
 	.kill_sb		= zpl_kill_sb,
 };