Linux 6.5 compat: register_sysctl_table removed

Additionally, the .child element of ctl_table has been removed in 6.5.
This change adds a new test for the pre-6.5 register_sysctl_table()
function, and uses the old code in that case. If it isn't found, then
the parentage entries in the tables are removed, and the register_sysctl
call is provided the paths of "kernel/spl", "kernel/spl/kmem", and
"kernel/spl/kstat" directly, to populate each subdirectory over three
calls, as is the new API.

Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: Coleman Kane <[email protected]>
Closes openzfs#15098

Author: ckane

config/kernel-register_sysctl_table.m4

@@ -0,0 +1,27 @@
+dnl #
+dnl # Linux 6.5 removes register_sysctl_table
+dnl #
+AC_DEFUN([ZFS_AC_KERNEL_SRC_REGISTER_SYSCTL_TABLE], [
+	ZFS_LINUX_TEST_SRC([has_register_sysctl_table], [
+		#include <linux/sysctl.h>
+
+		static struct ctl_table dummy_table[] = {
+			{}
+		};
+
+    ],[
+		struct ctl_table_header *h
+			__attribute((unused)) = register_sysctl_table(dummy_table);
+    ])
+])
+
+AC_DEFUN([ZFS_AC_KERNEL_REGISTER_SYSCTL_TABLE], [
+	AC_MSG_CHECKING([whether register_sysctl_table exists])
+	ZFS_LINUX_TEST_RESULT([has_register_sysctl_table], [
+		AC_MSG_RESULT([yes])
+		AC_DEFINE(HAVE_SYSCTL_REGISTER_TABLE, 1,
+			[sysctl_register_table exists])
+	],[
+		AC_MSG_RESULT([no])
+	])
+])

config/kernel.m4

@@ -160,6 +160,7 @@ AC_DEFUN([ZFS_AC_KERNEL_TEST_SRC], [
 	ZFS_AC_KERNEL_SRC_FILEMAP
 	ZFS_AC_KERNEL_SRC_WRITEPAGE_T
 	ZFS_AC_KERNEL_SRC_RECLAIMED
+	ZFS_AC_KERNEL_SRC_REGISTER_SYSCTL_TABLE
 	case "$host_cpu" in
 		powerpc*)
 			ZFS_AC_KERNEL_SRC_CPU_HAS_FEATURE
@@ -299,6 +300,7 @@ AC_DEFUN([ZFS_AC_KERNEL_TEST_RESULT], [
 	ZFS_AC_KERNEL_FILEMAP
 	ZFS_AC_KERNEL_WRITEPAGE_T
 	ZFS_AC_KERNEL_RECLAIMED
+	ZFS_AC_KERNEL_REGISTER_SYSCTL_TABLE
 	case "$host_cpu" in
 		powerpc*)
 			ZFS_AC_KERNEL_CPU_HAS_FEATURE

module/os/linux/spl/spl-proc.c

@@ -624,6 +624,7 @@ static struct ctl_table spl_table[] = {
 		.mode		= 0644,
 		.proc_handler	= &proc_dohostid,
 	},
+#ifdef HAVE_REGISTER_SYSCTL_TABLE
 	{
 		.procname	= "kmem",
 		.mode		= 0555,
@@ -634,9 +635,11 @@ static struct ctl_table spl_table[] = {
 		.mode		= 0555,
 		.child		= spl_kstat_table,
 	},
+#endif
 	{},
 };
 
+#ifdef HAVE_REGISTER_SYSCTL_TABLE
 static struct ctl_table spl_dir[] = {
 	{
 		.procname	= "spl",
@@ -648,21 +651,38 @@ static struct ctl_table spl_dir[] = {
 
 static struct ctl_table spl_root[] = {
 	{
-	.procname = "kernel",
-	.mode = 0555,
-	.child = spl_dir,
+		.procname	= "kernel",
+		.mode		= 0555,
+		.child		= spl_dir,
 	},
 	{}
 };
+#endif
 
 int
 spl_proc_init(void)
 {
 	int rc = 0;
 
+#ifdef HAVE_REGISTER_SYSCTL_TABLE
 	spl_header = register_sysctl_table(spl_root);
 	if (spl_header == NULL)
 		return (-EUNATCH);
+#else
+	spl_header = register_sysctl("kernel/spl", spl_table);
+	if (spl_header == NULL)
+		return (-EUNATCH);
+
+	if (register_sysctl("kernel/spl/kmem", spl_kmem_table) == NULL) {
+		rc = -EUNATCH;
+		goto out;
+	}
+
+	if (register_sysctl("kernel/spl/kstat", spl_kstat_table) == NULL) {
+		rc = -EUNATCH;
+		goto out;
+	}
+#endif
 
 	proc_spl = proc_mkdir("spl", NULL);
 	if (proc_spl == NULL) {