Linux 6.11: avoid passing "end" sentinel to register_sysctl()

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Sponsored-by: https://despairlabs.com/sponsor/
Closes #16400

config/kernel-register_sysctl_table.m4

@@ -26,6 +26,32 @@ AC_DEFUN([ZFS_AC_KERNEL_REGISTER_SYSCTL_TABLE], [
 	])
 ])
 
+dnl #
+dnl # Linux 6.11 register_sysctl() enforces that sysctl tables no longer
+dnl # supply a sentinel end-of-table element. 6.6 introduces
+dnl # register_sysctl_sz() to enable callers to choose, so we use it if
+dnl # available for backward compatibility.
+dnl #
+AC_DEFUN([ZFS_AC_KERNEL_SRC_REGISTER_SYSCTL_SZ], [
+	ZFS_LINUX_TEST_SRC([has_register_sysctl_sz], [
+		#include <linux/sysctl.h>
+	],[
+		struct ctl_table test_table[] __attribute__((unused)) = {0};
+		register_sysctl_sz("", test_table, 0);
+	])
+])
+
+AC_DEFUN([ZFS_AC_KERNEL_REGISTER_SYSCTL_SZ], [
+	AC_MSG_CHECKING([whether register_sysctl_sz exists])
+	ZFS_LINUX_TEST_RESULT([has_register_sysctl_sz], [
+		AC_MSG_RESULT([yes])
+		AC_DEFINE(HAVE_REGISTER_SYSCTL_SZ, 1,
+			[register_sysctl_sz exists])
+	],[
+		AC_MSG_RESULT([no])
+	])
+])
+
 dnl #
 dnl # Linux 6.11 makes const the ctl_table arg of proc_handler
 dnl #

config/kernel.m4

@@ -167,6 +167,7 @@ AC_DEFUN([ZFS_AC_KERNEL_TEST_SRC], [
 	ZFS_AC_KERNEL_SRC_WRITEPAGE_T
 	ZFS_AC_KERNEL_SRC_RECLAIMED
 	ZFS_AC_KERNEL_SRC_REGISTER_SYSCTL_TABLE
+	ZFS_AC_KERNEL_SRC_REGISTER_SYSCTL_SZ
 	ZFS_AC_KERNEL_SRC_PROC_HANDLER_CTL_TABLE_CONST
 	ZFS_AC_KERNEL_SRC_COPY_SPLICE_READ
 	ZFS_AC_KERNEL_SRC_SYNC_BDEV
@@ -321,6 +322,7 @@ AC_DEFUN([ZFS_AC_KERNEL_TEST_RESULT], [
 	ZFS_AC_KERNEL_WRITEPAGE_T
 	ZFS_AC_KERNEL_RECLAIMED
 	ZFS_AC_KERNEL_REGISTER_SYSCTL_TABLE
+	ZFS_AC_KERNEL_REGISTER_SYSCTL_SZ
 	ZFS_AC_KERNEL_PROC_HANDLER_CTL_TABLE_CONST
 	ZFS_AC_KERNEL_COPY_SPLICE_READ
 	ZFS_AC_KERNEL_SYNC_BDEV

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

@@ -22,6 +22,9 @@
  *
  *  Solaris Porting Layer (SPL) Proc Implementation.
  */
+/*
+ * Copyright (c) 2024, Rob Norris <robn@despairlabs.com>
+ */
 
 #include <sys/systeminfo.h>
 #include <sys/kstat.h>
@@ -694,6 +697,37 @@ static void spl_proc_cleanup(void)
 	}
 }
 
+#ifndef HAVE_REGISTER_SYSCTL_TABLE
+
+/*
+ * Traditionally, struct ctl_table arrays have been terminated by an "empty"
+ * sentinel element (specifically, one with .procname == NULL).
+ *
+ * Linux 6.6 began migrating away from this, adding register_sysctl_sz() so
+ * that callers could provide the size directly, and redefining
+ * register_sysctl() to just call register_sysctl_sz() with the array size. It
+ * retained support for the terminating element so that existing callers would
+ * continue to work.
+ *
+ * Linux 6.11 removed support for the terminating element, instead interpreting
+ * it as a real malformed element, and rejecting it.
+ *
+ * In order to continue support older kernels, we retain the terminating
+ * sentinel element for our sysctl tables, but instead detect availability of
+ * register_sysctl_sz(). If it exists, we pass it the array size -1, stopping
+ * the kernel from trying to process the terminator. For pre-6.6 kernels that
+ * don't have register_sysctl_sz(), we just use register_sysctl(), which can
+ * handle the terminating element as it always has.
+ */
+#ifdef HAVE_REGISTER_SYSCTL_SZ
+#define	spl_proc_register_sysctl(p, t)	\
+	register_sysctl_sz(p, t, ARRAY_SIZE(t)-1)
+#else
+#define	spl_proc_register_sysctl(p, t)	\
+	register_sysctl(p, t)
+#endif
+#endif
+
 int
 spl_proc_init(void)
 {
@@ -704,16 +738,17 @@ spl_proc_init(void)
 	if (spl_header == NULL)
 		return (-EUNATCH);
 #else
-	spl_header = register_sysctl("kernel/spl", spl_table);
+	spl_header = spl_proc_register_sysctl("kernel/spl", spl_table);
 	if (spl_header == NULL)
 		return (-EUNATCH);
 
-	spl_kmem = register_sysctl("kernel/spl/kmem", spl_kmem_table);
+	spl_kmem = spl_proc_register_sysctl("kernel/spl/kmem", spl_kmem_table);
 	if (spl_kmem == NULL) {
 		rc = -EUNATCH;
 		goto out;
 	}
-	spl_kstat = register_sysctl("kernel/spl/kstat", spl_kstat_table);
+	spl_kstat = spl_proc_register_sysctl("kernel/spl/kstat",
+	    spl_kstat_table);
 	if (spl_kstat == NULL) {
 		rc = -EUNATCH;
 		goto out;