sysctl: avoid spurious permanent empty tables

The test if a table is a permanently empty one, inspects the address of
the registered ctl_table argument.
However as sysctl_mount_point is an empty array and does not occupy and
space it can end up sharing an address with another object in memory.
If that other object itself is a "struct ctl_table" then registering
that table will fail as it's incorrectly recognized as permanently empty.

Avoid this issue by adding a dummy element to the array so that is not
empty anymore.
Explicitly register the table with zero elements as otherwise the dummy
element would be recognized as a sentinel element which would lead to a
runtime warning from the sysctl core.

While the issue seems not being encountered at this time, this seems
mostly to be due to luck.
Also a future change, constifying sysctl_mount_point and root_table, can
reliably trigger this issue on clang 18.

Given that empty arrays are non-standard in the first place it seems
prudent to avoid them if possible.

Fixes: 4a7b29f6 ("sysctl: move sysctl type to ctl_table_header")
Fixes: a35dd3a7 ("sysctl: drop now unnecessary out-of-bounds check")
Cc: stable@vger.kernel.org
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Closes: https://lore.kernel.org/oe-lkp/202408051453.f638857e-lkp@intel.comSigned-off-by: Joel Granados <j.granados@samsung.com>

fs/proc/proc_sysctl.c

@@ -29,8 +29,13 @@ static const struct inode_operations proc_sys_inode_operations;
 static const struct file_operations proc_sys_dir_file_operations;
 static const struct inode_operations proc_sys_dir_operations;
 
-/* Support for permanently empty directories */
-static struct ctl_table sysctl_mount_point[] = { };
+/*
+ * Support for permanently empty directories.
+ * Must be non-empty to avoid sharing an address with other tables.
+ */
+static struct ctl_table sysctl_mount_point[] = {
+	{ }
+};
 
 /**
  * register_sysctl_mount_point() - registers a sysctl mount point
@@ -42,7 +47,7 @@ static struct ctl_table sysctl_mount_point[] = { };
  */
 struct ctl_table_header *register_sysctl_mount_point(const char *path)
 {
-	return register_sysctl(path, sysctl_mount_point);
+	return register_sysctl_sz(path, sysctl_mount_point, 0);
 }
 EXPORT_SYMBOL(register_sysctl_mount_point);