[PATCH] ucounts: constify usage of ctl_table_header::ctl_table_arg

["Thomas Weißschuh" <linux@weissschuh.net>]

The sysctl core is preparing to only expose instances of
struct ctl_table as "const".
This will also affect the member
ctl_table_header::ctl_table_arg.

Prepare for that change to "struct ctl_table_header",
and already constify the usage of ctl_table_arg.

No functional change.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
---
Motivation
==========

Moving structures containing function pointers into unmodifiable .rodata
prevents attackers or bugs from corrupting and diverting those pointers.

Also the "struct ctl_table" exposed by the sysctl core were never meant
to be mutated by users.

Process
=======

* Drop ctl_table modifications from the sysctl core ([0], in -next)
* Constify arguments to ctl_table_root::{set_ownership,permissions}
  ([1], in -next)
* Migrate users of "ctl_table_header::ctl_table_arg" to "const".
  (in progress, this patch)
* Afterwards convert "ctl_table_header::ctl_table_arg" itself to const.
  (to be done)
* Prepare helpers used to implement proc_handlers throughout the tree to
  use "const struct ctl_table *". ([2], in progress)
* Afterwards switch over all proc_handlers callbacks to use
  "const struct ctl_table *" in one commit. ([2], in progress)
  Only custom handlers will be affected, the big commit avoids a
  disruptive and messy transition phase.
* Switch over the internals of the sysctl core to "const struct ctl_table *" (to be done)
* Switch include/linux/sysctl.h to "const struct ctl_table *" (to be done)
* Transition instances of "struct ctl_table" through the tree to const (to be done)

A work-in-progress view containging all the outlined changes can be found at
https://git.sr.ht/~t-8ch/linux sysctl-constfy

[0] https://lore.kernel.org/lkml/20240322-sysctl-empty-dir-v2-0-e559cf8ec7c0@weissschuh.net/
[1] https://lore.kernel.org/lkml/20240315-sysctl-const-ownership-v3-0-b86680eae02e@weissschuh.net/
[2] https://lore.kernel.org/lkml/20240423-sysctl-const-handler-v3-0-e0beccb836e2@weissschuh.net/

Cc: Joel Granados <j.granados@samsung.com>
Cc: Luis Chamberlain <mcgrof@kernel.org>
Cc: Eric W. Biederman <ebiederm@xmission.com>
---
 kernel/ucount.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/ucount.c b/kernel/ucount.c
index 4aa6166cb856..d9e283600f5c 100644
--- a/kernel/ucount.c
+++ b/kernel/ucount.c
@@ -119,7 +119,7 @@ bool setup_userns_sysctls(struct user_namespace *ns)
 void retire_userns_sysctls(struct user_namespace *ns)
 {
 #ifdef CONFIG_SYSCTL
-	struct ctl_table *tbl;
+	const struct ctl_table *tbl;
 
 	tbl = ns->sysctls->ctl_table_arg;
 	unregister_sysctl_table(ns->sysctls);

---
base-commit: cf87f46fd34d6c19283d9625a7822f20d90b64a4
change-id: 20240511-sysctl-const-table-arg-ucount-75027ec3fbf4

Best regards,
-- 
Thomas Weißschuh <linux@weissschuh.net>