Re: [PATCH v4 2/4] sysctl: Move some boundary constants form sysctl.c to sysctl_vals

[Xiaoming Ni <nixiaoming@xxxxxxxxxx>]

On 2020/5/19 12:44, Tetsuo Handa wrote:
> On 2020/05/19 12:31, Xiaoming Ni wrote:
> > Some boundary (.extra1 .extra2) constants (E.g: neg_one two) in
> > sysctl.c are used in multiple features. Move these variables to
> > sysctl_vals to avoid adding duplicate variables when cleaning up
> > sysctls table.
> >
> > Signed-off-by: Xiaoming Ni <nixiaoming@xxxxxxxxxx>
> > Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
>
> I feel that it is use of
>
> 	void *extra1;
> 	void *extra2;
>
> in "struct ctl_table" that requires constant values indirection.
> Can't we get rid of sysctl_vals using some "union" like below?
>
> struct ctl_table {
> 	const char *procname;           /* Text ID for /proc/sys, or zero */
> 	void *data;
> 	int maxlen;
> 	umode_t mode;
> 	struct ctl_table *child;        /* Deprecated */
> 	proc_handler *proc_handler;     /* Callback for text formatting */
> 	struct ctl_table_poll *poll;
> 	union {
> 		void *min_max_ptr[2];
> 		int min_max_int[2];
> 		long min_max_long[2];
> 	};
> } __randomize_layout;
>
> .

net/decnet/dn_dev.c:
static void dn_dev_sysctl_register(struct net_device *dev, struct 
dn_dev_parms *parms)
{
	struct dn_dev_sysctl_table *t;
	int i;

	char path[sizeof("net/decnet/conf/") + IFNAMSIZ];

	t = kmemdup(&dn_dev_sysctl, sizeof(*t), GFP_KERNEL);
	if (t == NULL)
		return;

	for(i = 0; i < ARRAY_SIZE(t->dn_dev_vars) - 1; i++) {
		long offset = (long)t->dn_dev_vars[i].data;
		t->dn_dev_vars[i].data = ((char *)parms) + offset;
	}

	snprintf(path, sizeof(path), "net/decnet/conf/%s",
		dev? dev->name : parms->name);

	t->dn_dev_vars[0].extra1 = (void *)dev;

	t->sysctl_header = register_net_sysctl(&init_net, path, t->dn_dev_vars);
	if (t->sysctl_header == NULL)
		kfree(t);
	else
		parms->sysctl = t;
}

A small amount of code is not used as a boundary value when using 
extra1. This scenario may not be suitable for renaming to min_max_ptr.

Should we add const to extra1 extra2 ?

--- a/include/linux/sysctl.h
+++ b/include/linux/sysctl.h
@@ -124,8 +124,8 @@ struct ctl_table {
        struct ctl_table *child;        /* Deprecated */
        proc_handler *proc_handler;     /* Callback for text formatting */
        struct ctl_table_poll *poll;
-       void *extra1;
-       void *extra2;
+       const void *extra1;
+       const void *extra2;
 } __randomize_layout;


Thanks
Xiaoming Ni