Re: [PATCH v3] proc/sysctl: add shared variables for range check

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 17 Apr 2019 15:15:31 +0200 Matteo Croce <mcroce@redhat.com> wrote:

> In the sysctl code the proc_dointvec_minmax() function is often used to
> validate the user supplied value between an allowed range. This function
> uses the extra1 and extra2 members from struct ctl_table as minimum and
> maximum allowed value.
> 
> On sysctl handler declaration, in every source file there are some readonly
> variables containing just an integer which address is assigned to the
> extra1 and extra2 members, so the sysctl range is enforced.
> 
> The special values 0, 1 and INT_MAX are very often used as range boundary,
> leading duplication of variables like zero=0, one=1, int_max=INT_MAX in
> different source files:
> 
>     $ git grep -E '\.extra[12].*&(zero|one|int_max)\b' |wc -l
>     245
> 
> This patch adds three const variables for the most commonly used values,
> and use them instead of creating a local one for every object file.
> 
> ...
>
> --- a/arch/s390/appldata/appldata_base.c
> +++ b/arch/s390/appldata/appldata_base.c
> @@ -220,15 +220,13 @@ appldata_timer_handler(struct ctl_table *ctl, int write,
>  			   void __user *buffer, size_t *lenp, loff_t *ppos)
>  {
>  	int timer_active = appldata_timer_active;
> -	int zero = 0;
> -	int one = 1;
>  	int rc;
>  	struct ctl_table ctl_entry = {
>  		.procname	= ctl->procname,
>  		.data		= &timer_active,
>  		.maxlen		= sizeof(int),
> -		.extra1		= &zero,
> -		.extra2		= &one,
> +		.extra1		= (void *)&sysctl_zero,
> +		.extra2		= (void *)&sysctl_one,
>  	};

Still not liking the casts :(

Did we decide whether making extra1&2 const void*'s was feasible?

I'm wondering if it would be better to do

extern const int sysctl_zero;
/* comment goes here */
#define SYSCTL_ZERO ((void *)&sysctl_zero)

and then use SYSCTL_ZERO everywhere.  That centralizes the ugliness and
makes it easier to switch over if/when extra1&2 are constified.

But it's all a bit sad and lame :(