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

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