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SubjectRe: [PATCH v2] proc/sysctl: add shared variables for range check
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On April 17, 2019 12:17:41 PM GMT+09:00, Andrew Morton <akpm@linux-foundation.org> wrote:
> On Wed, 17 Apr 2019 02:59:43 +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.
> >
>
> Nice. A few thoughts:
>
> > --- 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,
>
> The casts are ugly, and by casting away constness they introduce the
> risk that some errant could could change the value of 0, 1 and
> INT_MAX!
> Maybe - perhaps trying to do that would cause a segv but still,
> they're ugly.
>
> A proper fix would require changing extra1 and extra2 to const void *.
>
> Perhaps that would be unfeasibly extensive?
>

Hi Andrew,

I agree that the casts are ugly, but the "casts discards const qualifier" is way more ugly, so I have no choice.

I though about declaring extra1,2 as const, I quickly checked for code which write into these pointers and I found none, but I only looked for one, two and int_max values.

We could do a deeper search to see if other values are safe to turn to const.

> > ...
> >
> > --- a/fs/proc/proc_sysctl.c
> > +++ b/fs/proc/proc_sysctl.c
> > @@ -21,6 +21,11 @@ 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;
> >
> > +/* shared constants to be used in various sysctls */
> > +const int sysctl_zero = 0;
> > +const int sysctl_one = 1;
> > +const int sysctl_int_max = INT_MAX;
>
> Don't these require EXPORT_SYMBOL()?

Yes, for kernel modules, as the kbuild bot just pointed out.

Regards,
--
Matteo Croce
per aspera ad upstream

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