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

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Date:   Thu, 18 Apr 2019 23:28:10 +0900
From:   Matteo Croce <mcroce@...hat.com>
To:     Matthew Wilcox <willy@...radead.org>
CC:     LKML <linux-kernel@...r.kernel.org>, linux-fsdevel@...r.kernel.org,
        Kees Cook <keescook@...omium.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v3] proc/sysctl: add shared variables for range check

On April 18, 2019 12:49:00 AM GMT+09:00, Matthew Wilcox <willy@...radead.org> wrote:
> On Wed, Apr 17, 2019 at 03:15:31PM +0200, Matteo Croce 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.
> 
> Does this actually cause the kernel size to shrink?  EXPORT_SYMBOL
> isn't
> free, you know.

Hi Matthew,

In this case we have three __kstrtab_ symbols of size 11,12 and 15, and a bunch of u32 removed, so the size should shrink anyway.
I will try to calculate the exact saving with bloat-o-meter.

Regards,
-- 
Matteo Croce
per aspera ad upstream