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

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Date:   Wed, 17 Apr 2019 08:49:00 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Matteo Croce <mcroce@...hat.com>
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 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.