linux-kernel - Re: [PATCH v4 1/6] sysctl: Add flags to support min/max range clamping

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Message-ID: <53093737-776f-d455-5651-3a57d5ac7be9@redhat.com>
Date:   Mon, 12 Mar 2018 16:48:07 -0400
From:   Waiman Long <longman@...hat.com>
To:     "Luis R. Rodriguez" <mcgrof@...nel.org>
Cc:     Kees Cook <keescook@...omium.org>, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH v4 1/6] sysctl: Add flags to support min/max range
 clamping

On 03/12/2018 04:44 PM, Luis R. Rodriguez wrote:
> On Mon, Mar 12, 2018 at 04:15:39PM -0400, Waiman Long wrote:
>> When minimum/maximum values are specified for a sysctl parameter in
>> the ctl_table structure with proc_dointvec_minmax() handler, update
>> to that parameter will fail with error if the given value is outside
>> of the required range.
>>
>> There are use cases where it may be better to clamp the value of
>> the sysctl parameter to the given range without failing the update,
>> especially if the users are not aware of the actual range limits.
>> Reading the value back after the update will now be a good practice
>> to see if the provided value exceeds the range limits.
>>
>> To provide this less restrictive form of range checking, a new flags
>> field is added to the ctl_table structure.
>>
>> When the CTL_FLAGS_CLAMP_RANGE flag is set in the ctl_table
>> entry, any update from the userspace will be clamped to the given
>> range without error if either the proc_dointvec_minmax() or the
>> proc_douintvec_minmax() handlers is used.
> You keep missing to document both on commit log and kdoc which
> end on the range is selected if you happen to go over. To be clear
> it is unclear from reading the commit log if a default is set if
> you go over if you pick another value. In this case it is conditional
> if you go over we pick the high range max, and if you go below the
> lower range minimum set allowed.
>
> What happens if no low range is set and that is what the issue in
> terms of range triggers?

Sorry for missing that.

If a minimum value is not specified, no minimum checking and clamping
will be done. Similarly for maximum. I will clarify that.

Cheers,
Longman