linux-kernel - Re: [PATCH] kernel/sysctl: return -EINVAL if write invalid val to ulong type sysctl

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Date:   Fri, 02 Dec 2016 13:24:07 -0600
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Yisheng Xie <xieyisheng1@...wei.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        <linux-kernel@...r.kernel.org>, <acme@...hat.com>,
        <mgorman@...hsingularity.net>, <viro@...iv.linux.org.uk>,
        <hannes@...xchg.org>, <bristot@...hat.com>,
        <subashab@...eaurora.org>, <dcashman@...gle.com>, <w@....eu>,
        <arnd@...db.de>, <guohanjun@...wei.com>, <qiuxishi@...wei.com>
Subject: Re: [PATCH] kernel/sysctl: return -EINVAL if write invalid val to ulong type sysctl

Yisheng Xie <xieyisheng1@...wei.com> writes:

> On 2016/12/1 5:33, Andrew Morton wrote:
>> On Wed, 30 Nov 2016 18:30:52 +0800 Yisheng Xie <xieyisheng1@...wei.com> wrote:
>> 
>>> I tried to echo an invalid value to an unsigned long type sysctl on
>>> 4.9.0-rc6:
>>>    linux:~# cat /proc/sys/vm/user_reserve_kbytes
>>>    131072
>>>    linux:~# echo -1 > /proc/sys/vm/user_reserve_kbytes
>>>    linux:~# cat /proc/sys/vm/user_reserve_kbytes
>>>    131072
>>>
>>> The echo operation got error and the value do not write to
>>> user_reserve_kbytes, however, user do not know it until checking
>>> the value again.
>>>
>>> This patch return -EINVAL when write an invalid value to unsigned
>>> long type sysctl to make user know  what happened without
>>> checking its value once more, just as what proc_douintvec do.
>> 
>> hmpf.
>> 
>> # echo 18446744073709551615  > /proc/sys/vm/user_reserve_kbytes                                             
>> # cat /proc/sys/vm/user_reserve_kbytes
>> 18446744073709551615
>> 
>> I think that when taking in an unsigned long the kernel should simply
>> treat -1 as 0xffffffff (or 0xffffffffffffffff).  It's natural and
>> normal and everyone knows what it means?
>> 
> Hi Andrew,
> Thank you for your reply.
> Do you means it should be like this:
>   # echo -1 > /proc/sys/vm/user_reserve_kbytes
>   # cat /proc/sys/vm/user_reserve_kbytes
>   18446744073709551615
>
> I looks ok to me, however, I not sure whether other code in the kernel
> will also use its complement if user write a negative number for an
> unsigned long.  Does anyone have other opinion ?

Largely we need to be very careful with changing these functions as
they have been around for a long time, and have a very diverse set of
users.

So while changes are possible a reasonable argument needs to be made
that nothing in userspace cares.

Eric