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

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Date:   Tue, 6 Dec 2016 20:50:24 +0800
From:   Yisheng Xie <xieyisheng1@...wei.com>
To:     "Eric W. Biederman" <ebiederm@...ssion.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



On 2016/12/3 3:24, Eric W. Biederman wrote:
> 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.
Hi Eric,
Thanks for your reply.
That right.

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

So the patch's original aim that return -EINVAL when user write
invalid val to ulong type sysctl is more reasonable, Right?

Thanks,
Yisheng Xie

> Eric
> 
> .
>