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

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Date:   Fri, 2 Dec 2016 14:54:32 +0800
From:   Yisheng Xie <xieyisheng1@...wei.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
CC:     <linux-kernel@...r.kernel.org>, <acme@...hat.com>,
        <mgorman@...hsingularity.net>, <viro@...iv.linux.org.uk>,
        <hannes@...xchg.org>, <ebiederm@...ssion.com>,
        <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 2016/12/1 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 ?

Thanks
Yisheng Xie.

> 
> 
> 
> .
>