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

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Date:   Wed, 30 Nov 2016 13:33:58 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Yisheng Xie <xieyisheng1@...wei.com>
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 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?