Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Subject: Re: [PATCH] staging: comedi: cast to (unsigned int *)
To:     David Laight <David.Laight@ACULAB.COM>,
        Greg KH <gregkh@linuxfoundation.org>,
        Atul Gopinathan <atulgopinathan@gmail.com>
Cc:     "devel@driverdev.osuosl.org" <devel@driverdev.osuosl.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
References: <20210217165907.9777-1-atulgopinathan@gmail.com>
 <YC1T06VCh0K2BBW5@kroah.com> <20210217181000.GB10124@atulu-ubuntu>
 <YC1fzjVOwiqzO1nb@kroah.com> <3cfef23d-8d4a-205c-61e8-cbe8c9a0c0f4@mev.co.uk>
 <21a953261eb44e7ba302cfe74d8efa2d@AcuMS.aculab.com>
From:   Ian Abbott <abbotti@mev.co.uk>
Organization: MEV Ltd.
Message-ID: <94917044-ba8f-7044-42f6-7e8d5ef1e10b@mev.co.uk>
Date:   Fri, 19 Feb 2021 09:31:16 +0000
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101
 Thunderbird/78.7.1
MIME-Version: 1.0
In-Reply-To: <21a953261eb44e7ba302cfe74d8efa2d@AcuMS.aculab.com>
Content-Type: text/plain; charset=utf-8
Content-Language: en-GB
Content-Transfer-Encoding: 7bit
Precedence: bulk

On 19/02/2021 09:03, David Laight wrote:
>> It's kind of moot anyway because the patch is outdated.  But the reason
>> for the ___force is that the same `struct comedi_cmd` is used in both
>> user and kernel contexts.  In user contexts, the `chanlist` member
>> points to user memory and in kernel contexts it points to kernel memory
>> (copied from userspace).
> 
> Can't you use a union of the user and kernel pointers?
> (Possibly even anonymous?)
> Although, ideally, keeping them in separate fields is better.
> 8 bytes for a pointer isn't going make a fat lot of difference.

This is for a UAPI header (eventually), so cannot add a new field.  For
an anonymous union, one tagged with __user and one not, the __user tag
would be removed during conversion from UAPI headers to
/usr/include/linux headers, leaving a union of two identically typed
members, which would look a bit odd.  The union also kind of hides the
problem.

-- 
-=( Ian Abbott <abbotti@mev.co.uk> || MEV Ltd. is a company  )=-
-=( registered in England & Wales.  Regd. number: 02862268.  )=-
-=( Regd. addr.: S11 & 12 Building 67, Europa Business Park, )=-
-=( Bird Hall Lane, STOCKPORT, SK3 0XA, UK. || www.mev.co.uk )=-