linux-kernel - Re: [PATCH] staging: comedi: cast to (unsigned int *)

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Message-ID: <94917044-ba8f-7044-42f6-7e8d5ef1e10b@mev.co.uk>
Date:   Fri, 19 Feb 2021 09:31:16 +0000
From:   Ian Abbott <abbotti@....co.uk>
To:     David Laight <David.Laight@...LAB.COM>,
        Greg KH <gregkh@...uxfoundation.org>,
        Atul Gopinathan <atulgopinathan@...il.com>
Cc:     "devel@...verdev.osuosl.org" <devel@...verdev.osuosl.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] staging: comedi: cast to (unsigned int *)

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 It's kind of moot anyway because the patch is outdated.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.

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