linux-kernel - Re: __user with scalar data types

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Date:   Mon, 19 Jun 2017 21:27:20 +0200
From:   Luc Van Oostenryck <luc.vanoostenryck@...il.com>
To:     Jordan Crouse <jcrouse@...eaurora.org>
Cc:     linux-sparse@...r.kernel.org, linux-kernel@...r.kernel.org,
        dri-devel@...ts.freedesktop.org
Subject: Re: __user with scalar data types

On Mon, Jun 19, 2017 at 10:15:09AM -0600, Jordan Crouse wrote:
> A number of us over in DRM land have been using __u64 scalar types
> to store pointers for uapi structures in accordance with Daniel Vetter's
> now classic treatise on ioctls:
> 
> http://blog.ffwll.ch/2013/11/botching-up-ioctls.html
> 
> A smaller number of us have further been marking the __u64 with __user,
> to wit:
> 
> struct uapistruct {
> 	...
> 	__u64 __user myptr;
> 	---
> };

It wouldn't make sense to have this:
	struct uapistruct {
		__u64 __user myptr;
		__u64        anothermember;
	};
In other words, eiter all members are in the user address space
or none are. So, a struct member should not be marked __user
(exactly as for 'const' or 'volatile').

It wouldn't also make sense to move the __user to the whole struct,
giving something like:
	struct uapistruct {
		__u64 myptr;
		__u64 anothermember;
	} __user;
because it's not the type that belong to the user address space
but some specific objects.

Of course, your real problem here is that you're using a __u64 to
store a pointer and then expect this __u64 to have some properties
unique to pointers.


-- Luc Van Oostenryck (sparse hacker)