linux-kernel - Re: __user with scalar data types

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Date:   Mon, 19 Jun 2017 22:32:18 +0200
From:   Luc Van Oostenryck <luc.vanoostenryck@...il.com>
To:     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:
> struct uapistruct {
> 	...
> 	__u64 __user myptr;
> 	---
> };
> 
> And then converting it for use in the kernel as such:
> 
> {
> 	void __user *userptr = (void __user *)(uintptr_t)args->myptr;
> 
> 	copy_from_user(local, userptr, size);
> 	...
> }
> 
> The problem is that sparse doesn't like the momentary switch to
> uintptr_t:
> 
> 	warning: dereference of noderef expression

This warning doesn't come from the cast to uintptr_t but
simply from dereferencing the field which can't be dereferenced
since it's marked as '__user'. In other words, doing
'args->myptr' rightfully trigger the warning and no cast
will or should stop that.

Also, you can't expect the '__user' to be transmitted from
'myptr' to the pointer (without taking the address of 'myptr').
It's exactly like 'const int' vs. 'const int *': the '__user' or
the 'const' is not at the same level in the type hierarchy
('const object' vs. 'non-const pointer to const object').

-- Luc Van Oostenryck