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Re: __user with scalar data types

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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

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Thread

__user with scalar data types Jordan Crouse <jcrouse@codeaurora.org> - 2017-06-19 18:20 +0200
  Re: __user with scalar data types Al Viro <viro@ZenIV.linux.org.uk> - 2017-06-19 18:40 +0200
    Re: __user with scalar data types Daniel Vetter <daniel@ffwll.ch> - 2017-06-20 09:20 +0200
      Re: __user with scalar data types Gerd Hoffmann <kraxel@redhat.com> - 2017-06-20 09:50 +0200
        Re: __user with scalar data types Daniel Vetter <daniel@ffwll.ch> - 2017-06-20 10:40 +0200
  Re: __user with scalar data types Luc Van Oostenryck <luc.vanoostenryck@gmail.com> - 2017-06-19 21:30 +0200
  Re: __user with scalar data types Luc Van Oostenryck <luc.vanoostenryck@gmail.com> - 2017-06-19 22:40 +0200
    Re: __user with scalar data types Al Viro <viro@ZenIV.linux.org.uk> - 2017-06-19 22:50 +0200
      Re: __user with scalar data types Luc Van Oostenryck <luc.vanoostenryck@gmail.com> - 2017-06-20 00:40 +0200

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