Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > linux.kernel > #1670010

Re: __user with scalar data types

Show all headers | View raw

On Mon, Jun 19, 2017 at 09:46:37PM +0100, Al Viro wrote:
> On Mon, Jun 19, 2017 at 10:32:18PM +0200, Luc Van Oostenryck wrote:
> > 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').
> 
> Besides, suppose you add a special type for that.  How would it
> have to behave, really?  AFAICS, you want something similar to
> __bitwise, except that (assuming this type is T)
> 	T + integer => T
> 	T - integer => T
> 	T & integer => integer
> 	T | integer => T
> 	T - T => integer (quietly decay to underlying type for both
> arguments, then treat as normal -)
> 	T & T => T (probably, but might be worth a warning)
> 	T | T => T (ditto)
> 	comparison - same as for __bitwise
> 	constant conversion: 0 should convert clean, anything else - a warning
> 	cast to pointer => warn unless the target type is __user?  But that's
> not going to help with cast through uintptr_t...
> 	?: as usual
> 	any other arithmetics => warn and decay to underlying integer type

And how it should behave with typeof()?
Because it's already unclear to me what should be the result of:
	typeof(X {__user,__noderef,__nocast,__bitwise} [*])
and I don't think sparse do the right thing with this.

That said, I'm of the opinion that simply thinking about implementing this
special type is close to a capital sin.

-- Luc

Back to linux.kernel | Previous | Next — Previous in thread | Find similar | Unroll thread

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

csiph-web