linux-kernel - Re: __user with scalar data types

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Date:   Tue, 20 Jun 2017 00:39:41 +0200
From:   Luc Van Oostenryck <luc.vanoostenryck@...il.com>
To:     Al Viro <viro@...IV.linux.org.uk>
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 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