Re: [PATCH v2] uaccess: rust: use newtype for user pointers

[Boqun Feng <boqun.feng@gmail.com>]

On Wed, May 28, 2025 at 05:38:08PM +0200, Benno Lossin wrote:
> On Wed May 28, 2025 at 1:13 AM CEST, Boqun Feng wrote:
> > On Tue, May 27, 2025 at 11:12:11PM +0100, Al Viro wrote:
> >> On Tue, May 27, 2025 at 01:53:12PM +0000, Alice Ryhl wrote:
> >> > In C code we use sparse with the __user annotation to detect cases where
> >> > a user pointer is mixed up with other things. To replicate that, we
> >> > introduce a new struct UserPtr that serves the same purpose using the
> >> > newtype pattern.
> >> > 
> >> > The UserPtr type is not marked with #[derive(Debug)], which means that
> >> > it's not possible to print values of this type. This avoids ASLR
> >> > leakage.
> >> > 
> >> > The type is added to the prelude as it is a fairly fundamental type
> >> > similar to c_int. The wrapping_add() method is renamed to
> >> > wrapping_byte_add() for consistency with the method name found on raw
> >> > pointers.
> >> 
> >> That's considerably weaker than __user, though - with
> >> 	struct foo {struct bar x; struct baz y[2]; };
> >
> > Translate to Rust this is:
> >
> >     struct Foo {
> >         x: Bar,
> > 	y: Baz[2],
> >     }
> >
> >> 	struct foo __user *p;
> >
> > UserPtr should probably be generic over pointee, so:
> >
> >     pub struct UserPtr<T>(*mut c_void, PhantomData<*mut T>);
> >
> > and
> >
> >     let p: UserPtr<Foo> = ...;
> >
> >> 	void f(struct bar __user *);
> >
> > and this is:
> >
> >     pub fn f(bar: UserPtr<Bar>)
> >
> > and the checking should work, a (maybe unrelated) tricky part though..
> >
> >> sparse does figure out that f(&p->y[1]) is a type error - &p->y[1] is
> >
> > In Rust, you will need to play a little unsafe game to get &p->y[1]:
> >
> >     let foo_ptr: *mut Foo = p.as_mut_ptr();
> >     let y_ptr: *mut Baz = unsafe { addr_of_mut!((*foo_ptr).y[1]) };
> >     let y: UserPtr<Baz> = unsafe { UserPtr::from_ptr(y_ptr) };
> 
> Shouldn't this use `wrapping_add` since the pointer shouldn't be
> dereferenced?
> 

Good point, so:

     let foo_ptr: *mut Foo = p.as_mut_ptr();
     let y_ptr: *mut Baz = foo_ptr.wrapping_byte_add(offset_of!(Foo, y[1]));
     let y: UserPtr<Baz> = unsafe { UserPtr::from_ptr(y_ptr) };

(using wrapping_byte_add() + offset_of!())

Regards,
Boqun

> If we don't use `wrapping_add`, then the field projection operation for
> this type must be `unsafe`.
> 
> ---
> Cheers,
> Benno