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

[Al Viro <viro@zeniv.linux.org.uk>]

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]; };
	struct foo __user *p;
	void f(struct bar __user *);
sparse does figure out that f(&p->y[1]) is a type error - &p->y[1] is
struct baz __user * and f() expects struct bar __user *.

It's not just mixing userland pointers with other things - it's not mixing
userland pointers to different types, etc.

In practice I've seen quite a few brainos caught by that...