rust: uaccess: use newtype for user pointers

Currently, Rust code uses a typedef for unsigned long to represent
userspace addresses. This is unfortunate because it means that userspace
addresses could accidentally be mixed up with other integers. To
alleviate that, we introduce a new UserPtr struct that wraps a raw
pointer to represent a userspace address. By using a struct, type
checking enforces that userspace addresses cannot be mixed up with
anything else.

This is similar to the __user annotation in C that detects cases where
user pointers are mixed with non-user pointers.

Note that unlike __user pointers in C, this type is just a pointer
without a target type. This means that it can't detect cases such as
mixing up which struct this user pointer references. However, that is
okay due to the way this is intended to be used - generally, you create
a UserPtr in your ioctl callback from the provided usize *before*
dispatching on which ioctl is in use, and then after dispatching on the
ioctl you pass the UserPtr into a UserSliceReader or UserSliceWriter;
selecting the target type does not happen until you have obtained the
UserSliceReader/Writer.

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.

Reviewed-by: Benno Lossin <lossin@kernel.org>
Reviewed-by: Danilo Krummrich <dakr@kernel.org>
Reviewed-by: Christian Schrefl <chrisi.schrefl@gmail.com>
Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20250616-userptr-newtype-v3-1-5ff7b2d18d9e@google.com
[ Reworded title. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

1 files changed, 2 insertions, 0 deletions

diff --git a/samples/rust/rust_misc_device.rs b/samples/rust/rust_misc_device.rs
index c881fd6dbd08..e7ab77448f75 100644
--- a/samples/rust/rust_misc_device.rs
+++ b/samples/rust/rust_misc_device.rs
@@ -176,6 +176,8 @@ impl MiscDevice for RustMiscDevice {
     fn ioctl(me: Pin<&RustMiscDevice>, _file: &File, cmd: u32, arg: usize) -> Result<isize> {
         dev_info!(me.dev, "IOCTLing Rust Misc Device Sample\n");
 
+        // Treat the ioctl argument as a user pointer.
+        let arg = UserPtr::from_addr(arg);
         let size = _IOC_SIZE(cmd);
 
         match cmd {