[RFC PATCH v4 3/4] rust: validate: add `Validate` trait

[Benno Lossin <lossin@kernel.org>]

From: Benno Lossin <benno.lossin@proton.me>

Introduce the `Validate<Input>` trait and functions to validate
`Untrusted<T>` using said trait. This allows one to access the inner
value of `Untrusted<T>` via `validate{,_ref,_mut}` functions which
subsequently delegate the validation to user-implemented `Validate`
trait.

The `Validate` trait is the only entry point for validation code, making
it easy to spot where data is being validated.

The reason for restricting the types that can be inputs to
`Validate::validate` is to be able to have the `validate...` functions
on `Untrusted`. This is also the reason for the suggestions in the
`Usage in API Design` section in the commit that introduced
`Untrusted<T>`.

Signed-off-by: Benno Lossin <benno.lossin@proton.me>
---
 rust/kernel/validate.rs | 70 ++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 69 insertions(+), 1 deletion(-)

diff --git a/rust/kernel/validate.rs b/rust/kernel/validate.rs
index 1f75ccb79532..2a582a572aa5 100644
--- a/rust/kernel/validate.rs
+++ b/rust/kernel/validate.rs
@@ -11,6 +11,9 @@
 //! APIs that write back into userspace usually allow writing untrusted bytes directly, allowing
 //! direct copying of untrusted user data back into userspace without validation.
 //!
+//! The only way to access untrusted data is to [`Validate::validate`] it. This is facilitated by
+//! the [`Validate`] trait.
+//!
 //! # Rationale
 //!
 //! When reading data from an untrusted source, it must be validated before it can be used for
@@ -46,7 +49,7 @@
 /// untrusted, as it would otherwise violate normal Rust rules. For this reason, one can easily
 /// convert that reference to `&[Untrusted<u8>]`. Another such example is `Untrusted<KVec<T>>`, it
 /// derefs to `KVec<Untrusted<T>>`. Raw bytes however do not behave in this way, `Untrusted<u8>` is
-/// totally opaque.
+/// totally opaque and one can only access its value by calling [`Untrusted::validate()`].
 ///
 /// # Usage in API Design
 ///
@@ -101,6 +104,30 @@ pub fn new(value: T) -> Self
     {
         Self(value)
     }
+
+    /// Validate the underlying untrusted data.
+    ///
+    /// See the [`Validate`] trait for more information.
+    pub fn validate<V: Validate<Self>>(self) -> Result<V, V::Err>
+    where
+        T: Sized,
+    {
+        V::validate(self.0)
+    }
+
+    /// Validate the underlying untrusted data.
+    ///
+    /// See the [`Validate`] trait for more information.
+    pub fn validate_ref<'a, V: Validate<&'a Self>>(&'a self) -> Result<V, V::Err> {
+        V::validate(&self.0)
+    }
+
+    /// Validate the underlying untrusted data.
+    ///
+    /// See the [`Validate`] trait for more information.
+    pub fn validate_mut<'a, V: Validate<&'a mut Self>>(&'a mut self) -> Result<V, V::Err> {
+        V::validate(&mut self.0)
+    }
 }
 
 impl<T> Deref for Untrusted<[T]> {
@@ -140,3 +167,44 @@ fn deref_mut(&mut self) -> &mut Self::Target {
         unsafe { &mut *ptr }
     }
 }
+
+/// Marks valid input for the [`Validate`] trait.
+pub trait ValidateInput: private::Sealed {
+    /// Type of the inner data.
+    type Inner: ?Sized;
+}
+
+impl<T: ?Sized> ValidateInput for Untrusted<T> {
+    type Inner = T;
+}
+
+impl<'a, T: ?Sized> ValidateInput for &'a Untrusted<T> {
+    type Inner = &'a T;
+}
+
+impl<'a, T: ?Sized> ValidateInput for &'a mut Untrusted<T> {
+    type Inner = &'a mut T;
+}
+
+mod private {
+    use super::Untrusted;
+
+    pub trait Sealed {}
+
+    impl<T: ?Sized> Sealed for Untrusted<T> {}
+    impl<'a, T: ?Sized> Sealed for &'a Untrusted<T> {}
+    impl<'a, T: ?Sized> Sealed for &'a mut Untrusted<T> {}
+}
+
+/// Validate [`Untrusted`] data.
+///
+/// Care must be taken when implementing this trait, as unprotected access to unvalidated data is
+/// given to the [`Validate::validate`] function. The implementer must ensure that the data is only
+/// used for logic after successful validation.
+pub trait Validate<Input: ValidateInput>: Sized {
+    /// Validation error.
+    type Err;
+
+    /// Validate the raw input.
+    fn validate(raw: Input::Inner) -> Result<Self, Self::Err>;
+}
-- 
2.50.1