rust: ptr: add projection infrastructure

Add a generic infrastructure for performing field and index projections on
raw pointers. This will form the basis of performing I/O projections.

Pointers manipulations are intentionally using the safe wrapping variants
instead of the unsafe variants, as the latter requires pointers to be
inside an allocation which is not necessarily true for I/O pointers.

This projection macro protects against rogue `Deref` implementation, which
can causes the projected pointer to be outside the bounds of starting
pointer. This is extremely unlikely and Rust has a lint to catch this, but
is unsoundness regardless. The protection works by inducing type inference
ambiguity when `Deref` is implemented.

This projection macro also stops projecting into unaligned fields (i.e.
fields of `#[repr(packed)]` structs), as misaligned pointers require
special handling. This is implemented by attempting to create reference to
projected field inside a `if false` block. Despite being unreachable, Rust
still checks that they're not unaligned fields.

The projection macro supports both fallible and infallible index
projections. These are described in detail inside the documentation.

Signed-off-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Benno Lossin <lossin@kernel.org>
Acked-by: Miguel Ojeda <ojeda@kernel.org>
Link: https://patch.msgid.link/20260302164239.284084-3-gary@kernel.org
[ * Add intro-doc links where possible,
  * Fix typos and slightly improve wording, e.g. "as documentation
    describes" -> "as the documentation of [`Self::proj`] describes",
  * Add an empty line between regular and safety comments, before
    examples, and between logically independent comments,
  * Capitalize various safety comments.

    - Danilo ]
Signed-off-by: Danilo Krummrich <dakr@kernel.org>

4 files changed, 314 insertions, 1 deletions

diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index 510cc7fe4961..d93292d47420 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -38,6 +38,9 @@
 #![feature(const_ptr_write)]
 #![feature(const_refs_to_cell)]
 //
+// Stable since Rust 1.84.0.
+#![feature(strict_provenance)]
+//
 // Expected to become stable.
 #![feature(arbitrary_self_types)]
 //
diff --git a/rust/kernel/ptr.rs b/rust/kernel/ptr.rs
index e91de5f0d78f..bdc2d79ff669 100644
--- a/rust/kernel/ptr.rs
+++ b/rust/kernel/ptr.rs
@@ -2,6 +2,9 @@
 
 //! Types and functions to work with pointers and addresses.
 
+pub mod projection;
+pub use crate::project_pointer as project;
+
 use core::mem::{
     align_of,
     size_of, //
diff --git a/rust/kernel/ptr/projection.rs b/rust/kernel/ptr/projection.rs
new file mode 100644
index 000000000000..140ea8e21617
--- /dev/null
+++ b/rust/kernel/ptr/projection.rs
@@ -0,0 +1,305 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Infrastructure for handling projections.
+
+use core::{
+    mem::MaybeUninit,
+    ops::Deref, //
+};
+
+use crate::prelude::*;
+
+/// Error raised when a projection is attempted on an array or slice out of bounds.
+pub struct OutOfBound;
+
+impl From<OutOfBound> for Error {
+    #[inline(always)]
+    fn from(_: OutOfBound) -> Self {
+        ERANGE
+    }
+}
+
+/// A helper trait to perform index projection.
+///
+/// This is similar to [`core::slice::SliceIndex`], but operates on raw pointers safely and
+/// fallibly.
+///
+/// # Safety
+///
+/// The implementation of `index` and `get` (if [`Some`] is returned) must ensure that, if provided
+/// input pointer `slice` and returned pointer `output`, then:
+/// - `output` has the same provenance as `slice`;
+/// - `output.byte_offset_from(slice)` is between 0 to
+///   `KnownSize::size(slice) - KnownSize::size(output)`.
+///
+/// This means that if the input pointer is valid, then pointer returned by `get` or `index` is
+/// also valid.
+#[diagnostic::on_unimplemented(message = "`{Self}` cannot be used to index `{T}`")]
+#[doc(hidden)]
+pub unsafe trait ProjectIndex<T: ?Sized>: Sized {
+    type Output: ?Sized;
+
+    /// Returns an index-projected pointer, if in bounds.
+    fn get(self, slice: *mut T) -> Option<*mut Self::Output>;
+
+    /// Returns an index-projected pointer; fail the build if it cannot be proved to be in bounds.
+    #[inline(always)]
+    fn index(self, slice: *mut T) -> *mut Self::Output {
+        Self::get(self, slice).unwrap_or_else(|| build_error!())
+    }
+}
+
+// Forward array impl to slice impl.
+//
+// SAFETY: Safety requirement guaranteed by the forwarded impl.
+unsafe impl<T, I, const N: usize> ProjectIndex<[T; N]> for I
+where
+    I: ProjectIndex<[T]>,
+{
+    type Output = <I as ProjectIndex<[T]>>::Output;
+
+    #[inline(always)]
+    fn get(self, slice: *mut [T; N]) -> Option<*mut Self::Output> {
+        <I as ProjectIndex<[T]>>::get(self, slice)
+    }
+
+    #[inline(always)]
+    fn index(self, slice: *mut [T; N]) -> *mut Self::Output {
+        <I as ProjectIndex<[T]>>::index(self, slice)
+    }
+}
+
+// SAFETY: `get`-returned pointer has the same provenance as `slice` and the offset is checked to
+// not exceed the required bound.
+unsafe impl<T> ProjectIndex<[T]> for usize {
+    type Output = T;
+
+    #[inline(always)]
+    fn get(self, slice: *mut [T]) -> Option<*mut T> {
+        if self >= slice.len() {
+            None
+        } else {
+            Some(slice.cast::<T>().wrapping_add(self))
+        }
+    }
+}
+
+// SAFETY: `get`-returned pointer has the same provenance as `slice` and the offset is checked to
+// not exceed the required bound.
+unsafe impl<T> ProjectIndex<[T]> for core::ops::Range<usize> {
+    type Output = [T];
+
+    #[inline(always)]
+    fn get(self, slice: *mut [T]) -> Option<*mut [T]> {
+        let new_len = self.end.checked_sub(self.start)?;
+        if self.end > slice.len() {
+            return None;
+        }
+        Some(core::ptr::slice_from_raw_parts_mut(
+            slice.cast::<T>().wrapping_add(self.start),
+            new_len,
+        ))
+    }
+}
+
+// SAFETY: Safety requirement guaranteed by the forwarded impl.
+unsafe impl<T> ProjectIndex<[T]> for core::ops::RangeTo<usize> {
+    type Output = [T];
+
+    #[inline(always)]
+    fn get(self, slice: *mut [T]) -> Option<*mut [T]> {
+        (0..self.end).get(slice)
+    }
+}
+
+// SAFETY: Safety requirement guaranteed by the forwarded impl.
+unsafe impl<T> ProjectIndex<[T]> for core::ops::RangeFrom<usize> {
+    type Output = [T];
+
+    #[inline(always)]
+    fn get(self, slice: *mut [T]) -> Option<*mut [T]> {
+        (self.start..slice.len()).get(slice)
+    }
+}
+
+// SAFETY: `get` returned the pointer as is, so it always has the same provenance and offset of 0.
+unsafe impl<T> ProjectIndex<[T]> for core::ops::RangeFull {
+    type Output = [T];
+
+    #[inline(always)]
+    fn get(self, slice: *mut [T]) -> Option<*mut [T]> {
+        Some(slice)
+    }
+}
+
+/// A helper trait to perform field projection.
+///
+/// This trait has a `DEREF` generic parameter so it can be implemented twice for types that
+/// implement [`Deref`]. This will cause an ambiguity error and thus block [`Deref`] types being
+/// used as base of projection, as they can inject unsoundness. Users therefore must not specify
+/// `DEREF` and should always leave it to be inferred.
+///
+/// # Safety
+///
+/// `proj` may only invoke `f` with a valid allocation, as the documentation of [`Self::proj`]
+/// describes.
+#[doc(hidden)]
+pub unsafe trait ProjectField<const DEREF: bool> {
+    /// Project a pointer to a type to a pointer of a field.
+    ///
+    /// `f` may only be invoked with a valid allocation so it can safely obtain raw pointers to
+    /// fields using `&raw mut`.
+    ///
+    /// This is needed because `base` might not point to a valid allocation, while `&raw mut`
+    /// requires pointers to be in bounds of a valid allocation.
+    ///
+    /// # Safety
+    ///
+    /// `f` must return a pointer in bounds of the provided pointer.
+    unsafe fn proj<F>(base: *mut Self, f: impl FnOnce(*mut Self) -> *mut F) -> *mut F;
+}
+
+// NOTE: in theory, this API should work for `T: ?Sized` and `F: ?Sized`, too. However, we cannot
+// currently support that as we need to obtain a valid allocation that `&raw const` can operate on.
+//
+// SAFETY: `proj` invokes `f` with valid allocation.
+unsafe impl<T> ProjectField<false> for T {
+    #[inline(always)]
+    unsafe fn proj<F>(base: *mut Self, f: impl FnOnce(*mut Self) -> *mut F) -> *mut F {
+        // Create a valid allocation to start projection, as `base` is not necessarily so. The
+        // memory is never actually used so it will be optimized out, so it should work even for
+        // very large `T` (`memoffset` crate also relies on this). To be extra certain, we also
+        // annotate `f` closure with `#[inline(always)]` in the macro.
+        let mut place = MaybeUninit::uninit();
+        let place_base = place.as_mut_ptr();
+        let field = f(place_base);
+        // SAFETY: `field` is in bounds from `base` per safety requirement.
+        let offset = unsafe { field.byte_offset_from(place_base) };
+        // Use `wrapping_byte_offset` as `base` does not need to be of valid allocation.
+        base.wrapping_byte_offset(offset).cast()
+    }
+}
+
+// SAFETY: Vacuously satisfied.
+unsafe impl<T: Deref> ProjectField<true> for T {
+    #[inline(always)]
+    unsafe fn proj<F>(_: *mut Self, _: impl FnOnce(*mut Self) -> *mut F) -> *mut F {
+        build_error!("this function is a guard against `Deref` impl and is never invoked");
+    }
+}
+
+/// Create a projection from a raw pointer.
+///
+/// The projected pointer is within the memory region marked by the input pointer. There is no
+/// requirement that the input raw pointer needs to be valid, so this macro may be used for
+/// projecting pointers outside normal address space, e.g. I/O pointers. However, if the input
+/// pointer is valid, the projected pointer is also valid.
+///
+/// Supported projections include field projections and index projections.
+/// It is not allowed to project into types that implement custom [`Deref`] or
+/// [`Index`](core::ops::Index).
+///
+/// The macro has basic syntax of `kernel::ptr::project!(ptr, projection)`, where `ptr` is an
+/// expression that evaluates to a raw pointer which serves as the base of projection. `projection`
+/// can be a projection expression of form `.field` (normally identifier, or numeral in case of
+/// tuple structs) or of form `[index]`.
+///
+/// If a mutable pointer is needed, the macro input can be prefixed with the `mut` keyword, i.e.
+/// `kernel::ptr::project!(mut ptr, projection)`. By default, a const pointer is created.
+///
+/// `ptr::project!` macro can perform both fallible indexing and build-time checked indexing.
+/// `[index]` form performs build-time bounds checking; if compiler fails to prove `[index]` is in
+/// bounds, compilation will fail. `[index]?` can be used to perform runtime bounds checking;
+/// `OutOfBound` error is raised via `?` if the index is out of bounds.
+///
+/// # Examples
+///
+/// Field projections are performed with `.field_name`:
+///
+/// ```
+/// struct MyStruct { field: u32, }
+/// let ptr: *const MyStruct = core::ptr::dangling();
+/// let field_ptr: *const u32 = kernel::ptr::project!(ptr, .field);
+///
+/// struct MyTupleStruct(u32, u32);
+///
+/// fn proj(ptr: *const MyTupleStruct) {
+///     let field_ptr: *const u32 = kernel::ptr::project!(ptr, .1);
+/// }
+/// ```
+///
+/// Index projections are performed with `[index]`:
+///
+/// ```
+/// fn proj(ptr: *const [u8; 32]) -> Result {
+///     let field_ptr: *const u8 = kernel::ptr::project!(ptr, [1]);
+///     // The following invocation, if uncommented, would fail the build.
+///     //
+///     // kernel::ptr::project!(ptr, [128]);
+///
+///     // This will raise an `OutOfBound` error (which is convertible to `ERANGE`).
+///     kernel::ptr::project!(ptr, [128]?);
+///     Ok(())
+/// }
+/// ```
+///
+/// If you need to match on the error instead of propagate, put the invocation inside a closure:
+///
+/// ```
+/// let ptr: *const [u8; 32] = core::ptr::dangling();
+/// let field_ptr: Result<*const u8> = (|| -> Result<_> {
+///     Ok(kernel::ptr::project!(ptr, [128]?))
+/// })();
+/// assert!(field_ptr.is_err());
+/// ```
+///
+/// For mutable pointers, put `mut` as the first token in macro invocation.
+///
+/// ```
+/// let ptr: *mut [(u8, u16); 32] = core::ptr::dangling_mut();
+/// let field_ptr: *mut u16 = kernel::ptr::project!(mut ptr, [1].1);
+/// ```
+#[macro_export]
+macro_rules! project_pointer {
+    (@gen $ptr:ident, ) => {};
+    // Field projection. `$field` needs to be `tt` to support tuple index like `.0`.
+    (@gen $ptr:ident, .$field:tt $($rest:tt)*) => {
+        // SAFETY: The provided closure always returns an in-bounds pointer.
+        let $ptr = unsafe {
+            $crate::ptr::projection::ProjectField::proj($ptr, #[inline(always)] |ptr| {
+                // Check unaligned field. Not all users (e.g. DMA) can handle unaligned
+                // projections.
+                if false {
+                    let _ = &(*ptr).$field;
+                }
+                // SAFETY: `$field` is in bounds, and no implicit `Deref` is possible (if the
+                // type implements `Deref`, Rust cannot infer the generic parameter `DEREF`).
+                &raw mut (*ptr).$field
+            })
+        };
+        $crate::ptr::project!(@gen $ptr, $($rest)*)
+    };
+    // Fallible index projection.
+    (@gen $ptr:ident, [$index:expr]? $($rest:tt)*) => {
+        let $ptr = $crate::ptr::projection::ProjectIndex::get($index, $ptr)
+            .ok_or($crate::ptr::projection::OutOfBound)?;
+        $crate::ptr::project!(@gen $ptr, $($rest)*)
+    };
+    // Build-time checked index projection.
+    (@gen $ptr:ident, [$index:expr] $($rest:tt)*) => {
+        let $ptr = $crate::ptr::projection::ProjectIndex::index($index, $ptr);
+        $crate::ptr::project!(@gen $ptr, $($rest)*)
+    };
+    (mut $ptr:expr, $($proj:tt)*) => {{
+        let ptr: *mut _ = $ptr;
+        $crate::ptr::project!(@gen ptr, $($proj)*);
+        ptr
+    }};
+    ($ptr:expr, $($proj:tt)*) => {{
+        let ptr = <*const _>::cast_mut($ptr);
+        // We currently always project using mutable pointer, as it is not decided whether `&raw
+        // const` allows the resulting pointer to be mutated (see documentation of `addr_of!`).
+        $crate::ptr::project!(@gen ptr, $($proj)*);
+        ptr.cast_const()
+    }};
+}
diff --git a/scripts/Makefile.build b/scripts/Makefile.build
index 32e209bc7985..3652b85be545 100644
--- a/scripts/Makefile.build
+++ b/scripts/Makefile.build
@@ -310,16 +310,18 @@ $(obj)/%.lst: $(obj)/%.c FORCE
 
 # The features in this list are the ones allowed for non-`rust/` code.
 #
+#   - Stable since Rust 1.79.0: `feature(slice_ptr_len)`.
 #   - Stable since Rust 1.81.0: `feature(lint_reasons)`.
 #   - Stable since Rust 1.82.0: `feature(asm_const)`,
 #     `feature(offset_of_nested)`, `feature(raw_ref_op)`.
+#   - Stable since Rust 1.84.0: `feature(strict_provenance)`.
 #   - Stable since Rust 1.87.0: `feature(asm_goto)`.
 #   - Expected to become stable: `feature(arbitrary_self_types)`.
 #   - To be determined: `feature(used_with_arg)`.
 #
 # Please see https://github.com/Rust-for-Linux/linux/issues/2 for details on
 # the unstable features in use.
-rust_allowed_features := asm_const,asm_goto,arbitrary_self_types,lint_reasons,offset_of_nested,raw_ref_op,used_with_arg
+rust_allowed_features := asm_const,asm_goto,arbitrary_self_types,lint_reasons,offset_of_nested,raw_ref_op,slice_ptr_len,strict_provenance,used_with_arg
 
 # `--out-dir` is required to avoid temporaries being created by `rustc` in the
 # current working directory, which may be not accessible in the out-of-tree