rust: ptr: add panicking index projection variant

There have been a few cases where the programmer knows that the indices are
in bounds but the compiler cannot deduce that. This is also
compiler-version-dependent, so using build indexing here can be
problematic. On the other hand, it is also not ideal to use the fallible
variant, as it adds an error handling path that is never hit.

Add a new panicking index projection for this scenario. Like all panicking
operations, this should be used carefully only in cases where the user
knows the index is going to be in bounds, and panicking would indicate
something is catastrophically wrong.

To signify this, require users to explicitly denote the type of index being
used. The existing two types of index projections also gain the keyworded
version, which will be the recommended way going forward.

The keyworded syntax also paves the way of perhaps adding more flavors in
the future, e.g. `unsafe` index projection. However, unless the code is
extremely performance sensitive and bounds checking cannot be tolerated,
the panicking variant is safer and should be preferred, so it will be left
to the future when demand arises.

Signed-off-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Reviewed-by: Andreas Hindborg <a.hindborg@kernel.org>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Acked-by: Danilo Krummrich <dakr@kernel.org>
Link: https://patch.msgid.link/20260602-projection-syntax-rework-v2-3-6989470f5440@garyguo.net
[ Fixed broken intra-doc link. Added a few extra intra-doc links. Reworded
  some docs. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: nbdd0121

rust/kernel/dma.rs

@@ -1207,6 +1207,9 @@ macro_rules! dma_write {
     (@parse [$dma:expr] [$($proj:tt)*] [.$field:tt $($rest:tt)*]) => {
         $crate::dma_write!(@parse [$dma] [$($proj)* .$field] [$($rest)*])
     };
+    (@parse [$dma:expr] [$($proj:tt)*] [[$flavor:ident: $index:expr] $($rest:tt)*]) => {
+        $crate::dma_write!(@parse [$dma] [$($proj)* [$flavor: $index]] [$($rest)*])
+    };
     (@parse [$dma:expr] [$($proj:tt)*] [[$index:expr]? $($rest:tt)*]) => {
         $crate::dma_write!(@parse [$dma] [$($proj)* [$index]?] [$($rest)*])
     };

rust/kernel/ptr/projection.rs

@@ -26,14 +26,14 @@ impl From<OutOfBound> for Error {
 ///
 /// # Safety
 ///
-/// For given input pointer `slice` and return value `output`, the implementation of `build_index`
-/// and `get` (if [`Some`] is returned) must ensure that:
+/// For a given input pointer `slice` and return value `output`, the implementation of `index`,
+/// `build_index` and `get` (if [`Some`] is returned) must ensure that:
 /// - `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 `build_index`
-/// is also valid.
+/// This means that if the input pointer is valid, then the pointer returned by `get`, `index`
+/// or `build_index` is also valid.
 #[diagnostic::on_unimplemented(message = "`{Self}` cannot be used to index `{T}`")]
 #[doc(hidden)]
 pub unsafe trait ProjectIndex<T: ?Sized>: Sized {
@@ -42,6 +42,9 @@ pub unsafe trait ProjectIndex<T: ?Sized>: Sized {
     /// Returns an index-projected pointer, if in bounds.
     fn get(self, slice: *mut T) -> Option<*mut Self::Output>;
 
+    /// Returns an index-projected pointer; panic if out of bounds.
+    fn index(self, slice: *mut T) -> *mut Self::Output;
+
     /// Returns an index-projected pointer; fail the build if it cannot be proved to be in bounds.
     #[inline(always)]
     fn build_index(self, slice: *mut T) -> *mut Self::Output {
@@ -66,6 +69,11 @@ where
         <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)
+    }
+
     #[inline(always)]
     fn build_index(self, slice: *mut [T; N]) -> *mut Self::Output {
         <I as ProjectIndex<[T]>>::build_index(self, slice)
@@ -85,6 +93,16 @@ unsafe impl<T> ProjectIndex<[T]> for usize {
             Some(slice.cast::<T>().wrapping_add(self))
         }
     }
+
+    #[inline(always)]
+    fn index(self, slice: *mut [T]) -> *mut T {
+        // Leverage Rust built-in operators for bounds checking.
+        // SAFETY: All non-null and aligned pointers are valid for ZST read.
+        let zst_slice =
+            unsafe { core::slice::from_raw_parts::<()>(core::ptr::dangling(), slice.len()) };
+        let () = zst_slice[self];
+        slice.cast::<T>().wrapping_add(self)
+    }
 }
 
 // SAFETY: `get`-returned pointer has the same provenance as `slice` and the offset is checked to
@@ -103,6 +121,18 @@ unsafe impl<T> ProjectIndex<[T]> for core::ops::Range<usize> {
             new_len,
         ))
     }
+
+    #[inline(always)]
+    fn index(self, slice: *mut [T]) -> *mut [T] {
+        // Leverage Rust built-in operators for bounds checking.
+        // SAFETY: All non-null and aligned pointers are valid for ZST read.
+        let zst_slice =
+            unsafe { core::slice::from_raw_parts::<()>(core::ptr::dangling(), slice.len()) };
+        _ = zst_slice[self.clone()];
+
+        // SAFETY: Bounds checked.
+        unsafe { self.get(slice).unwrap_unchecked() }
+    }
 }
 
 // SAFETY: Safety requirement guaranteed by the forwarded impl.
@@ -113,6 +143,11 @@ unsafe impl<T> ProjectIndex<[T]> for core::ops::RangeTo<usize> {
     fn get(self, slice: *mut [T]) -> Option<*mut [T]> {
         (0..self.end).get(slice)
     }
+
+    #[inline(always)]
+    fn index(self, slice: *mut [T]) -> *mut [T] {
+        (0..self.end).index(slice)
+    }
 }
 
 // SAFETY: Safety requirement guaranteed by the forwarded impl.
@@ -123,6 +158,11 @@ unsafe impl<T> ProjectIndex<[T]> for core::ops::RangeFrom<usize> {
     fn get(self, slice: *mut [T]) -> Option<*mut [T]> {
         (self.start..slice.len()).get(slice)
     }
+
+    #[inline(always)]
+    fn index(self, slice: *mut [T]) -> *mut [T] {
+        (self.start..slice.len()).index(slice)
+    }
 }
 
 // SAFETY: `get` returned the pointer as is, so it always has the same provenance and offset of 0.
@@ -133,6 +173,11 @@ unsafe impl<T> ProjectIndex<[T]> for core::ops::RangeFull {
     fn get(self, slice: *mut [T]) -> Option<*mut [T]> {
         Some(slice)
     }
+
+    #[inline(always)]
+    fn index(self, slice: *mut [T]) -> *mut [T] {
+        slice
+    }
 }
 
 /// A helper trait to perform field projection.
@@ -210,10 +255,13 @@ unsafe impl<T: Deref> ProjectField<true> for T {
 /// 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.
+/// The `ptr::project!` macro can perform both fallible indexing and build-time checked indexing.
+/// The syntax is of the form `[<flavor>: index]` where `flavor` indicates the way of handling
+/// index out-of-bounds errors.
+/// - `try` will raise an [`OutOfBound`] error (which is convertible to [`ERANGE`]).
+/// - `build` will use the [`build_assert!`] mechanism to have the compiler validate the index is
+///   in bounds.
+/// - `panic` will cause a Rust [`panic!`] if the index goes out of bounds.
 ///
 /// # Examples
 ///
@@ -231,17 +279,21 @@ unsafe impl<T: Deref> ProjectField<true> for T {
 /// }
 /// ```
 ///
-/// Index projections are performed with `[index]`:
+/// Index projections are performed with `[<flavor>: index]`, where `flavor` is `try`, `build` or
+/// `panic`:
 ///
 /// ```
 /// fn proj(ptr: *const [u8; 32]) -> Result {
-///     let field_ptr: *const u8 = kernel::ptr::project!(ptr, [1]);
+///     let field_ptr: *const u8 = kernel::ptr::project!(ptr, [build: 1]);
 ///     // The following invocation, if uncommented, would fail the build.
 ///     //
-///     // kernel::ptr::project!(ptr, [128]);
+///     // kernel::ptr::project!(ptr, [build: 128]);
 ///
 ///     // This will raise an `OutOfBound` error (which is convertible to `ERANGE`).
-///     kernel::ptr::project!(ptr, [128]?);
+///     kernel::ptr::project!(ptr, [try: 128]);
+///
+///     // This will panic at runtime if executed.
+///     kernel::ptr::project!(ptr, [panic: 128]);
 ///     Ok(())
 /// }
 /// ```
@@ -251,7 +303,7 @@ unsafe impl<T: Deref> ProjectField<true> for T {
 /// ```
 /// let ptr: *const [u8; 32] = core::ptr::dangling();
 /// let field_ptr: Result<*const u8> = (|| -> Result<_> {
-///     Ok(kernel::ptr::project!(ptr, [128]?))
+///     Ok(kernel::ptr::project!(ptr, [try: 128]))
 /// })();
 /// assert!(field_ptr.is_err());
 /// ```
@@ -260,7 +312,7 @@ unsafe impl<T: Deref> ProjectField<true> for T {
 ///
 /// ```
 /// let ptr: *mut [(u8, u16); 32] = core::ptr::dangling_mut();
-/// let field_ptr: *mut u16 = kernel::ptr::project!(mut ptr, [1].1);
+/// let field_ptr: *mut u16 = kernel::ptr::project!(mut ptr, [build: 1].1);
 /// ```
 #[macro_export]
 macro_rules! project_pointer {
@@ -283,16 +335,30 @@ macro_rules! project_pointer {
         $crate::ptr::project!(@gen $ptr, $($rest)*)
     };
     // Fallible index projection.
-    (@gen $ptr:ident, [$index:expr]? $($rest:tt)*) => {
+    (@gen $ptr:ident, [try: $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)*)
     };
+    // Panicking index projection.
+    (@gen $ptr:ident, [panic: $index:expr] $($rest:tt)*) => {
+        let $ptr = $crate::ptr::projection::ProjectIndex::index($index, $ptr);
+        $crate::ptr::project!(@gen $ptr, $($rest)*)
+    };
     // Build-time checked index projection.
-    (@gen $ptr:ident, [$index:expr] $($rest:tt)*) => {
+    (@gen $ptr:ident, [build: $index:expr] $($rest:tt)*) => {
         let $ptr = $crate::ptr::projection::ProjectIndex::build_index($index, $ptr);
         $crate::ptr::project!(@gen $ptr, $($rest)*)
     };
+
+    // For compatibility
+    (@gen $ptr:ident, [$index:expr]? $($rest:tt)*) => {
+        $crate::ptr::project!(@gen $ptr, [try: $index] $($rest)*)
+    };
+    (@gen $ptr:ident, [$index:expr] $($rest:tt)*) => {
+        $crate::ptr::project!(@gen $ptr, [build: $index] $($rest)*)
+    };
+
     (mut $ptr:expr, $($proj:tt)*) => {{
         let ptr: *mut _ = $ptr;
         $crate::ptr::project!(@gen ptr, $($proj)*);