[PATCH 01/20] rust: alloc: add `Allocator` trait

[Danilo Krummrich <dakr@redhat.com>]

Add a kernel specific `Allocator` trait, that in contrast to the one in
Rust's core library doesn't require unstable features and supports GFP
flags.

Subsequent patches add the following trait implementors: `Kmalloc`,
`Vmalloc` and `KVmalloc`.

Signed-off-by: Danilo Krummrich <dakr@redhat.com>
---
 rust/kernel/alloc.rs | 73 ++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 73 insertions(+)

diff --git a/rust/kernel/alloc.rs b/rust/kernel/alloc.rs
index 531b5e471cb1..462e00982510 100644
--- a/rust/kernel/alloc.rs
+++ b/rust/kernel/alloc.rs
@@ -11,6 +11,7 @@
 /// Indicates an allocation error.
 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
 pub struct AllocError;
+use core::{alloc::Layout, ptr, ptr::NonNull};
 
 /// Flags to be used when allocating memory.
 ///
@@ -71,3 +72,75 @@ pub mod flags {
     /// small allocations.
     pub const GFP_NOWAIT: Flags = Flags(bindings::GFP_NOWAIT);
 }
+
+/// The kernel's [`Allocator`] trait.
+///
+/// An implementation of [`Allocator`] can allocate, re-allocate and free memory buffer described
+/// via [`Layout`].
+///
+/// [`Allocator`] is designed to be implemented on ZSTs; its safety requirements to not allow for
+/// keeping a state throughout an instance.
+///
+/// # Safety
+///
+/// Memory returned from an allocator must point to a valid memory buffer and remain valid until
+/// its explicitly freed.
+///
+/// Copying, cloning, or moving the allocator must not invalidate memory blocks returned from this
+/// allocator. A copied, cloned or even new allocator of the same type must behave like the same
+/// allocator, and any pointer to a memory buffer which is currently allocated may be passed to any
+/// other method of the allocator.
+pub unsafe trait Allocator {
+    /// Allocate memory based on `layout` and `flags`.
+    ///
+    /// On success, returns a buffer represented as `NonNull<[u8]>` that satisfies the size an
+    /// alignment requirements of layout, but may exceed the requested size.
+    ///
+    /// This function is equivalent to `realloc` when called with a NULL pointer and an `old_size`
+    /// of `0`.
+    fn alloc(&self, layout: Layout, flags: Flags) -> Result<NonNull<[u8]>, AllocError> {
+        // SAFETY: Passing a NULL pointer to `realloc` is valid by it's safety requirements and asks
+        // for a new memory allocation.
+        unsafe { self.realloc(ptr::null_mut(), 0, layout, flags) }
+    }
+
+    /// Re-allocate an existing memory allocation to satisfy the requested `layout`. If the
+    /// requested size is zero, `realloc` behaves equivalent to `free`.
+    ///
+    /// If the requested size is larger than `old_size`, a successful call to `realloc` guarantees
+    /// that the new or grown buffer has at least `Layout::size` bytes, but may also be larger.
+    ///
+    /// If the requested size is smaller than `old_size`, `realloc` may or may not shrink the
+    /// buffer; this is implementation specific to the allocator.
+    ///
+    /// On allocation failure, the existing buffer, if any, remains valid.
+    ///
+    /// The buffer is represented as `NonNull<[u8]>`.
+    ///
+    /// # Safety
+    ///
+    /// `ptr` must point to an existing and valid memory allocation created by this allocator
+    /// instance of a size of at least `old_size`.
+    ///
+    /// Additionally, `ptr` is allowed to be a NULL pointer; in this case a new memory allocation is
+    /// created.
+    unsafe fn realloc(
+        &self,
+        ptr: *mut u8,
+        old_size: usize,
+        layout: Layout,
+        flags: Flags,
+    ) -> Result<NonNull<[u8]>, AllocError>;
+
+    /// Free an existing memory allocation.
+    ///
+    /// # Safety
+    ///
+    /// `ptr` must point to an existing and valid memory allocation created by this `Allocator`
+    /// instance.
+    unsafe fn free(&self, ptr: *mut u8) {
+        // SAFETY: `ptr` is guaranteed to be previously allocated with this `Allocator` or NULL.
+        // Calling `realloc` with a buffer size of zero, frees the buffer `ptr` points to.
+        let _ = unsafe { self.realloc(ptr, 0, Layout::new::<()>(), Flags(0)) };
+    }
+}
-- 
2.45.2