rust: alloc: implement Cmalloc in module allocator_test

So far the kernel's `Box` and `Vec` types can't be used by userspace
test cases, since all users of those types (e.g. `CString`) use kernel
allocators for instantiation.

In order to allow userspace test cases to make use of such types as
well, implement the `Cmalloc` allocator within the allocator_test module
and type alias all kernel allocators to `Cmalloc`. The `Cmalloc`
allocator uses libc's realloc() function as allocator backend.

Signed-off-by: Danilo Krummrich <dakr@kernel.org>
Link: https://lore.kernel.org/r/20240911225449.152928-23-dakr@kernel.org

Author: Danilo Krummrich

rust/kernel/alloc/allocator_test.rs

@@ -1,21 +1,200 @@
 // SPDX-License-Identifier: GPL-2.0
 
+//! So far the kernel's `Box` and `Vec` types can't be used by userspace test cases, since all users
+//! of those types (e.g. `CString`) use kernel allocators for instantiation.
+//!
+//! In order to allow userspace test cases to make use of such types as well, implement the
+//! `Cmalloc` allocator within the allocator_test module and type alias all kernel allocators to
+//! `Cmalloc`. The `Cmalloc` allocator uses libc's realloc() function as allocator backend.
+
 #![allow(missing_docs)]
 
-use super::{AllocError, Allocator, Flags};
+use super::{flags::*, AllocError, Allocator, Flags};
 use core::alloc::Layout;
+use core::cmp;
+use core::mem;
+use core::ptr;
 use core::ptr::NonNull;
 
-pub struct Kmalloc;
+/// The userspace allocator based on libc.
+pub struct Cmalloc;
+
+pub type Kmalloc = Cmalloc;
 pub type Vmalloc = Kmalloc;
 pub type KVmalloc = Kmalloc;
 
-unsafe impl Allocator for Kmalloc {
+extern "C" {
+    #[link_name = "aligned_alloc"]
+    fn libc_aligned_alloc(align: usize, size: usize) -> *mut core::ffi::c_void;
+
+    #[link_name = "free"]
+    fn libc_free(ptr: *mut core::ffi::c_void);
+}
+
+struct CmallocData {
+    // The actual size as requested through `Cmalloc::alloc` or `Cmalloc::realloc`.
+    size: usize,
+    // The offset from the pointer returned to the caller of `Cmalloc::alloc` or `Cmalloc::realloc`
+    // to the actual base address of the allocation.
+    offset: usize,
+}
+
+impl Cmalloc {
+    /// Adjust the size and alignment such that we can additionally store `CmallocData` right
+    /// before the actual data described by `layout`.
+    ///
+    /// Example:
+    ///
+    /// - For `CmallocData` assume an alignment of 8 and a size of 16.
+    /// - For `layout` assume and alignment of 16 and a size of 64.
+    ///
+    ///```text
+    /// 0                16               32                                               96
+    /// |----------------|----------------|------------------------------------------------|
+    ///        empty         CmallocData                         data
+    ///```
+    ///
+    /// For this example the returned `Layout` has an alignment of 32 and a size of 96.
+    fn layout_adjust(layout: Layout) -> Result<Layout, AllocError> {
+        let layout = layout.pad_to_align();
+
+        // Ensure that `CmallocData` fits into half the alignment. Additionally, this guarantees
+        // that advancing a pointer aligned to `align` by `align / 2` we still satisfy or exceed
+        // the alignment requested through `layout`.
+        let align = cmp::max(
+            layout.align(),
+            mem::size_of::<CmallocData>().next_power_of_two(),
+        ) * 2;
+
+        // Add the additional space required for `CmallocData`.
+        let size = layout.size() + mem::size_of::<CmallocData>();
+
+        Ok(Layout::from_size_align(size, align)
+            .map_err(|_| AllocError)?
+            .pad_to_align())
+    }
+
+    fn alloc_store_data(layout: Layout) -> Result<NonNull<u8>, AllocError> {
+        let requested_size = layout.size();
+
+        let layout = Self::layout_adjust(layout)?;
+        let min_align = layout.align() / 2;
+
+        // SAFETY: Returns either NULL or a pointer to a memory allocation that satisfies or
+        // exceeds the given size and alignment requirements.
+        let raw_ptr = unsafe { libc_aligned_alloc(layout.align(), layout.size()) } as *mut u8;
+
+        let priv_ptr = NonNull::new(raw_ptr).ok_or(AllocError)?;
+
+        // SAFETY:
+        // - By adding `min_align` the pointer remains within the allocated object and `min_align`
+        //   can not exceed `isize`.
+        //
+        // GUARANTEE:
+        // - The adjustments from `Self::layout_adjust` ensure that after this operation the
+        // original size and alignment requirements are still satisfied or exceeded.
+        let ptr = unsafe { priv_ptr.as_ptr().add(min_align) };
+
+        // SAFETY: `min_align` is greater than or equal to the size of `CmallocData`, hence we
+        // don't exceed the allocation boundaries.
+        let data_ptr: *mut CmallocData = unsafe { ptr.sub(mem::size_of::<CmallocData>()) }.cast();
+
+        let data = CmallocData {
+            size: requested_size,
+            offset: min_align,
+        };
+
+        // SAFETY: `data_ptr` is properly aligned and within the allocation boundaries reserved for
+        // `CmallocData`.
+        unsafe { data_ptr.write(data) };
+
+        NonNull::new(ptr).ok_or(AllocError)
+    }
+
+    /// # Safety
+    ///
+    /// - `ptr` must have been previously allocated with `Self::alloc_store_data`.
+    unsafe fn data(ptr: NonNull<u8>) -> CmallocData {
+        // SAFETY: `Self::alloc_store_data` stores the `CmallocData` right before the address
+        // returned to callers of `Self::alloc_store_data`.
+        let data_ptr: *mut CmallocData =
+            unsafe { ptr.as_ptr().sub(mem::size_of::<CmallocData>()) }.cast();
+
+        // `CmallocData` has been previously stored at this offset with `Self::alloc_store_data`.
+        //
+        // SAFETY:
+        // - `data_ptr` points to a properly aligned and initialized value of `CmallocData`.
+        unsafe { core::ptr::read(data_ptr) }
+    }
+
+    /// # Safety
+    ///
+    /// This function must not be called more than once for the same allocation.
+    ///
+    /// - `ptr` must have been previously allocated with `Self::alloc_store_data`.
+    unsafe fn free_read_data(ptr: NonNull<u8>) {
+        // SAFETY: `ptr` has been created by `Self::alloc_store_data`.
+        let data = unsafe { Self::data(ptr) };
+
+        // SAFETY: `ptr` has been created by `Self::alloc_store_data`.
+        let priv_ptr = unsafe { ptr.as_ptr().sub(data.offset) };
+
+        // SAFETY: `priv_ptr` has previously been allocatored with this `Allocator`.
+        unsafe { libc_free(priv_ptr.cast()) };
+    }
+}
+
+unsafe impl Allocator for Cmalloc {
+    fn alloc(layout: Layout, flags: Flags) -> Result<NonNull<[u8]>, AllocError> {
+        if layout.size() == 0 {
+            return Ok(NonNull::slice_from_raw_parts(NonNull::dangling(), 0));
+        }
+
+        let ptr = Self::alloc_store_data(layout)?;
+
+        if flags.contains(__GFP_ZERO) {
+            // SAFETY: `Self::alloc_store_data` guarantees that `ptr` points to memory of at least
+            // `layout.size()` bytes.
+            unsafe { ptr.as_ptr().write_bytes(0, layout.size()) };
+        }
+
+        Ok(NonNull::slice_from_raw_parts(ptr, layout.size()))
+    }
+
     unsafe fn realloc(
-        _ptr: Option<NonNull<u8>>,
-        _layout: Layout,
-        _flags: Flags,
+        ptr: Option<NonNull<u8>>,
+        layout: Layout,
+        flags: Flags,
     ) -> Result<NonNull<[u8]>, AllocError> {
-        panic!();
+        let Some(src) = ptr else {
+            return Self::alloc(layout, flags);
+        };
+
+        if layout.size() == 0 {
+            // SAFETY: `src` has been created by `Self::alloc_store_data`.
+            unsafe { Self::free_read_data(src) };
+
+            return Ok(NonNull::slice_from_raw_parts(NonNull::dangling(), 0));
+        }
+
+        let dst = Self::alloc(layout, flags)?;
+
+        // SAFETY: `src` has been created by `Self::alloc_store_data`.
+        let data = unsafe { Self::data(src) };
+
+        // SAFETY: `src` has previously been allocated with this `Allocator`; `dst` has just been
+        // newly allocated. Copy up to the smaller of both sizes.
+        unsafe {
+            ptr::copy_nonoverlapping(
+                src.as_ptr(),
+                dst.as_ptr().cast(),
+                cmp::min(layout.size(), data.size),
+            )
+        };
+
+        // SAFETY: `src` has been created by `Self::alloc_store_data`.
+        unsafe { Self::free_read_data(src) };
+
+        Ok(dst)
     }
 }