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.

Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Reviewed-by: Gary Guo <gary@garyguo.net>
Signed-off-by: Danilo Krummrich <dakr@kernel.org>
Link: https://lore.kernel.org/r/20241004154149.93856-26-dakr@kernel.org
[ Removed the temporary `allow(dead_code)` as discussed in the list and
  fixed typo, added backticks. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

rust/kernel/alloc.rs

@@ -215,7 +215,6 @@ unsafe fn free(ptr: NonNull<u8>, layout: Layout) {
     }
 }
 
-#[allow(dead_code)]
 /// Returns a properly aligned dangling pointer from the given `layout`.
 pub(crate) fn dangling_from_layout(layout: Layout) -> NonNull<u8> {
     let ptr = layout.align() as *mut u8;

rust/kernel/alloc/allocator_test.rs

@@ -1,22 +1,95 @@
 // 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::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);
+}
+
+// SAFETY:
+// - memory remains valid until it is explicitly freed,
+// - passing a pointer to a valid memory allocation created by this `Allocator` is always OK,
+// - `realloc` provides the guarantees as provided in the `# Guarantees` section.
+unsafe impl Allocator for Cmalloc {
     unsafe fn realloc(
-        _ptr: Option<NonNull<u8>>,
-        _layout: Layout,
-        _old_layout: Layout,
-        _flags: Flags,
+        ptr: Option<NonNull<u8>>,
+        layout: Layout,
+        old_layout: Layout,
+        flags: Flags,
     ) -> Result<NonNull<[u8]>, AllocError> {
-        panic!();
+        let src = match ptr {
+            Some(src) => {
+                if old_layout.size() == 0 {
+                    ptr::null_mut()
+                } else {
+                    src.as_ptr()
+                }
+            }
+            None => ptr::null_mut(),
+        };
+
+        if layout.size() == 0 {
+            // SAFETY: `src` is either NULL or was previously allocated with this `Allocator`
+            unsafe { libc_free(src.cast()) };
+
+            return Ok(NonNull::slice_from_raw_parts(
+                crate::alloc::dangling_from_layout(layout),
+                0,
+            ));
+        }
+
+        // SAFETY: Returns either NULL or a pointer to a memory allocation that satisfies or
+        // exceeds the given size and alignment requirements.
+        let dst = unsafe { libc_aligned_alloc(layout.align(), layout.size()) } as *mut u8;
+        let dst = NonNull::new(dst).ok_or(AllocError)?;
+
+        if flags.contains(__GFP_ZERO) {
+            // SAFETY: The preceding calls to `libc_aligned_alloc` and `NonNull::new`
+            // guarantee that `dst` points to memory of at least `layout.size()` bytes.
+            unsafe { dst.as_ptr().write_bytes(0, layout.size()) };
+        }
+
+        if !src.is_null() {
+            // SAFETY:
+            // - `src` has previously been allocated with this `Allocator`; `dst` has just been
+            //   newly allocated, hence the memory regions do not overlap.
+            // - both` src` and `dst` are properly aligned and valid for reads and writes
+            unsafe {
+                ptr::copy_nonoverlapping(
+                    src,
+                    dst.as_ptr(),
+                    cmp::min(layout.size(), old_layout.size()),
+                )
+            };
+        }
+
+        // SAFETY: `src` is either NULL or was previously allocated with this `Allocator`
+        unsafe { libc_free(src.cast()) };
+
+        Ok(NonNull::slice_from_raw_parts(dst, layout.size()))
     }
 }