2 files changed, 60 insertions, 15 deletions

diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helper.h
index 3e601ce2548d4..058954961bfc0 100644
--- a/rust/bindings/bindings_helper.h
+++ b/rust/bindings/bindings_helper.h
@@ -13,5 +13,6 @@
 #include <linux/sched.h>
 
 /* `bindgen` gets confused at certain things. */
+const size_t BINDINGS_ARCH_SLAB_MINALIGN = ARCH_SLAB_MINALIGN;
 const gfp_t BINDINGS_GFP_KERNEL = GFP_KERNEL;
 const gfp_t BINDINGS___GFP_ZERO = __GFP_ZERO;
diff --git a/rust/kernel/allocator.rs b/rust/kernel/allocator.rs
index 397a3dd57a9b1..9363b527be664 100644
--- a/rust/kernel/allocator.rs
+++ b/rust/kernel/allocator.rs
@@ -9,6 +9,36 @@ use crate::bindings;
 
 struct KernelAllocator;
 
+/// Calls `krealloc` with a proper size to alloc a new object aligned to `new_layout`'s alignment.
+///
+/// # Safety
+///
+/// - `ptr` can be either null or a pointer which has been allocated by this allocator.
+/// - `new_layout` must have a non-zero size.
+unsafe fn krealloc_aligned(ptr: *mut u8, new_layout: Layout, flags: bindings::gfp_t) -> *mut u8 {
+    // Customized layouts from `Layout::from_size_align()` can have size < align, so pad first.
+    let layout = new_layout.pad_to_align();
+
+    let mut size = layout.size();
+
+    if layout.align() > bindings::BINDINGS_ARCH_SLAB_MINALIGN {
+        // The alignment requirement exceeds the slab guarantee, thus try to enlarge the size
+        // to use the "power-of-two" size/alignment guarantee (see comments in `kmalloc()` for
+        // more information).
+        //
+        // Note that `layout.size()` (after padding) is guaranteed to be a multiple of
+        // `layout.align()`, so `next_power_of_two` gives enough alignment guarantee.
+        size = size.next_power_of_two();
+    }
+
+    // SAFETY:
+    // - `ptr` is either null or a pointer returned from a previous `k{re}alloc()` by the
+    //   function safety requirement.
+    // - `size` is greater than 0 since it's either a `layout.size()` (which cannot be zero
+    //    according to the function safety requirement) or a result from `next_power_of_two()`.
+    unsafe { bindings::krealloc(ptr as *const core::ffi::c_void, size, flags) as *mut u8 }
+}
+
 unsafe impl GlobalAlloc for KernelAllocator {
     unsafe fn alloc(&self, layout: Layout) -> *mut u8 {
         // `krealloc()` is used instead of `kmalloc()` because the latter is
@@ -30,10 +60,20 @@ static ALLOCATOR: KernelAllocator = KernelAllocator;
 // to extract the object file that has them from the archive. For the moment,
 // let's generate them ourselves instead.
 //
+// Note: Although these are *safe* functions, they are called by the compiler
+// with parameters that obey the same `GlobalAlloc` function safety
+// requirements: size and align should form a valid layout, and size is
+// greater than 0.
+//
 // Note that `#[no_mangle]` implies exported too, nowadays.
 #[no_mangle]
-fn __rust_alloc(size: usize, _align: usize) -> *mut u8 {
-    unsafe { bindings::krealloc(core::ptr::null(), size, bindings::GFP_KERNEL) as *mut u8 }
+fn __rust_alloc(size: usize, align: usize) -> *mut u8 {
+    // SAFETY: See assumption above.
+    let layout = unsafe { Layout::from_size_align_unchecked(size, align) };
+
+    // SAFETY: `ptr::null_mut()` is null, per assumption above the size of `layout` is greater
+    // than 0.
+    unsafe { krealloc_aligned(ptr::null_mut(), layout, bindings::GFP_KERNEL) }
 }
 
 #[no_mangle]
@@ -42,23 +82,27 @@ fn __rust_dealloc(ptr: *mut u8, _size: usize, _align: usize) {
 }
 
 #[no_mangle]
-fn __rust_realloc(ptr: *mut u8, _old_size: usize, _align: usize, new_size: usize) -> *mut u8 {
-    unsafe {
-        bindings::krealloc(
-            ptr as *const core::ffi::c_void,
-            new_size,
-            bindings::GFP_KERNEL,
-        ) as *mut u8
-    }
+fn __rust_realloc(ptr: *mut u8, _old_size: usize, align: usize, new_size: usize) -> *mut u8 {
+    // SAFETY: See assumption above.
+    let new_layout = unsafe { Layout::from_size_align_unchecked(new_size, align) };
+
+    // SAFETY: Per assumption above, `ptr` is allocated by `__rust_*` before, and the size of
+    // `new_layout` is greater than 0.
+    unsafe { krealloc_aligned(ptr, new_layout, bindings::GFP_KERNEL) }
 }
 
 #[no_mangle]
-fn __rust_alloc_zeroed(size: usize, _align: usize) -> *mut u8 {
+fn __rust_alloc_zeroed(size: usize, align: usize) -> *mut u8 {
+    // SAFETY: See assumption above.
+    let layout = unsafe { Layout::from_size_align_unchecked(size, align) };
+
+    // SAFETY: `ptr::null_mut()` is null, per assumption above the size of `layout` is greater
+    // than 0.
     unsafe {
-        bindings::krealloc(
-            core::ptr::null(),
-            size,
+        krealloc_aligned(
+            ptr::null_mut(),
+            layout,
             bindings::GFP_KERNEL | bindings::__GFP_ZERO,
-        ) as *mut u8
+        )
     }
 }