[PATCH v2] rust: alloc: satisfy `aligned_alloc` requirements

[Tamir Duberstein <tamird@gmail.com>]

The implementation added in commit dd09538fb409 ("rust: alloc: implement
`Cmalloc` in module allocator_test") did not honor the documented
requirements of `aligned_alloc`. These requirements may not be enforced
on all systems, but they are on macOS. Ensure that alignment is at least
`sizeof(void *)` and round size up to the nearest multiple of that
value.

Fixes: dd09538fb409 ("rust: alloc: implement `Cmalloc` in module allocator_test")

Signed-off-by: Tamir Duberstein <tamird@gmail.com>
---
Changes in v2:
- Shorten some variable names. (Danilo Krummrich)
- Replace shadowing alignment variable with a second call to
  Layout::align. (Danilo Krummrich)
- Link to v1: https://lore.kernel.org/r/20250201-aligned-alloc-v1-1-c99a73f3cbd4@gmail.com
---
 rust/kernel/alloc/allocator_test.rs | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/rust/kernel/alloc/allocator_test.rs b/rust/kernel/alloc/allocator_test.rs
index e3240d16040b..69f79e246891 100644
--- a/rust/kernel/alloc/allocator_test.rs
+++ b/rust/kernel/alloc/allocator_test.rs
@@ -62,9 +62,21 @@ unsafe fn realloc(
             ));
         }
 
+        // According to `man aligned_alloc`:
+        //
+        // aligned_alloc() returns a NULL pointer and sets errno to EINVAL if size is not an
+        // integral multiple of alignment, or if alignment is not a power of 2 at least as large as
+        // sizeof(void *).
+        let min_align = core::mem::size_of::<*const crate::ffi::c_void>();
+        let (align, size) = if layout.align() < min_align {
+            (min_align, layout.size().div_ceil(min_align) * min_align)
+        } else {
+            (layout.align(), layout.size())
+        };
+
         // 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 = unsafe { libc_aligned_alloc(align, size) } as *mut u8;
         let dst = NonNull::new(dst).ok_or(AllocError)?;
 
         if flags.contains(__GFP_ZERO) {

---
base-commit: 89a010129b2a60185d34d7377ef8aec7fbb92e76
change-id: 20250201-aligned-alloc-b52cb2353c82

Best regards,
-- 
Tamir Duberstein <tamird@gmail.com>