[PATCH v5] rust: alloc: satisfy POSIX alignment requirement

[Tamir Duberstein <tamird@gmail.com>]

ISO C's `aligned_alloc` is partially implementation-defined; on some
systems it inherits stricter requirements from POSIX's `posix_memalign`.

This causes the call added in commit dd09538fb409 ("rust: alloc:
implement `Cmalloc` in module allocator_test") to fail on macOS because
it doesn't meet the requirements of `posix_memalign`.

Adjust the call to meet the POSIX requirement and add a comment. This
fixes failures in `make rusttest` on macOS.

Acked-by: Danilo Krummrich <dakr@kernel.org>
Fixes: dd09538fb409 ("rust: alloc: implement `Cmalloc` in module allocator_test")
Signed-off-by: Tamir Duberstein <tamird@gmail.com>
---
Changes in v5:
- Remove errant newline in commit message. (Miguel Ojeda)
- Use more succinct expression. (Gary Guo)
- Drop and then add Danilo's Acked-by again.
- Link to v4: https://lore.kernel.org/r/20250210-aligned-alloc-v4-1-609c3a6fe139@gmail.com

Changes in v4:
- Revert to `aligned_alloc` and correct rationale. (Miguel Ojeda)
- Apply Danilo's Acked-by from v2.
- Rebase on v6.14-rc2.
- Link to v3: https://lore.kernel.org/r/20250206-aligned-alloc-v3-1-0cbc0ab0306d@gmail.com

Changes in v3:
- Replace `aligned_alloc` with `posix_memalign` for portability.
- Link to v2: https://lore.kernel.org/r/20250202-aligned-alloc-v2-1-5af0b5fdd46f@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 | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/rust/kernel/alloc/allocator_test.rs b/rust/kernel/alloc/allocator_test.rs
index e3240d16040b..17a475380253 100644
--- a/rust/kernel/alloc/allocator_test.rs
+++ b/rust/kernel/alloc/allocator_test.rs
@@ -62,6 +62,26 @@ unsafe fn realloc(
             ));
         }
 
+        // ISO C (ISO/IEC 9899:2011) defines `aligned_alloc`:
+        //
+        // > The value of alignment shall be a valid alignment supported by the implementation
+        // [...].
+        //
+        // As an example of the "supported by the implementation" requirement, POSIX.1-2001 (IEEE
+        // 1003.1-2001) defines `posix_memalign`:
+        //
+        // > The value of alignment shall be a power of two multiple of sizeof (void *).
+        //
+        // and POSIX-based implementations of `aligned_alloc` inherit this requirement. At the time
+        // of writing, this is known to be the case on macOS (but not in glibc).
+        //
+        // Satisfy the stricter requirement to avoid spurious test failures on some platforms.
+        let min_align = core::mem::size_of::<*const crate::ffi::c_void>();
+        let layout = layout.align_to(min_align).unwrap_or_else(|_err| {
+            crate::build_error!("invalid alignment")
+        });
+        let layout = layout.pad_to_align();
+
         // 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;

---
base-commit: a64dcfb451e254085a7daee5fe51bf22959d52d3
change-id: 20250201-aligned-alloc-b52cb2353c82

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