Re: [PATCH V2] mm/slab: ensure all metadata in slab object are word-aligned

[Harry Yoo]

On Mon, Oct 27, 2025 at 09:00:28PM +0900, Harry Yoo wrote:
> When the SLAB_STORE_USER debug flag is used, any metadata placed after
> the original kmalloc request size (orig_size) is not properly aligned
> on 64-bit architectures because its type is unsigned int. When both KASAN
> and SLAB_STORE_USER are enabled, kasan_alloc_meta is misaligned.
> 
> Note that 64-bit architectures without HAVE_EFFICIENT_UNALIGNED_ACCESS
> are assumed to require 64-bit accesses to be 64-bit aligned.
> See HAVE_64BIT_ALIGNED_ACCESS and commit adab66b71abf ("Revert:
> "ring-buffer: Remove HAVE_64BIT_ALIGNED_ACCESS"") for more details.
> 
> Because not all architectures support unaligned memory accesses,
> ensure that all metadata (track, orig_size, kasan_{alloc,free}_meta)
> in a slab object are word-aligned. struct track, kasan_{alloc,free}_meta
> are aligned by adding __aligned(__alignof__(unsigned long)).
> 
> For orig_size, use ALIGN(sizeof(unsigned int), sizeof(unsigned long)) to
                                                 ^ Uh, here I intended
						 to say:
                                                 __aligneof__(unsigned long))

> make clear that its size remains unsigned int but it must be aligned to
> a word boundary. On 64-bit architectures, this reserves 8 bytes for
> orig_size, which is acceptable since kmalloc's original request size
> tracking is intended for debugging rather than production use.
> 
> Cc: stable@xxxxxxxxxxxxxxx
> Fixes: 6edf2576a6cc ("mm/slub: enable debugging memory wasting of kmalloc")
> Acked-by: Andrey Konovalov <andreyknvl@xxxxxxxxx>
> Signed-off-by: Harry Yoo <harry.yoo@xxxxxxxxxx>
> ---
> 
> v1 -> v2:
> - Added Andrey's Acked-by.
> - Added references to HAVE_64BIT_ALIGNED_ACCESS and the commit that
>   resurrected it.
> - Used __alignof__() instead of sizeof(), as suggested by Pedro (off-list).
>   Note: either __alignof__ or sizeof() produces the exactly same mm/slub.o
>   files, so there's no functional difference.
> 
> Thanks!
> 
>  mm/kasan/kasan.h |  4 ++--
>  mm/slub.c        | 16 +++++++++++-----
>  2 files changed, 13 insertions(+), 7 deletions(-)

-- 
Cheers,
Harry / Hyeonggon