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

[Harry Yoo]

On Wed, Oct 29, 2025 at 03:36:28PM +0100, Andrey Ryabinin wrote:
> 
> 
> On 10/27/25 1:00 PM, 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.
> > 
> 
> kasan_alloc_meta is properly aligned. It consists of 4 32-bit words,
> so the proper alignment is 32bit regardless of architecture bitness.

Right.

> kasan_free_meta however requires 'unsigned long' alignment
> and could be misaligned if placed at 32-bit boundary on 64-bit arch

Right.

> > 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)).
> > 
> 
> __aligned() attribute ensures nothing. It tells compiler what alignment to expect
> and affects compiler controlled placement of struct in memory (e.g. stack/.bss/.data)
> But it can't enforce placement in dynamic memory.

Right.

> Also for struct kasan_free_meta, struct track alignof(unsigned long) already dictated
> by C standard, so adding this __aligned() have zero effect.

Right.

> And there is no reason to increase alignment requirement for kasan_alloc_meta struct.

Right.

> > For orig_size, use ALIGN(sizeof(unsigned int), sizeof(unsigned long)) to
> > 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.
>
> I would suggest to use 'unsigned long' for orig_size. It changes nothing for 32-bit,
> and it shouldn't increase memory usage for 64-bit since we currently wasting it anyway
> to align next object to ARCH_KMALLOC_MINALIGN.

Sounds fair! Patch soon. Thanks.

-- 
Cheers,
Harry / Hyeonggon