linux-kernel - Re: [PATCH V5 1/8] mm/slab: use unsigned long for orig

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Message-ID: <CAG_fn=XCx9-uYOhRQXTde7ud=H9kwM_Sf3ZjHQd9hfYDspzeOA@mail.gmail.com>
Date: Thu, 8 Jan 2026 12:39:22 +0100
From: Alexander Potapenko <glider@...gle.com>
To: Harry Yoo <harry.yoo@...cle.com>
Cc: akpm@...ux-foundation.org, vbabka@...e.cz, andreyknvl@...il.com, 
	cl@...two.org, dvyukov@...gle.com, hannes@...xchg.org, linux-mm@...ck.org, 
	mhocko@...nel.org, muchun.song@...ux.dev, rientjes@...gle.com, 
	roman.gushchin@...ux.dev, ryabinin.a.a@...il.com, shakeel.butt@...ux.dev, 
	surenb@...gle.com, vincenzo.frascino@....com, yeoreum.yun@....com, 
	tytso@....edu, adilger.kernel@...ger.ca, linux-ext4@...r.kernel.org, 
	linux-kernel@...r.kernel.org, cgroups@...r.kernel.org, hao.li@...ux.dev, 
	stable@...r.kernel.org
Subject: Re: [PATCH V5 1/8] mm/slab: use unsigned long for orig_size to ensure
 proper metadata align

On Mon, Jan 5, 2026 at 9:02 AM Harry Yoo <harry.yoo@...cle.com> wrote:
>
> When both KASAN and SLAB_STORE_USER are enabled, accesses to
> struct kasan_alloc_meta fields can be misaligned on 64-bit architectures.
> This occurs because orig_size is currently defined as unsigned int,
> which only guarantees 4-byte alignment. When struct kasan_alloc_meta is
> placed after orig_size, it may end up at a 4-byte boundary rather than
> the required 8-byte boundary on 64-bit systems.
>
> 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.
>
> Change orig_size from unsigned int to unsigned long to ensure proper
> alignment for any subsequent metadata. This should not waste additional
> memory because kmalloc objects are already aligned to at least
> ARCH_KMALLOC_MINALIGN.
>
> Suggested-by: Andrey Ryabinin <ryabinin.a.a@...il.com>
> Cc: stable@...r.kernel.org
> Fixes: 6edf2576a6cc ("mm/slub: enable debugging memory wasting of kmalloc")
> Signed-off-by: Harry Yoo <harry.yoo@...cle.com>
> ---
>  mm/slub.c | 14 +++++++-------
>  1 file changed, 7 insertions(+), 7 deletions(-)
>
> diff --git a/mm/slub.c b/mm/slub.c
> index ad71f01571f0..1c747435a6ab 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -857,7 +857,7 @@ static inline bool slab_update_freelist(struct kmem_cache *s, struct slab *slab,
>   * request size in the meta data area, for better debug and sanity check.
>   */
>  static inline void set_orig_size(struct kmem_cache *s,
> -                               void *object, unsigned int orig_size)
> +                               void *object, unsigned long orig_size)
>  {
>         void *p = kasan_reset_tag(object);
>
> @@ -867,10 +867,10 @@ static inline void set_orig_size(struct kmem_cache *s,
>         p += get_info_end(s);
>         p += sizeof(struct track) * 2;
>
> -       *(unsigned int *)p = orig_size;
> +       *(unsigned long *)p = orig_size;

Instead of calculating the offset of the original size in several
places, should we maybe introduce a function that returns a pointer to
it?