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

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Message-ID: <1372138e-5837-4634-81de-447a1ef0a5ad@suse.cz>
Date: Wed, 7 Jan 2026 12:43:17 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: Harry Yoo <harry.yoo@...cle.com>, akpm@...ux-foundation.org
Cc: andreyknvl@...il.com, cl@...two.org, dvyukov@...gle.com,
 glider@...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 1/5/26 09:02, Harry Yoo 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.

Oops.

> 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.

I'll add:

Closes: https://lore.kernel.org/all/aPrLF0OUK651M4dk@hyeyoo/

since that's useful context and discussion.

> 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>

As the problem was introduced in 6.1, doesn't seem urgent to push as 6.19 rc
fix, so keeping it as part of the series (where it's a necessary
prerequisity per the Closes: link above) and stable backporting later seems
indeed sufficient. Thanks.