Re: [PATCH V5 1/8] mm/slab: use unsigned long for orig_size to ensure proper metadata align

[Harry Yoo <harry.yoo@oracle.com>]

On Wed, Jan 07, 2026 at 12:43:17PM +0100, Vlastimil Babka wrote:
> 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.

Looks good to me.

> > Suggested-by: Andrey Ryabinin <ryabinin.a.a@gmail.com>
> > Cc: stable@vger.kernel.org
> > Fixes: 6edf2576a6cc ("mm/slub: enable debugging memory wasting of kmalloc")
> > Signed-off-by: Harry Yoo <harry.yoo@oracle.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

Yeah, no need to hurry.

> (where it's a necessary prerequisity per the Closes: link above)

Technically it's not a necessary prerequisite anymore because the series
doesn't unpoison slabobj_ext anymore, but later patches depend on it
because of the change in object layout.

> and stable backporting later seems indeed sufficient. Thanks.

backporting later sounds reasonable.

Thanks!

-- 
Cheers,
Harry / Hyeonggon