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

[Harry Yoo]

On Thu, Jan 08, 2026 at 12:39:22PM +0100, Alexander Potapenko wrote:
> On Mon, Jan 5, 2026 at 9:02 AM Harry Yoo <harry.yoo@xxxxxxxxxx> 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@xxxxxxxxx>
> > Cc: stable@xxxxxxxxxxxxxxx
> > Fixes: 6edf2576a6cc ("mm/slub: enable debugging memory wasting of kmalloc")
> > Signed-off-by: Harry Yoo <harry.yoo@xxxxxxxxxx>
> > ---
> >  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?

Good point.

The calculation of various metadata offset (including the original size)
is repeated in several places, and perhaps it's worth cleaning up,
something like this:

enum {
  FREE_POINTER_OFFSET,
  ALLOC_TRACK_OFFSET,
  FREE_TRACK_OFFSET,
  ORIG_SIZE_OFFSET,
  KASAN_ALLOC_META_OFFSET,
  OBJ_EXT_OFFSET,
  FINAL_ALIGNMENT_PADDING_OFFSET,
  ...
};

orig_size = *(unsigned long *)get_metadata_ptr(p, ORIG_SIZE_OFFSET);

... of course, perhaps as a follow-up rather than
as part of this series.

-- 
Cheers,
Harry / Hyeonggon