Re: [PATCH 1/2] mm/kasan: Fix KASAN poisoning in vrealloc()

[Andrey Konovalov <andreyknvl@gmail.com>]

On Fri, Jan 16, 2026 at 2:26 PM Andrey Ryabinin <ryabinin.a.a@gmail.com> wrote:
>
> So something like bellow I guess.

Yeah, looks good.

> I think this would actually have the opposite effect and make the code harder to follow.
> Introducing an extra wrapper adds another layer of indirection and more boilerplate, which
> makes the control flow less obvious and the code harder to navigate and grep.
>
> And what's the benefit here? I don't clearly see it.

One functional benefit is when HW_TAGS mode enabled in .config but
disabled via command-line, we avoid a function call into KASAN
runtime.

From the readability perspective, what we had before the recent
clean-up was an assortment of kasan_enabled/kasan_arch_ready checks in
lower-level KASAN functions, which made it hard to figure out what
actually happens when KASAN is not enabled. And these high-level
checks make it more clear. At least in my opinion.


>
> ---
>  include/linux/kasan.h | 10 +++++++++-
>  mm/kasan/shadow.c     |  5 +----
>  2 files changed, 10 insertions(+), 5 deletions(-)
>
> diff --git a/include/linux/kasan.h b/include/linux/kasan.h
> index ff27712dd3c8..338a1921a50a 100644
> --- a/include/linux/kasan.h
> +++ b/include/linux/kasan.h
> @@ -641,9 +641,17 @@ kasan_unpoison_vmap_areas(struct vm_struct **vms, int nr_vms,
>                 __kasan_unpoison_vmap_areas(vms, nr_vms, flags);
>  }
>
> -void kasan_vrealloc(const void *start, unsigned long old_size,
> +void __kasan_vrealloc(const void *start, unsigned long old_size,
>                 unsigned long new_size);
>
> +static __always_inline void kasan_vrealloc(const void *start,
> +                                       unsigned long old_size,
> +                                       unsigned long new_size)
> +{
> +       if (kasan_enabled())
> +               __kasan_vrealloc(start, old_size, new_size);
> +}
> +
>  #else /* CONFIG_KASAN_VMALLOC */
>
>  static inline void kasan_populate_early_vm_area_shadow(void *start,
> diff --git a/mm/kasan/shadow.c b/mm/kasan/shadow.c
> index e9b6b2d8e651..29b0d0d38b40 100644
> --- a/mm/kasan/shadow.c
> +++ b/mm/kasan/shadow.c
> @@ -651,12 +651,9 @@ void __kasan_poison_vmalloc(const void *start, unsigned long size)
>         kasan_poison(start, size, KASAN_VMALLOC_INVALID, false);
>  }
>
> -void kasan_vrealloc(const void *addr, unsigned long old_size,
> +void __kasan_vrealloc(const void *addr, unsigned long old_size,
>                 unsigned long new_size)
>  {
> -       if (!kasan_enabled())
> -               return;
> -
>         if (new_size < old_size) {
>                 kasan_poison_last_granule(addr, new_size);
>
> --
> 2.52.0
>
>