Re: [RFC PATCH] Introduce sysfs interface to disable kfence for selected slabs.

[Marco Elver]

On Thu, 11 Aug 2022 at 09:26, Imran Khan <imran.f.khan@xxxxxxxxxx> wrote:
>
> By default kfence allocation can happen for any slab object, whose size
> is up to PAGE_SIZE, as long as that allocation is the first allocation
> after expiration of kfence sample interval. But in certain debugging
> scenarios we may be interested in debugging corruptions involving
> some specific slub objects like dentry or ext4_* etc. In such cases
> limiting kfence for allocations involving only specific slub objects
> will increase the probablity of catching the issue since kfence pool
> will not be consumed by other slab objects.
>
> This patch introduces a sysfs interface '/sys/kernel/slab/<name>/skip_kfence'
> to disable kfence for specific slabs. Having the interface work in this
> way does not impact current/default behavior of kfence and allows us to
> use kfence for specific slabs (when needed) as well. The decision to
> skip/use kfence is taken depending on whether kmem_cache.flags has
> (newly introduced) SLAB_SKIP_KFENCE flag set or not.
>
> Signed-off-by: Imran Khan <imran.f.khan@xxxxxxxxxx>
> ---
>
> This RFC patch is implementing the sysfs work mentioned in [1]. Since the
> approach taken in [1] was not proper, I am sending this RFC patch as a
> separate change.

This design is much simpler and looks good to me. Feel free to send as
a non-RFC.

> [1]: https://lore.kernel.org/lkml/20220727234241.1423357-1-imran.f.khan@xxxxxxxxxx/
>
>  include/linux/slab.h |  6 ++++++
>  mm/kfence/core.c     |  7 +++++++
>  mm/slub.c            | 27 +++++++++++++++++++++++++++
>  3 files changed, 40 insertions(+)
>
> diff --git a/include/linux/slab.h b/include/linux/slab.h
> index 0fefdf528e0d..947d912fd08c 100644
> --- a/include/linux/slab.h
> +++ b/include/linux/slab.h
> @@ -119,6 +119,12 @@
>   */
>  #define SLAB_NO_USER_FLAGS     ((slab_flags_t __force)0x10000000U)
>
> +#ifdef CONFIG_KFENCE
> +#define SLAB_SKIP_KFENCE            ((slab_flags_t __force)0x20000000U)
> +#else
> +#define SLAB_SKIP_KFENCE            0
> +#endif
> +
>  /* The following flags affect the page allocator grouping pages by mobility */
>  /* Objects are reclaimable */
>  #define SLAB_RECLAIM_ACCOUNT   ((slab_flags_t __force)0x00020000U)
> diff --git a/mm/kfence/core.c b/mm/kfence/core.c
> index c252081b11df..8c08ae2101d7 100644
> --- a/mm/kfence/core.c
> +++ b/mm/kfence/core.c
> @@ -1003,6 +1003,13 @@ void *__kfence_alloc(struct kmem_cache *s, size_t size, gfp_t flags)
>                 return NULL;
>         }
>
> +       /*
> +        * Skip allocations for this slab, if KFENCE has been disabled for
> +        * this slab.
> +        */
> +       if (s->flags & SLAB_SKIP_KFENCE)
> +               return NULL;
> +
>         if (atomic_inc_return(&kfence_allocation_gate) > 1)
>                 return NULL;
>  #ifdef CONFIG_KFENCE_STATIC_KEYS
> diff --git a/mm/slub.c b/mm/slub.c
> index 862dbd9af4f5..ee8b48327536 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -5745,6 +5745,30 @@ STAT_ATTR(CPU_PARTIAL_NODE, cpu_partial_node);
>  STAT_ATTR(CPU_PARTIAL_DRAIN, cpu_partial_drain);
>  #endif /* CONFIG_SLUB_STATS */
>
> +#ifdef CONFIG_KFENCE
> +static ssize_t skip_kfence_show(struct kmem_cache *s, char *buf)
> +{
> +       return sysfs_emit(buf, "%d\n", !!(s->flags & SLAB_SKIP_KFENCE));
> +}
> +
> +static ssize_t skip_kfence_store(struct kmem_cache *s,
> +                       const char *buf, size_t length)
> +{
> +       int ret = length;
> +
> +       if (buf[0] == '0')
> +               s->flags &= ~SLAB_SKIP_KFENCE;
> +       else if (buf[0] == '1')
> +               s->flags |= SLAB_SKIP_KFENCE;
> +       else
> +               ret = -EINVAL;
> +
> +       return ret;
> +}
> +SLAB_ATTR(skip_kfence);
> +

^ unnecessary space?


> +#endif
> +
>  static struct attribute *slab_attrs[] = {
>         &slab_size_attr.attr,
>         &object_size_attr.attr,
> @@ -5812,6 +5836,9 @@ static struct attribute *slab_attrs[] = {
>         &failslab_attr.attr,
>  #endif
>         &usersize_attr.attr,
> +#ifdef CONFIG_KFENCE
> +       &skip_kfence_attr.attr,
> +#endif
>
>         NULL
>  };
>
> base-commit: 40d43a7507e1547dd45cb02af2e40d897c591870
> --
> 2.30.2
>