Re: [PATCH v2] Introduce sysfs interface to disable kfence for selected slabs.

[Hyeonggon Yoo]

On Thu, Aug 11, 2022 at 06:59:38PM +1000, Imran Khan 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>
> ---
> 
> Changes since v1:
>  - Remove RFC tag
> 
>  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);
> +
> +#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

No strong opinion on its interface, but from view of correctness:

Looks good to me.
Reviewed-by: Hyeonggon Yoo <42.hyeyoo@xxxxxxxxx>

-- 
Thanks,
Hyeonggon