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
