Re: [PATCH v2 2/2] mm/slub, kunit: Add a test case for kmalloc redzone check

[Marco Elver]

On Tue, 29 Nov 2022 at 13:53, Feng Tang <feng.tang@xxxxxxxxx> wrote:
>
> On Tue, Nov 29, 2022 at 08:02:51PM +0800, Vlastimil Babka wrote:
> > On 11/29/22 12:48, Marco Elver wrote:
> > > On Tue, 29 Nov 2022 at 12:01, Vlastimil Babka <vbabka@xxxxxxx> wrote:
> > >>
> > >> On 11/29/22 10:31, Marco Elver wrote:
> > >> > On Tue, 29 Nov 2022 at 07:37, Feng Tang <feng.tang@xxxxxxxxx> wrote:
> > >
> > >> For SLAB_SKIP_KFENCE, we could also add the flag after creation to avoid
> > >> this trouble? After all there is a sysfs file to control it at runtime
> > >> anyway (via skip_kfence_store()).
> > >> In that case patch 1 would have to wrap kmem_cache_create() and the flag
> > >> addition with a new function to avoid repeating. That function could also be
> > >> adding SLAB_NO_USER_FLAGS to kmem_cache_create(), instead of the #define
> > >> DEFAULT_FLAGS.
> > >
> > > I wouldn't overcomplicate it, all we need is a way to say "this flag
> > > should not be used directly" - and only have it available via an
> > > indirect step. Availability via sysfs is one such step.
> > >
> > > And for tests, there are 2 options:
> > >
> > > 1. we could provide a function "kmem_cache_set_test_flags(cache,
> > > gfp_flags)" and define SLAB_TEST_FLAGS (which would include
> > > SLAB_SKIP_KFENCE). This still allows to set it generally, but should
> > > make abuse less likely due to the "test" in the name of that function.
> > >
> > > 2. just set it directly, s->flags |= SLAB_SKIP_KFENCE.
> > >
> > > If you're fine with #2, that seems simplest and would be my preference.
> >
> > Yeah, that's what I meant. But slub_kunit.c could still have own internal
> > cache creation function so the "|SLAB_NO_USER_FLAGS" and "s->flags |=
> > SLAB_SKIP_KFENCE" is not repeated X times.
>
> I just quickly tried adding a new wrapper, like
>
>   struct kmem_cache *debug_kmem_cache_create(const char *name, unsigned int size,
>                         unsigned int align, slab_flags_t flags,
>                         void (*ctor)(void *), slab_flags_t debug_flags);
>
> and found that, IIUC, both SLAB_KMALLOC and SLAB_NO_USER are creation
> time flag, while SLAB_SKIP_KFENCE is an allocation runtime flag which
> could be set after creation.
>
> So how about use the initial suggestion from Vlastimil to set the
> SKIP_KFENCE flag through an internal wrapper in slub_kunit.c?
>
>         /* Only for debug and test use, to skip kfence allocation */
>         static inline void kmem_cache_skip_kfence(struct kmem_cache *s)
>         {
>                 s->flags |= SLAB_SKIP_KFENCE;
>         }

Yes, that's fine - as long as it's local to slub_kunit.c, this seems
very reasonable.

Thanks,
-- Marco