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

[Marco Elver]

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:

> >> diff --git a/mm/slab.h b/mm/slab.h
> >> index c71590f3a22b..b6cd98b16ba7 100644
> >> --- a/mm/slab.h
> >> +++ b/mm/slab.h
> >> @@ -327,7 +327,8 @@ static inline slab_flags_t kmem_cache_flags(unsigned int object_size,
> >>  /* Legal flag mask for kmem_cache_create(), for various configurations */
> >>  #define SLAB_CORE_FLAGS (SLAB_HWCACHE_ALIGN | SLAB_CACHE_DMA | \
> >>                          SLAB_CACHE_DMA32 | SLAB_PANIC | \
> >> -                        SLAB_TYPESAFE_BY_RCU | SLAB_DEBUG_OBJECTS )
> >> +                        SLAB_KMALLOC | SLAB_SKIP_KFENCE | \
> >> +                        SLAB_TYPESAFE_BY_RCU | SLAB_DEBUG_OBJECTS)
> >
> > Shouldn't this hunk be in the previous patch, otherwise that patch
> > alone will fail?
>
> Good point.
>
> > This will also make SLAB_SKIP_KFENCE generally available to be used
> > for cache creation. This is a significant change, and before it wasn't
> > possible. Perhaps add a brief note to the commit message (or have a
> > separate patch). We were trying to avoid making this possible, as it
> > might be abused - however, given it's required for tests like these, I
> > suppose there's no way around it.
>
> 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.

> For SLAB_KMALLOC there's probably no such way unless we abuse the internal
> APIs even more and call e.g. create_boot_cache() instead of
> kmem_cache_create(). But that one is __init, so probably not. If we do
> instead allow the flag, I wouldn't add it to SLAB_CORE_FLAGS but rather
> SLAB_CACHE_FLAGS and SLAB_FLAGS_PERMITTED.

I'd probably go with the simplest solution here.