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

[Feng Tang]

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; 
	}

Thanks,
Feng

> > 
> >> 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.
> 
> Agreed.