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

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Message-ID: <c531cc8e-9c37-3de7-e5c6-7fd316a9c9bb@suse.cz>
Date:   Tue, 29 Nov 2022 18:48:05 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Marco Elver <elver@...gle.com>, Feng Tang <feng.tang@...el.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Oliver Glitta <glittao@...il.com>,
        Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Hyeonggon Yoo <42.hyeyoo@...il.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 2/2] mm/slub, kunit: Add a test case for kmalloc
 redzone check

On 11/29/22 13:56, Marco Elver wrote:
> On Tue, 29 Nov 2022 at 13:53, Feng Tang <feng.tang@...el.com> 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@...e.cz> wrote:
>> > >>
>> > >> On 11/29/22 10:31, Marco Elver wrote:
>> > >> > On Tue, 29 Nov 2022 at 07:37, Feng Tang <feng.tang@...el.com> 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.

Wrapping just |= SLAB_SKIP_KFENCE won't help that much as you'd need to add
a call to kmem_cache_skip_kfence() after each kmem_cache_create() in
slub_kunit.c. That's why I propose a wrapper, *also internally defined in
slub_kunit.c*, that calls kmem_cache_create() with flags
|SLAB_NO_USER_FLAGS, then does s->flags |= SLAB_SKIP_KFENCE; then returns s.
At this point said wrapper wouldn't even need align and ctor parameters and
could pass 0 and NULL to kmem_cache_create() by itself, as no test uses
different values.

> Thanks,
> -- Marco