linux-kernel - Re: [PATCH v1 1/8] kasan: Change the behavior of kmalloc_large_oob

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Date:	Tue, 2 Feb 2016 17:25:21 +0100
From:	Alexander Potapenko <glider@...gle.com>
To:	Andrey Ryabinin <ryabinin.a.a@...il.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Andrey Konovalov <adech.fo@...il.com>,
	Christoph Lameter <cl@...ux.com>,
	Dmitriy Vyukov <dvyukov@...gle.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	kasan-dev@...glegroups.com, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [PATCH v1 1/8] kasan: Change the behavior of kmalloc_large_oob_right
 test

The intention was to detect the situation in which a new allocator
appears for which we don't know how it behaves if we allocate more
than KMALLOC_MAX_CACHE_SIZE.
I agree this makes little sense and we can just stick to
CONFIG_SLAB/CONFIG_SLUB cases.

However I think it's better to keep 'size = KMALLOC_MAX_CACHE_SIZE +
something' to keep this code working in the case the value of
KMALLOC_MAX_CACHE_SIZE changes.

On Tue, Feb 2, 2016 at 4:29 PM, Andrey Ryabinin <ryabinin.a.a@...il.com> wrote:
>
>
> On 02/02/2016 08:34 AM, Andrew Morton wrote:
>> On Wed, 27 Jan 2016 19:25:06 +0100 Alexander Potapenko <glider@...gle.com> wrote:
>>
>>> depending on which allocator (SLAB or SLUB) is being used
>>>
>>> ...
>>>
>>> --- a/lib/test_kasan.c
>>> +++ b/lib/test_kasan.c
>>> @@ -68,7 +68,22 @@ static noinline void __init kmalloc_node_oob_right(void)
>>>  static noinline void __init kmalloc_large_oob_right(void)
>>>  {
>>>      char *ptr;
>>> -    size_t size = KMALLOC_MAX_CACHE_SIZE + 10;
>>> +    size_t size;
>>> +
>>> +    if (KMALLOC_MAX_CACHE_SIZE == KMALLOC_MAX_SIZE) {
>>> +            /*
>>> +             * We're using the SLAB allocator. Allocate a chunk that fits
>>> +             * into a slab.
>>> +             */
>>> +            size = KMALLOC_MAX_CACHE_SIZE - 256;
>>> +    } else {
>>> +            /*
>>> +             * KMALLOC_MAX_SIZE > KMALLOC_MAX_CACHE_SIZE.
>>> +             * We're using the SLUB allocator. Allocate a chunk that does
>>> +             * not fit into a slab to trigger the page allocator.
>>> +             */
>>> +            size = KMALLOC_MAX_CACHE_SIZE + 10;
>>> +    }
>>
>> This seems a weird way of working out whether we're using SLAB or SLUB.
>>
>> Can't we use, umm, #ifdef CONFIG_SLAB?  If not that then let's cook up
>> something standardized rather than a weird just-happens-to-work like
>> this.
>>
>
> Actually it would be simpler to not use KMALLOC_MAX_CACHE_SIZE at all.
> Simply replace it with 2 or 3 PAGE_SIZEs.

-- 
Alexander Potapenko
Software Engineer

Google Germany GmbH
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