linux-kernel - Re: [PATCH v2] mm/slub: clean up create_unique

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Date:   Mon, 26 Sep 2022 16:24:08 +0800
From:   Chao Yu <chao@...nel.org>
To:     Vlastimil Babka <vbabka@...e.cz>, Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Hugh Dickins <hugh@...itas.com>
Cc:     Chao Yu <chao.yu@...o.com>,
        Christophe JAILLET <christophe.jaillet@...adoo.fr>,
        Hyeonggon Yoo <42.hyeyoo@...il.com>,
        Roman Gushchin <roman.gushchin@...ux.dev>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] mm/slub: clean up create_unique_id()

On 2022/9/26 16:22, Vlastimil Babka wrote:
> On 9/25/22 09:12, Chao Yu wrote:
>> From: Chao Yu <chao.yu@...o.com>
>>
>> As Christophe JAILLET suggested:
>>
>> In create_unique_id(),
>>
>> "looks that ID_STR_LENGTH could even be reduced to 32 or 16.
>>
>> The 2nd BUG_ON at the end of the function could certainly be just
>> removed as well or remplaced by a:
>>          if (p > name + ID_STR_LENGTH - 1) {
>>                  kfree(name);
>>                  return -E<something>;
>>          }
>> "
>>
>> According to above suggestion, let's do below cleanups:
>> 1. reduce ID_STR_LENGTH to 32, as the buffer size should be enough;
>> 2. use WARN_ON instead of BUG_ON() and return error if check condition
>> is true;
>> 3. use snprintf instead of sprintf to avoid overflow.
>>
>> Link: https://lore.kernel.org/linux-mm/2025305d-16db-abdf-6cd3-1fb93371c2b4@wanadoo.fr/
>> Fixes: 81819f0fc828 ("SLUB core")
>> Suggested-by: Christophe JAILLET <christophe.jaillet@...adoo.fr>
>> Reviewed-by: Hyeonggon Yoo <42.hyeyoo@...il.com>
>> Signed-off-by: Chao Yu <chao.yu@...o.com>
>> ---
>> v2:
>> - add WARN_ON() instead of return error silently;
>> - use snprintf instead of sprintf to avoid overflow.
>>   mm/slub.c | 10 +++++++---
>>   1 file changed, 7 insertions(+), 3 deletions(-)
>>
>> diff --git a/mm/slub.c b/mm/slub.c
>> index 4b98dff9be8e..3d37a8a7b965 100644
>> --- a/mm/slub.c
>> +++ b/mm/slub.c
>> @@ -5890,7 +5890,7 @@ static inline struct kset *cache_kset(struct kmem_cache *s)
>>       return slab_kset;
>>   }
>> -#define ID_STR_LENGTH 64
>> +#define ID_STR_LENGTH 32
>>   /* Create a unique string id for a slab cache:
>>    *
>> @@ -5924,9 +5924,13 @@ static char *create_unique_id(struct kmem_cache *s)
>>           *p++ = 'A';
>>       if (p != name + 1)
>>           *p++ = '-';
>> -    p += sprintf(p, "%07u", s->size);
>> +    p += snprintf(p, ID_STR_LENGTH - 1 - (p - name), "%07u", s->size);
> 
> I think we don't need "- 1" here as snprintf() says:
> @size: The size of the buffer, including the trailing null space

Oops, my bad.

> 
>> -    BUG_ON(p > name + ID_STR_LENGTH - 1);
>> +    if (p > name + ID_STR_LENGTH - 1) {
>> +        WARN_ON(1);
> 
> This would be shorter: if (WARN_ON(p > name...))

Thanks for the comments, will update soon.

Thanks,

> 
>> +        kfree(name);
>> +        return ERR_PTR(-EINVAL);
>> +    }
>>       return name;
>>   }
>