Re: [PATCH] mm/slub: clean up create_unique_id()

[Chao Yu <chao@kernel.org>]

On 2022/9/23 4:47, Vlastimil Babka wrote:
> On 9/18/22 11:21, Chao Yu wrote:
>> From: Chao Yu <chao.yu@oppo.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. remove BUG_ON() and return error if check condition is true.
> 
> I'd leave a WARN_ON there as we really don't expect this to happen, so if it
> does, we should be loud about it and not silently fail.

Agreed.

> 
>> Link: https://lore.kernel.org/linux-mm/2025305d-16db-abdf-6cd3-1fb93371c2b4@wanadoo.fr/
>> Suggested-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
>> Fixes: 81819f0fc828 ("SLUB core")
>> Signed-off-by: Chao Yu <chao.yu@oppo.com>
>> ---
>>   mm/slub.c | 7 +++++--
>>   1 file changed, 5 insertions(+), 2 deletions(-)
>>
>> diff --git a/mm/slub.c b/mm/slub.c
>> index 5ba6db62a5ab..a045c1ca8772 100644
>> --- a/mm/slub.c
>> +++ b/mm/slub.c
>> @@ -5883,7 +5883,7 @@ 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:
>>    *
>> @@ -5919,7 +5919,10 @@ static char *create_unique_id(struct kmem_cache *s)
>>   		*p++ = '-';
>>   	p += sprintf(p, "%07u", s->size);
> 
> Hm but sprintf() will happily overflow, so if we only detect that
> afterwards, it's kinda too late to gracefully fail.
> Should use snprintf() then?

Correct.

Updated in v2.

Thanks,

> 
>> -	BUG_ON(p > name + ID_STR_LENGTH - 1);
>> +	if (p > name + ID_STR_LENGTH - 1) {
>> +		kfree(name);
>> +		return ERR_PTR(-EINVAL);
>> +	}
>>   	return name;
>>   }
>>   
>