Re: [PATCH v2] mm/slub: clean up create_unique_id()

[Vlastimil Babka <vbabka@suse.cz>]

On 9/25/22 09:12, 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. 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@wanadoo.fr>
> Reviewed-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
> Signed-off-by: Chao Yu <chao.yu@oppo.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

>   
> -	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...))

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