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

[Vlastimil Babka]

On 9/18/22 11:21, Chao Yu wrote:
> From: Chao Yu <chao.yu@xxxxxxxx>
> 
> 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.

> Link: https://lore.kernel.org/linux-mm/2025305d-16db-abdf-6cd3-1fb93371c2b4@xxxxxxxxxx/
> Suggested-by: Christophe JAILLET <christophe.jaillet@xxxxxxxxxx>
> Fixes: 81819f0fc828 ("SLUB core")
> Signed-off-by: Chao Yu <chao.yu@xxxxxxxx>
> ---
>  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?

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