Re: [PATCH] slab: kmalloc_size_roundup() must not return 0 for non-zero size

[Vlastimil Babka]

Please Cc: also R: folks in MAINTAINERS, adding them now.

On 9/6/23 10:18, David Laight wrote:
> The typical use of kmalloc_size_roundup() is:
> 	ptr = kmalloc(sz = kmalloc_size_roundup(size), ...);
> 	if (!ptr) return -ENOMEM.
> This means it is vitally important that the returned value isn't
> less than the argument even if the argument is insane.
> In particular if kmalloc_slab() fails or the value is above
> (MAX_ULONG - PAGE_SIZE) zero is returned and kmalloc() will return
> it's single zero-length buffer.
> 
> Fix by returning the input size on error or if the size exceeds
> a 'sanity' limit.
> kmalloc() will then return NULL is the size really is too big.
> 
> Signed-off-by: David Laight <david.laight@xxxxxxxxxx>
> Fixes: 05a940656e1eb ("slab: Introduce kmalloc_size_roundup()")
> ---
> The 'sanity limit' value doesn't really matter (even if too small)
> It could be 'MAX_ORDER + PAGE_SHIFT' but one ppc64 has MAX_ORDER 16
> and I don't know if that also has large pages.

Well we do have KMALLOC_MAX_SIZE, which is based on MAX_ORDER + PAGE_SHIFT
(and no issues on ppc64 so I'd expect the combination of MAX_ORDER and
PAGE_SHIFT should always be such that it doesn't overflow on the particular
arch) so I think it would be the most straightforward to simply use that.

> Maybe it could be 1ul << 30 on 64bit, but it really doesn't matter
> if it is too big.
> 
> The original patch also added kmalloc_size_roundup() to mm/slob.c
> that can also round up a value to zero - but has since been removed.
> 
>  mm/slab_common.c | 29 ++++++++++++++---------------
>  1 file changed, 14 insertions(+), 15 deletions(-)
> 
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index cd71f9581e67..8418eccda8cf 100644
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -747,22 +747,21 @@ size_t kmalloc_size_roundup(size_t size)
>  {
>  	struct kmem_cache *c;
>  
> -	/* Short-circuit the 0 size case. */
> -	if (unlikely(size == 0))
> -		return 0;
> -	/* Short-circuit saturated "too-large" case. */
> -	if (unlikely(size == SIZE_MAX))
> -		return SIZE_MAX;
> -	/* Above the smaller buckets, size is a multiple of page size. */
> -	if (size > KMALLOC_MAX_CACHE_SIZE)
> -		return PAGE_SIZE << get_order(size);
> +	if (size && size <= KMALLOC_MAX_CACHE_SIZE) {

I guess the whole test could all be likely().

Also this patch could probably be just replacing the SIZE_MAX test with >=
KMALLOC_MAX_SIZE, but since the majority is expected to be 0 < size <=
KMALLOC_MAX_CACHE_SIZE, your reordering makes sense to me.

> +		/*
> +		 * The flags don't matter since size_index is common to all.
> +		 * Neither does the caller for just getting ->object_size.
> +		 */
> +		c = kmalloc_slab(size, GFP_KERNEL, 0);
> +		return likely(c) ? c->object_size : size;
> +	}
>  
> -	/*
> -	 * The flags don't matter since size_index is common to all.
> -	 * Neither does the caller for just getting ->object_size.
> -	 */
> -	c = kmalloc_slab(size, GFP_KERNEL, 0);
> -	return c ? c->object_size : 0;
> +	/* Return 'size' for 0 and very large - kmalloc() may fail. */
> +	if (unlikely((size - 1) >> (sizeof (long) == 8 ? 34 : 30)))

So I'd just test for size == 0 || size > KMALLOC_MAX_SIZE?

> +		return size;
> +
> +	/* Above the smaller buckets, size is a multiple of page size. */
> +	return PAGE_SIZE << get_order(size);
>  }
>  EXPORT_SYMBOL(kmalloc_size_roundup);
>