Re: [PATCH RFC] slab: introduce knalloc/kxnalloc

[Pekka Enberg]

On Thu, Feb 9, 2012 at 2:41 PM, Xi Wang <xi.wang@xxxxxxxxx> wrote:
> This patch introduces knalloc/kxnalloc wrappers that perform integer
> overflow checks without zeroing the memory.
>
> knalloc(n, size, flags) is the non-zeroing version of kcalloc(),
> which allocates n * size bytes.
>
> kxnalloc(xsize, n, size, flags) allocates xsize + n * size bytes.
> It is useful to allocate a structure ending with a zero-length array,
> which is a commonly used pattern.  For example, in posix_acl_alloc()
> to allocate a posix_acl object one could call
>
>    kxnalloc(sizeof(struct posix_acl),
>             count, sizeof(struct posix_acl_entry), flags);
>
> to avoid overflowing the allocation size.
>
> Suggested-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Xi Wang <xi.wang@xxxxxxxxx>

Are there really enough potential users to justify adding both?

> ---
> I coined the name knalloc() for Andrew's wtf_do_i_call_this() in the
> previous email.  A better name is welcome.
>
> I additionally added kxnalloc() since it's a common idiom.
> ---
>  include/linux/slab.h |   31 +++++++++++++++++++++++++++----
>  1 files changed, 27 insertions(+), 4 deletions(-)
>
> diff --git a/include/linux/slab.h b/include/linux/slab.h
> index 573c809..e3a1e81 100644
> --- a/include/linux/slab.h
> +++ b/include/linux/slab.h
> @@ -190,7 +190,8 @@ size_t ksize(const void *);
>  #endif
>
>  /**
> - * kcalloc - allocate memory for an array. The memory is set to zero.
> + * kxnalloc - allocate memory for an array with an extra header.
> + * @xsize: extra header size.
>  * @n: number of elements.
>  * @size: element size.
>  * @flags: the type of memory to allocate.
> @@ -240,11 +241,33 @@ size_t ksize(const void *);
>  * for general use, and so are not documented here. For a full list of
>  * potential flags, always refer to linux/gfp.h.
>  */
> -static inline void *kcalloc(size_t n, size_t size, gfp_t flags)
> +static inline void *kxnalloc(size_t xsize, size_t n, size_t size, gfp_t flags)
>  {
> -       if (size != 0 && n > ULONG_MAX / size)
> +       if (size != 0 && n > (ULONG_MAX - xsize) / size)
>                return NULL;
> -       return __kmalloc(n * size, flags | __GFP_ZERO);
> +       return __kmalloc(xsize + n * size, flags);
> +}
> +
> +/**
> + * knalloc - allocate memory for an array.
> + * @n: number of elements.
> + * @size: element size.
> + * @flags: the type of memory to allocate (see kmalloc).
> + */
> +static inline void *knalloc(size_t n, size_t size, gfp_t flags)
> +{
> +       return kxnalloc(0, n, size, flags);
> +}

> +/**
> + * kcalloc - allocate memory for an array. The memory is set to zero.
> + * @n: number of elements.
> + * @size: element size.
> + * @flags: the type of memory to allocate (see kmalloc).
> + */
> +static inline void *kcalloc(size_t n, size_t size, gfp_t flags)
> +{
> +       return knalloc(n, size, flags | __GFP_ZERO);
>  }
>
>  #if !defined(CONFIG_NUMA) && !defined(CONFIG_SLOB)
--
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