Re: [PATCH] mm: slab: Avoid the use of one-element array and use struct_size() helper

From: Gustavo A. R. Silva
Date: Wed Jul 29 2020 - 20:28:29 EST


Hi,

If you are going to copy/paste this, please at least CC the people
that is originally working on these changes, in this case _me_. One
needs to be very careful when doing these transformations. This
code doesn't even exist in linux-next.

If you want to land your first kernel patch, I encourage you to start
in drivers/staging/, while you get the hang of the kernel development
process and you know how to work with linux-next. :)

Thanks
--
Gustavo

On 7/29/20 09:50, Qianli Zhao wrote:
> From: Qianli Zhao <zhaoqianli@xxxxxxxxxx>
>
> There is a regular need in the kernel to provide a way to declare having a
> dynamically sized set of trailing elements in a structure. Kernel code should
> always use âflexible array membersâ[1] for these cases. The older style of
> one-element or zero-length arrays should no longer be used[2].
>
> Make use of the struct_size() helper instead of an open-coded version
> in order to avoid any potential type mistakes.
>
> [1] https://en.wikipedia.org/wiki/Flexible_array_member
> [2] https://github.com/KSPP/linux/issues/21
>
> Signed-off-by: Qianli Zhao <zhaoqianli@xxxxxxxxxx>
> ---
> mm/slab.h | 2 +-
> mm/slab_common.c | 7 ++-----
> 2 files changed, 3 insertions(+), 6 deletions(-)
>
> diff --git a/mm/slab.h b/mm/slab.h
> index 74f7e09..c12fb65 100644
> --- a/mm/slab.h
> +++ b/mm/slab.h
> @@ -34,7 +34,7 @@ struct kmem_cache {
>
> struct memcg_cache_array {
> struct rcu_head rcu;
> - struct kmem_cache *entries[0];
> + struct kmem_cache *entries[];
> };
>
> /*
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index fe8b684..56f4818 100644
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -166,9 +166,7 @@ static int init_memcg_params(struct kmem_cache *s,
> if (!memcg_nr_cache_ids)
> return 0;
>
> - arr = kvzalloc(sizeof(struct memcg_cache_array) +
> - memcg_nr_cache_ids * sizeof(void *),
> - GFP_KERNEL);
> + arr = kvzalloc(struct_size(arr, entries, memcg_nr_cache_ids), GFP_KERNEL);
> if (!arr)
> return -ENOMEM;
>
> @@ -199,8 +197,7 @@ static int update_memcg_params(struct kmem_cache *s, int new_array_size)
> {
> struct memcg_cache_array *old, *new;
>
> - new = kvzalloc(sizeof(struct memcg_cache_array) +
> - new_array_size * sizeof(void *), GFP_KERNEL);
> + new = kvzalloc(struct_size(new, entries, new_array_size), GFP_KERNEL);
> if (!new)
> return -ENOMEM;
>
>