linux-kernel - Re: [PATCH] ubifs: super: Use struct

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Date:   Sun, 15 Sep 2019 23:58:28 +0200
From:   Richard Weinberger <richard.weinberger@...il.com>
To:     "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Cc:     Richard Weinberger <richard@....at>,
        Artem Bityutskiy <dedekind1@...il.com>,
        Adrian Hunter <adrian.hunter@...el.com>,
        linux-mtd@...ts.infradead.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ubifs: super: Use struct_size() helper

On Thu, Aug 29, 2019 at 2:50 AM Gustavo A. R. Silva
<gustavo@...eddedor.com> wrote:
>
> One of the more common cases of allocation size calculations is finding
> the size of a structure that has a zero-sized array at the end, along
> with memory for some number of elements for that array. For example:
>
> struct ubifs_znode {
>         ...
>         struct ubifs_zbranch zbranch[];
> };
>
> Make use of the struct_size() helper instead of an open-coded version
> in order to avoid any potential type mistakes.
>
> So, replace the following form:
>
> sizeof(struct ubifs_znode) + c->fanout * sizeof(struct ubifs_zbranch)
>
> with:
>
> struct_size(c->cnext, zbranch, c->fanout)
>
> This code was detected with the help of Coccinelle.
>
> Signed-off-by: Gustavo A. R. Silva <gustavo@...eddedor.com>
> ---
>  fs/ubifs/super.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
>
> diff --git a/fs/ubifs/super.c b/fs/ubifs/super.c
> index 2706f13e3eb9..ca86489048c8 100644
> --- a/fs/ubifs/super.c
> +++ b/fs/ubifs/super.c
> @@ -661,8 +661,7 @@ static int init_constants_sb(struct ubifs_info *c)
>         long long tmp64;
>
>         c->main_bytes = (long long)c->main_lebs * c->leb_size;
> -       c->max_znode_sz = sizeof(struct ubifs_znode) +
> -                               c->fanout * sizeof(struct ubifs_zbranch);
> +       c->max_znode_sz = struct_size(c->cnext, zbranch, c->fanout);

First of all, c->fanout is bound checked.
I had to lookup how struct_size() works to understand this
single line of code and why you use the completely unrelated c->cnext here.
Sorry this change does not make the code any better just harder to read.

-- 
Thanks,
//richard