netdev - Re: [PATCH net-next] net: sched: cls_u32: Use flexible-array member

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Message-ID: <CAM_iQpXi6JAOK7bWMazvOEwa=S4U3=5L5cqwKST9FKFfr+GkzQ@mail.gmail.com>
Date:   Thu, 16 Jan 2020 20:43:55 -0800
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Cc:     Jamal Hadi Salim <jhs@...atatu.com>, Jiri Pirko <jiri@...nulli.us>,
        "David S. Miller" <davem@...emloft.net>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net-next] net: sched: cls_u32: Use flexible-array member

On Thu, Jan 16, 2020 at 1:36 PM Gustavo A. R. Silva
<gustavo@...eddedor.com> wrote:
>
> Old code in the kernel uses 1-byte and 0-byte arrays to indicate the
> presence of a "variable length array":
>
> struct something {
>     int length;
>     u8 data[1];
> };
>
> struct something *instance;
>
> instance = kmalloc(sizeof(*instance) + size, GFP_KERNEL);
> instance->length = size;
> memcpy(instance->data, source, size);

Hmm, your patch is not correct, for u32 it is length+1, so you have
to allocate size+1 after you switch to zero-length array.

Take a look at u32_walk() if you have any doubt.

Thanks.