linux-kernel - Re: [PATCH][next] mtd: rawnand: Replace one-element array with flexible-array member

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Date:   Wed, 30 Sep 2020 23:10:43 +0200
From:   Jann Horn <jannh@...gle.com>
To:     "Gustavo A. R. Silva" <gustavoars@...nel.org>
Cc:     Stefan Agner <stefan@...er.ch>, Lucas Stach <dev@...xeye.de>,
        Miquel Raynal <miquel.raynal@...tlin.com>,
        Richard Weinberger <richard@....at>,
        Vignesh Raghavendra <vigneshr@...com>,
        Thierry Reding <thierry.reding@...il.com>,
        Jonathan Hunter <jonathanh@...dia.com>,
        linux-mtd@...ts.infradead.org, linux-tegra@...r.kernel.org,
        kernel list <linux-kernel@...r.kernel.org>,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH][next] mtd: rawnand: Replace one-element array with
 flexible-array member

On Wed, Sep 30, 2020 at 11:02 PM Gustavo A. R. Silva
<gustavoars@...nel.org> wrote:
> 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 “flexible array members”[1] for these cases. The older
> style of one-element or zero-length arrays should no longer be used[2].

But this is not such a case, right? Isn't this a true fixed-size
array? It sounds like you're just changing it because it
pattern-matched on "array of length 1 at the end of a struct".

> Refactor the code according to the use of a flexible-array member
> instead of a one-element array. Also, make use of the struct_size()
> helper to calculate the size of the allocation for _nand_. In order
> to keep the code as maintainable as possible and to keep _cs_ as an
> array, add a new macro CS_N to aid in the allocation size calculation,
> in case there is a need for more Chip Select IDs in the future. In the
> meantime, the macro is set to 1. This also avoids having to use a magic
> number '1' as the last argument for struct_size().
[...]
> diff --git a/drivers/mtd/nand/raw/tegra_nand.c b/drivers/mtd/nand/raw/tegra_nand.c
[...]
> +/* Number of Chip Selects. Currently, only one. */
> +#define CS_N                   1
> +
>  struct tegra_nand_controller {
>         struct nand_controller controller;
>         struct device *dev;
> @@ -183,7 +186,7 @@ struct tegra_nand_chip {
>         u32 config;
>         u32 config_ecc;
>         u32 bch_config;
> -       int cs[1];
> +       int cs[];
>  };
[...]