linux-kernel - Re: [PATCH] PNPBIOS: Replace zero-length array with flexible-array

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Message-ID: <CAJZ5v0hCERp+5m33qhXOkq1LDEj87CbZkiz7t_oD7HmD9cA+Xw@mail.gmail.com>
Date:   Fri, 15 May 2020 18:23:57 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     "Gustavo A. R. Silva" <gustavoars@...nel.org>
Cc:     "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] PNPBIOS: Replace zero-length array with flexible-array

On Thu, May 7, 2020 at 9:02 PM Gustavo A. R. Silva
<gustavoars@...nel.org> wrote:
>
> The current codebase makes use of the zero-length array language
> extension to the C90 standard, but the preferred mechanism to declare
> variable-length types such as these ones is a flexible array member[1][2],
> introduced in C99:
>
> struct foo {
>         int stuff;
>         struct boo array[];
> };
>
> By making use of the mechanism above, we will get a compiler warning
> in case the flexible array does not occur last in the structure, which
> will help us prevent some kind of undefined behavior bugs from being
> inadvertently introduced[3] to the codebase from now on.
>
> Also, notice that, dynamic memory allocations won't be affected by
> this change:
>
> "Flexible array members have incomplete type, and so the sizeof operator
> may not be applied. As a quirk of the original implementation of
> zero-length arrays, sizeof evaluates to zero."[1]
>
> sizeof(flexible-array-member) triggers a warning because flexible array
> members have incomplete type[1]. There are some instances of code in
> which the sizeof operator is being incorrectly/erroneously applied to
> zero-length arrays and the result is zero. Such instances may be hiding
> some bugs. So, this work (flexible-array member conversions) will also
> help to get completely rid of those sorts of issues.
>
> This issue was found with the help of Coccinelle.
>
> [1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
> [2] https://github.com/KSPP/linux/issues/21
> [3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")
>
> Signed-off-by: Gustavo A. R. Silva <gustavoars@...nel.org>
> ---
>  drivers/pnp/pnpbios/pnpbios.h |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/pnp/pnpbios/pnpbios.h b/drivers/pnp/pnpbios/pnpbios.h
> index 37acb8378f39..2ce739ff9c1a 100644
> --- a/drivers/pnp/pnpbios/pnpbios.h
> +++ b/drivers/pnp/pnpbios/pnpbios.h
> @@ -107,7 +107,7 @@ struct pnp_bios_node {
>         __u32 eisa_id;
>         __u8 type_code[3];
>         __u16 flags;
> -       __u8 data[0];
> +       __u8 data[];
>  };
>  #pragma pack()

Applied as 5.8 material, thanks!