netdev - Re: [PATCH] treewide: Replace zero-length arrays with flexible-array member

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Date:   Thu, 13 Feb 2020 12:09:08 +0100
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Kees Cook <keescook@...omium.org>,
        "David S. Miller" <davem@...emloft.net>,
        Greg KH <gregkh@...uxfoundation.org>,
        Linux Crypto Mailing List <linux-crypto@...r.kernel.org>,
        netdev <netdev@...r.kernel.org>,
        USB list <linux-usb@...r.kernel.org>
Subject: Re: [PATCH] treewide: Replace zero-length arrays with flexible-array member

Hi Gustavo,

On Tue, Feb 11, 2020 at 10:49 PM Gustavo A. R. Silva
<gustavo@...eddedor.com> 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
> unadvertenly introduced[3] to the codebase from now on.
>
> All these instances of code were found with the help of the following
> Coccinelle script:
>
> @@
> identifier S, member, array;
> type T1, T2;
> @@
>
> struct S {
>   ...
>   T1 member;
>   T2 array[
> - 0
>   ];
> };

I've stumbled across one more in include/uapi/linux/usb/ch9.h:

    struct usb_key_descriptor {
            __u8  bLength;
            __u8  bDescriptorType;

            __u8  tTKID[3];
            __u8  bReserved;
            __u8  bKeyData[0];
    } __attribute__((packed));

And it seems people are (ab)using one-sized arrays for flexible arrays, too:

    struct usb_string_descriptor {
            __u8  bLength;
            __u8  bDescriptorType;

            __le16 wData[1];                /* UTF-16LE encoded */
    } __attribute__ ((packed));

As this is UAPI, we have to be careful for regressions, though.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds