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

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Message-ID: <CAKv+Gu-eRn+H2xj=iYW8gqKRCWWzeOTbC=9W5nKae0ytq5NYGA@mail.gmail.com>
Date:   Thu, 13 Feb 2020 11:11:59 +0000
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        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

On Thu, 13 Feb 2020 at 12:09, Geert Uytterhoeven <geert@...ux-m68k.org> wrote:
>
> 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.
>

These were probably taken straight from the specification. The [1]
trick is used a lot in the UEFI specification as well, for instance.