linux-kernel - Re: [PATCH] drm/qxl: replace zero-length array with flexible-array member

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Message-ID: <20200213105218.xsjy3bv2d6c2y2mc@sirius.home.kraxel.org>
Date:   Thu, 13 Feb 2020 11:52:18 +0100
From:   Gerd Hoffmann <kraxel@...hat.com>
To:     "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Cc:     Dave Airlie <airlied@...hat.com>, David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        virtualization@...ts.linux-foundation.org,
        spice-devel@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm/qxl: replace zero-length array with flexible-array
 member

On Wed, Feb 12, 2020 at 01:33:44PM -0600, Gustavo A. R. Silva 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
> inadvertenly 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]
> 
> 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")

Pushed to drm-misc-next.

thanks,
  Gerd