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Message-ID: <bbe8a83fade9c0cffbf439f06f94e8b5bc7fcfd3.camel@pengutronix.de>
Date: Thu, 05 Mar 2020 12:17:48 +0100
From: Lucas Stach <l.stach@...gutronix.de>
To: "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
Russell King <linux+etnaviv@...linux.org.uk>,
Christian Gmeiner <christian.gmeiner@...il.com>,
David Airlie <airlied@...ux.ie>,
Daniel Vetter <daniel@...ll.ch>
Cc: etnaviv@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH][next] drm: etnaviv_gem.h: Replace zero-length array
with flexible-array member
Hi Gustavo,
I've adjusted the subject a bit to match the style of the other etnaviv
driver commits and applied this to the etnaviv/next branch.
Regards,
Lucas
On Do, 2020-03-05 at 04:51 -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
> 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]
>
> 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 <gustavo@...eddedor.com>
> ---
> drivers/gpu/drm/etnaviv/etnaviv_gem.h | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/etnaviv/etnaviv_gem.h b/drivers/gpu/drm/etnaviv/etnaviv_gem.h
> index 6b68fe16041b..98e60df882b6 100644
> --- a/drivers/gpu/drm/etnaviv/etnaviv_gem.h
> +++ b/drivers/gpu/drm/etnaviv/etnaviv_gem.h
> @@ -105,7 +105,7 @@ struct etnaviv_gem_submit {
> unsigned int nr_pmrs;
> struct etnaviv_perfmon_request *pmrs;
> unsigned int nr_bos;
> - struct etnaviv_gem_submit_bo bos[0];
> + struct etnaviv_gem_submit_bo bos[];
> /* No new members here, the previous one is variable-length! */
> };
>
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