linux-kernel - Re: [PATCH] rpmsg: virtio_rpmsg_bus: Replace zero-length array with flexible-array member

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Message-ID: <6d7bd66d-ebea-dc4a-03ff-5da9d313a75d@embeddedor.com>
Date:   Thu, 27 Feb 2020 13:10:24 -0600
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     Ohad Ben-Cohen <ohad@...ery.com>,
        Bjorn Andersson <bjorn.andersson@...aro.org>
Cc:     linux-remoteproc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rpmsg: virtio_rpmsg_bus: Replace zero-length array with
 flexible-array member

Hi all,

Friendly ping: Who can take this?

Thanks
--
Gustavo

On 2/12/20 18:22, 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/rpmsg/virtio_rpmsg_bus.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/rpmsg/virtio_rpmsg_bus.c b/drivers/rpmsg/virtio_rpmsg_bus.c
> index 376ebbf880d6..07d4f3374098 100644
> --- a/drivers/rpmsg/virtio_rpmsg_bus.c
> +++ b/drivers/rpmsg/virtio_rpmsg_bus.c
> @@ -89,7 +89,7 @@ struct rpmsg_hdr {
>  	u32 reserved;
>  	u16 len;
>  	u16 flags;
> -	u8 data[0];
> +	u8 data[];
>  } __packed;
>  
>  /**
>