linux-kernel - Re: [PATCH][next] media: venus: hfi_cmds.h: Replace zero-length array with flexible-array member

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Message-ID: <849ddb2c-d022-bd13-89c2-b0c1a4363a23@linaro.org>
Date:   Wed, 1 Apr 2020 11:37:30 +0300
From:   Stanimir Varbanov <stanimir.varbanov@...aro.org>
To:     "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        Stanimir Varbanov <stanimir.varbanov@...aro.org>,
        Andy Gross <agross@...nel.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Mauro Carvalho Chehab <mchehab@...nel.org>
Cc:     linux-media@...r.kernel.org, linux-arm-msm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH][next] media: venus: hfi_cmds.h: Replace zero-length array
 with flexible-array member

Hi Gustavo,

Thanks for the patch!

On 3/20/20 12:21 AM, 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/media/platform/qcom/venus/hfi_cmds.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 

Acked-by: Stanimir Varbanov <staninmir.varbanov@...aro.org>

> diff --git a/drivers/media/platform/qcom/venus/hfi_cmds.h b/drivers/media/platform/qcom/venus/hfi_cmds.h
> index cae9d5d61c0c..83705e237f1c 100644
> --- a/drivers/media/platform/qcom/venus/hfi_cmds.h
> +++ b/drivers/media/platform/qcom/venus/hfi_cmds.h
> @@ -107,7 +107,7 @@ struct hfi_session_abort_pkt {
>  struct hfi_session_set_property_pkt {
>  	struct hfi_session_hdr_pkt shdr;
>  	u32 num_properties;
> -	u32 data[0];
> +	u32 data[];
>  };
>  
>  struct hfi_session_set_buffers_pkt {
> 

-- 
regards,
Stan