linux-kernel - Re: [PATCH] ALSA: fireworks: Replace zero-length array with flexible-array

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Message-ID: <20200508022737.GA332087@workstation>
Date:   Fri, 8 May 2020 11:27:37 +0900
From:   Takashi Sakamoto <o-takashi@...amocchi.jp>
To:     "Gustavo A. R. Silva" <gustavoars@...nel.org>
Cc:     Clemens Ladisch <clemens@...isch.de>, alsa-devel@...a-project.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ALSA: fireworks: Replace zero-length array with
 flexible-array

On Thu, May 07, 2020 at 01:52:45PM -0500, 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]
> 
> sizeof(flexible-array-member) triggers a warning because flexible array
> members have incomplete type[1]. There are some instances of code in
> which the sizeof operator is being incorrectly/erroneously applied to
> zero-length arrays and the result is zero. Such instances may be hiding
> some bugs. So, this work (flexible-array member conversions) will also
> help to get completely rid of those sorts of issues.
> 
> 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 <gustavoars@...nel.org>
> ---
>  sound/firewire/fireworks/fireworks.h |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
 
Acked-by: Takashi Sakamoto <o-takashi@...amocchi.jp>

> diff --git a/sound/firewire/fireworks/fireworks.h b/sound/firewire/fireworks/fireworks.h
> index dda797209a27..654e28a6669f 100644
> --- a/sound/firewire/fireworks/fireworks.h
> +++ b/sound/firewire/fireworks/fireworks.h
> @@ -177,7 +177,7 @@ struct snd_efw_phys_meters {
>  	u32 in_meters;
>  	u32 reserved4;
>  	u32 reserved5;
> -	u32 values[0];
> +	u32 values[];
>  } __packed;
>  enum snd_efw_clock_source {
>  	SND_EFW_CLOCK_SOURCE_INTERNAL	= 0,


Thanks

Takashi Sakamoto