netdev - Re: [PATCH][next] sfc: Replace zero-length array with flexible-array member

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Message-ID: <6a100eb2-4ba1-32ac-35de-d02c38503785@solarflare.com>
Date:   Tue, 25 Feb 2020 13:42:34 +0000
From:   Martin Habets <mhabets@...arflare.com>
To:     "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        "David S. Miller" <davem@...emloft.net>
CC:     Solarflare linux maintainers <linux-net-drivers@...arflare.com>,
        "Edward Cree" <ecree@...arflare.com>, <netdev@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH][next] sfc: Replace zero-length array with flexible-array
 member

On 25/02/2020 00:06, 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]

These padding fields are only used to enforce alignment of the struct.
But the patch is still ok.

> 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>

Acked-by: Martin Habets <mhabets@...arflare.com>

> ---
>  drivers/net/ethernet/sfc/falcon/net_driver.h | 2 +-
>  drivers/net/ethernet/sfc/net_driver.h        | 2 +-
>  2 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/net/ethernet/sfc/falcon/net_driver.h b/drivers/net/ethernet/sfc/falcon/net_driver.h
> index a49ea2e719b6..a529ff395ead 100644
> --- a/drivers/net/ethernet/sfc/falcon/net_driver.h
> +++ b/drivers/net/ethernet/sfc/falcon/net_driver.h
> @@ -288,7 +288,7 @@ struct ef4_rx_buffer {
>  struct ef4_rx_page_state {
>  	dma_addr_t dma_addr;
>  
> -	unsigned int __pad[0] ____cacheline_aligned;
> +	unsigned int __pad[] ____cacheline_aligned;
>  };
>  
>  /**
> diff --git a/drivers/net/ethernet/sfc/net_driver.h b/drivers/net/ethernet/sfc/net_driver.h
> index 9f9886f222c8..392bd5b7017e 100644
> --- a/drivers/net/ethernet/sfc/net_driver.h
> +++ b/drivers/net/ethernet/sfc/net_driver.h
> @@ -336,7 +336,7 @@ struct efx_rx_buffer {
>  struct efx_rx_page_state {
>  	dma_addr_t dma_addr;
>  
> -	unsigned int __pad[0] ____cacheline_aligned;
> +	unsigned int __pad[] ____cacheline_aligned;
>  };
>  
>  /**
>