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Subject: Re: [PATCH][next] sfc: Replace zero-length array with flexible-array
 member
To:     Martin Habets <mhabets@solarflare.com>,
        "David S. Miller" <davem@davemloft.net>
Cc:     Solarflare linux maintainers <linux-net-drivers@solarflare.com>,
        Edward Cree <ecree@solarflare.com>, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org
References: <20200225000647.GA17795@embeddedor>
 <6a100eb2-4ba1-32ac-35de-d02c38503785@solarflare.com>
From:   "Gustavo A. R. Silva" <gustavo@embeddedor.com>
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Message-ID: <7963b6dc-cafe-f427-e634-a043c688c78a@embeddedor.com>
Date:   Tue, 25 Feb 2020 08:01:56 -0600
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On 2/25/20 07:42, Martin Habets wrote:
> 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.
> 

Correct.

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

Thanks, Martin.
--
Gustavo

>> ---
>>  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;
>>  };
>>  
>>  /**
>>