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

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

[Gustavo A. R. Silva]

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@embeddedor.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;
 };
 
 /**
-- 
2.25.0

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

[Martin Habets]

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@embeddedor.com>

Acked-by: Martin Habets <mhabets@solarflare.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;
>  };
>  
>  /**
> 

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

[Gustavo A. R. Silva]

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

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

[David Miller]

From: "Gustavo A. R. Silva" <gustavo@embeddedor.com>
Date: Mon, 24 Feb 2020 18:06:47 -0600

> 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@embeddedor.com>

Applied.