[PATCH][next] drm: etnaviv_gem.h: Replace zero-length array with flexible-array member

[PATCH][next] drm: etnaviv_gem.h: 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/gpu/drm/etnaviv/etnaviv_gem.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/etnaviv/etnaviv_gem.h b/drivers/gpu/drm/etnaviv/etnaviv_gem.h
index 6b68fe16041b..98e60df882b6 100644
--- a/drivers/gpu/drm/etnaviv/etnaviv_gem.h
+++ b/drivers/gpu/drm/etnaviv/etnaviv_gem.h
@@ -105,7 +105,7 @@ struct etnaviv_gem_submit {
 	unsigned int nr_pmrs;
 	struct etnaviv_perfmon_request *pmrs;
 	unsigned int nr_bos;
-	struct etnaviv_gem_submit_bo bos[0];
+	struct etnaviv_gem_submit_bo bos[];
 	/* No new members here, the previous one is variable-length! */
 };
 
-- 
2.25.0

Re: [PATCH][next] drm: etnaviv_gem.h: Replace zero-length array with flexible-array member

[Lucas Stach]

Hi Gustavo,

I've adjusted the subject a bit to match the style of the other etnaviv
driver commits and applied this to the etnaviv/next branch.

Regards,
Lucas

On Do, 2020-03-05 at 04:51 -0600, 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@embeddedor.com>
> ---
>  drivers/gpu/drm/etnaviv/etnaviv_gem.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/etnaviv/etnaviv_gem.h b/drivers/gpu/drm/etnaviv/etnaviv_gem.h
> index 6b68fe16041b..98e60df882b6 100644
> --- a/drivers/gpu/drm/etnaviv/etnaviv_gem.h
> +++ b/drivers/gpu/drm/etnaviv/etnaviv_gem.h
> @@ -105,7 +105,7 @@ struct etnaviv_gem_submit {
>  	unsigned int nr_pmrs;
>  	struct etnaviv_perfmon_request *pmrs;
>  	unsigned int nr_bos;
> -	struct etnaviv_gem_submit_bo bos[0];
> +	struct etnaviv_gem_submit_bo bos[];
>  	/* No new members here, the previous one is variable-length! */
>  };
>  

Re: [PATCH][next] drm: etnaviv_gem.h: Replace zero-length array with flexible-array member

[Gustavo A. R. Silva]

Hi Lucas,

On 3/5/20 05:17, Lucas Stach wrote:
> Hi Gustavo,
> 
> I've adjusted the subject a bit to match the style of the other etnaviv
> driver commits and applied this to the etnaviv/next branch.
> 

Great. Thanks a lot for that. :)
--
Gustavo

> Regards,
> Lucas
> 
> On Do, 2020-03-05 at 04:51 -0600, 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@embeddedor.com>
>> ---
>>  drivers/gpu/drm/etnaviv/etnaviv_gem.h | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/drivers/gpu/drm/etnaviv/etnaviv_gem.h b/drivers/gpu/drm/etnaviv/etnaviv_gem.h
>> index 6b68fe16041b..98e60df882b6 100644
>> --- a/drivers/gpu/drm/etnaviv/etnaviv_gem.h
>> +++ b/drivers/gpu/drm/etnaviv/etnaviv_gem.h
>> @@ -105,7 +105,7 @@ struct etnaviv_gem_submit {
>>  	unsigned int nr_pmrs;
>>  	struct etnaviv_perfmon_request *pmrs;
>>  	unsigned int nr_bos;
>> -	struct etnaviv_gem_submit_bo bos[0];
>> +	struct etnaviv_gem_submit_bo bos[];
>>  	/* No new members here, the previous one is variable-length! */
>>  };
>>  
>