[PATCH][next] nvdimm: nd.h: Replace zero-length array with flexible-array member

[PATCH][next] nvdimm: nd.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/nvdimm/nd.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/nvdimm/nd.h b/drivers/nvdimm/nd.h
index c4d69c1cce55..85dbb2a322b9 100644
--- a/drivers/nvdimm/nd.h
+++ b/drivers/nvdimm/nd.h
@@ -39,7 +39,7 @@ struct nd_region_data {
 	int ns_count;
 	int ns_active;
 	unsigned int hints_shift;
-	void __iomem *flush_wpq[0];
+	void __iomem *flush_wpq[];
 };
 
 static inline void __iomem *ndrd_get_flush_wpq(struct nd_region_data *ndrd,
@@ -157,7 +157,7 @@ struct nd_region {
 	struct nd_interleave_set *nd_set;
 	struct nd_percpu_lane __percpu *lane;
 	int (*flush)(struct nd_region *nd_region, struct bio *bio);
-	struct nd_mapping mapping[0];
+	struct nd_mapping mapping[];
 };
 
 struct nd_blk_region {
-- 
2.23.0
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Re: [PATCH][next] nvdimm: nd.h: Replace zero-length array with flexible-array member

[Ira Weiny]

On Thu, Mar 19, 2020 at 06:09:37PM -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:

"won't" be affected?

My reading of [1] indicates that this change will break the allocation in
nd_region_activate() because sizeof() can no longer be used on the base
structure?

What am I missing?

Ira

> 
> "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/nvdimm/nd.h | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/nvdimm/nd.h b/drivers/nvdimm/nd.h
> index c4d69c1cce55..85dbb2a322b9 100644
> --- a/drivers/nvdimm/nd.h
> +++ b/drivers/nvdimm/nd.h
> @@ -39,7 +39,7 @@ struct nd_region_data {
>  	int ns_count;
>  	int ns_active;
>  	unsigned int hints_shift;
> -	void __iomem *flush_wpq[0];
> +	void __iomem *flush_wpq[];
>  };
>  
>  static inline void __iomem *ndrd_get_flush_wpq(struct nd_region_data *ndrd,
> @@ -157,7 +157,7 @@ struct nd_region {
>  	struct nd_interleave_set *nd_set;
>  	struct nd_percpu_lane __percpu *lane;
>  	int (*flush)(struct nd_region *nd_region, struct bio *bio);
> -	struct nd_mapping mapping[0];
> +	struct nd_mapping mapping[];
>  };
>  
>  struct nd_blk_region {
> -- 
> 2.23.0
> 
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Re: [PATCH][next] nvdimm: nd.h: Replace zero-length array with flexible-array member

[Gustavo A. R. Silva]

On 3/19/20 6:47 PM, Ira Weiny wrote:
> On Thu, Mar 19, 2020 at 06:09:37PM -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:
> 
> "won't" be affected?
> 

Yep. They won't:

"As a quirk of the original implementation of zero-length arrays, sizeof evaluates to zero."

> My reading of [1] indicates that this change will break the allocation inha

The allocation will remain the same as with *flush_wpg[0]. See this example: https://godbolt.org/z/oohwmB

> nd_region_activate() because sizeof() can no longer be used on the base
> structure?
> 

In this case the sizeof operator cannot be applied to flush_wpg, not the base structure, which is
nd_region_data.


> What am I missing?
>

I think you are confusing the base structure with the actual flexible-array member.

Thanks
--
Gustavo

> Ira
> 
>>
>> "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/nvdimm/nd.h | 4 ++--
>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/nvdimm/nd.h b/drivers/nvdimm/nd.h
>> index c4d69c1cce55..85dbb2a322b9 100644
>> --- a/drivers/nvdimm/nd.h
>> +++ b/drivers/nvdimm/nd.h
>> @@ -39,7 +39,7 @@ struct nd_region_data {
>>  	int ns_count;
>>  	int ns_active;
>>  	unsigned int hints_shift;
>> -	void __iomem *flush_wpq[0];
>> +	void __iomem *flush_wpq[];
>>  };
>>  
>>  static inline void __iomem *ndrd_get_flush_wpq(struct nd_region_data *ndrd,
>> @@ -157,7 +157,7 @@ struct nd_region {
>>  	struct nd_interleave_set *nd_set;
>>  	struct nd_percpu_lane __percpu *lane;
>>  	int (*flush)(struct nd_region *nd_region, struct bio *bio);
>> -	struct nd_mapping mapping[0];
>> +	struct nd_mapping mapping[];
>>  };
>>  
>>  struct nd_blk_region {
>> -- 
>> 2.23.0
>>
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