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

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Date:   Thu, 19 Mar 2020 19:03:22 -0500
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     Ira Weiny <ira.weiny@...el.com>
Cc:     Dan Williams <dan.j.williams@...el.com>,
        Vishal Verma <vishal.l.verma@...el.com>,
        Dave Jiang <dave.jiang@...el.com>, linux-nvdimm@...ts.01.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH][next] nvdimm: nd.h: Replace zero-length array with
 flexible-array member



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@...eddedor.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
>>