From: "Gustavo A. R. Silva" <gustavo@embeddedor.com>
To: Ira Weiny <ira.weiny@intel.com>
Cc: linux-nvdimm@lists.01.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH][next] nvdimm: nd.h: Replace zero-length array with flexible-array member
Date: Thu, 19 Mar 2020 19:03:22 -0500 [thread overview]
Message-ID: <e1b07806-04c5-83ca-568b-9edcafada52c@embeddedor.com> (raw)
In-Reply-To: <20200319234744.GA1688758@iweiny-DESK2.sc.intel.com>
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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prev parent reply other threads:[~2020-03-20 0:03 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-03-19 23:09 [PATCH][next] nvdimm: nd.h: Replace zero-length array with flexible-array member Gustavo A. R. Silva
2020-03-19 23:47 ` Ira Weiny
2020-03-20 0:03 ` Gustavo A. R. Silva [this message]
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