linux-kernel - Re: [PATCH] regulator: da9062: Replace zero-length array with flexible-array member

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Message-ID: <b5270b23-45aa-9830-7249-272e82454280@embeddedor.com>
Date:   Wed, 12 Feb 2020 08:28:35 -0600
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     Adam Thomson <Adam.Thomson.Opensource@...semi.com>,
        Support Opensource <Support.Opensource@...semi.com>,
        Liam Girdwood <lgirdwood@...il.com>,
        Mark Brown <broonie@...nel.org>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] regulator: da9062: Replace zero-length array with
 flexible-array member



On 2/12/20 05:12, Adam Thomson wrote:
> On 11 February 2020 23:46, 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
>> inadvertenly introduced[3] to the codebase from now on.
>>
>> 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/regulator/da9062-regulator.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/drivers/regulator/da9062-regulator.c b/drivers/regulator/da9062-
>> regulator.c
>> index b064d8a19d4c..c3b6ba9bafdf 100644
>> --- a/drivers/regulator/da9062-regulator.c
>> +++ b/drivers/regulator/da9062-regulator.c
>> @@ -86,7 +86,7 @@ struct da9062_regulators {
>>  	int					irq_ldo_lim;
>>  	unsigned				n_regulators;
>>  	/* Array size to be defined during init. Keep at end. */
>> -	struct da9062_regulator			regulator[0];
>> +	struct da9062_regulator			regulator[];
> 
> I don't think is the correct change here for this driver. In the probe
> 'struct_size()' is used to determine the actual size requested from 'malloc()'
> when allocating memory for this structure. It's not statically initialised.
> Your change will break that code I believe.
> 

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]

[1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html

Thanks
--
Gustavo