linux-kernel - Re: [PATCH] regulator: da9063: Replace zero-length array with flexible-array member

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Date:   Wed, 12 Feb 2020 08:29:06 -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: da9063: Replace zero-length array with
 flexible-array member



On 2/12/20 05:14, Adam Thomson wrote:
> On 11 February 2020 23:47, 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/da9063-regulator.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/drivers/regulator/da9063-regulator.c b/drivers/regulator/da9063-
>> regulator.c
>> index 2b0c7a85306a..368f8ad2a9f9 100644
>> --- a/drivers/regulator/da9063-regulator.c
>> +++ b/drivers/regulator/da9063-regulator.c
>> @@ -119,7 +119,7 @@ struct da9063_regulator {
>>  struct da9063_regulators {
>>  	unsigned				n_regulators;
>>  	/* Array size to be defined during init. Keep at end. */
>> -	struct da9063_regulator			regulator[0];
>> +	struct da9063_regulator			regulator[];
> 
> Same comment as for da9062. The probe uses malloc and does not statically
> initialise for this struct so this will break the probe.
> 

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