linux-kernel - Re: [PATCH] sched/fair: Replace zero-length array with flexible-array member

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Date:   Thu, 13 Feb 2020 10:58:31 -0600
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...hat.com>, Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched/fair: Replace zero-length array with flexible-array
 member



On 2/13/20 10:45, Peter Zijlstra wrote:
> On Thu, Feb 13, 2020 at 09:19:51AM -0600, 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:
>>
>> "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>
>> ---
>>  kernel/sched/fair.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index f38ff5a335d3..12a424878b23 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -1081,7 +1081,7 @@ struct numa_group {
>>  	 * more by CPU use than by memory faults.
>>  	 */
>>  	unsigned long *faults_cpu;
>> -	unsigned long faults[0];
>> +	unsigned long faults[];
>>  };
> 
> Hurmph, and where are all the other similar changes for kernel/sched/ ?
> Because this really isn't the only such usage and I really don't see the
> point in having a separate patch for every single one of them.
> 

Yeah. I can do that. I'll send a patch for the whole kernel/sched.

> Also; couldn't you've taught the compiler to also warn about [0] ?
> There's really no other purpose to having a zero length array.
> 

Yeah, this is something we'd like to see in the short future.
Unfortunately, for now, the only way for the compiler to warn
about zero-length arrays in through the use of "-pedantic".
And we definitely don't want to follow this path.

What we can do, in the meantime, is to add a test for it to
checkpatch.

Thanks
--
Gustavo