linux-kernel - Re: [PATCH][next] perf: Replace zero-length array with flexible-array member

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Date:   Tue, 3 Mar 2020 11:24:59 -0600
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH][next] perf: Replace zero-length array with flexible-array
 member



On 3/2/20 07:04, Peter Zijlstra wrote:
> On Mon, Mar 02, 2020 at 06:48:32AM -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]
>>
> 
>> diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h
>> index 397cfd65b3fe..d71023c46058 100644
>> --- a/include/uapi/linux/perf_event.h
>> +++ b/include/uapi/linux/perf_event.h
>> @@ -453,7 +453,7 @@ struct perf_event_query_bpf {
>>  	/*
>>  	 * User provided buffer to store program ids
>>  	 */
>> -	__u32	ids[0];
>> +	__u32	ids[];
>>  };
>>  
>>  /*
> 
> Just to be absolutely sure; there is no ABI difference (or any actual
> difference in generated code), right?
> 

Yeah; we should have no problem with that:

https://godbolt.org/z/oohwmB

Thanks
--
Gustavo