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

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Date:   Mon, 2 Mar 2020 14:04:31 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Gustavo A. R. Silva" <gustavo@...eddedor.com>
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 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?