linux-kernel - Re: [PATCH 1/5] perf: fix broken union in perf_event_mmap

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Message-ID: <20130628152224.GG6626@twins.programming.kicks-ass.net>
Date:	Fri, 28 Jun 2013 17:22:24 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Adrian Hunter <adrian.hunter@...el.com>
Cc:	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	H Peter Anvin <hpa@...or.com>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	linux-kernel@...r.kernel.org, David Ahern <dsahern@...il.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Jiri Olsa <jolsa@...hat.com>, Mike Galbraith <efault@....de>,
	Namhyung Kim <namhyung@...il.com>,
	Paul Mackerras <paulus@...ba.org>,
	Stephane Eranian <eranian@...gle.com>
Subject: Re: [PATCH 1/5] perf: fix broken union in perf_event_mmap_page

On Fri, Jun 28, 2013 at 04:22:17PM +0300, Adrian Hunter wrote:
> The capabilities bits must not be "union'ed" together.
> Put them in a separate struct.
> 
> Signed-off-by: Adrian Hunter <adrian.hunter@...el.com>
> ---
>  include/uapi/linux/perf_event.h | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h
> index 0b1df41..19f6ee5 100644
> --- a/include/uapi/linux/perf_event.h
> +++ b/include/uapi/linux/perf_event.h
> @@ -375,9 +375,11 @@ struct perf_event_mmap_page {
>  	__u64	time_running;		/* time event on cpu */
>  	union {
>  		__u64	capabilities;
> -		__u64	cap_usr_time  : 1,
> -			cap_usr_rdpmc : 1,
> -			cap_____res   : 62;
> +		struct {
> +			__u64	cap_usr_time		: 1,
> +				cap_usr_rdpmc		: 1,
> +				cap_____res		: 62;
> +		};
>  	};

Ick, it did that!? and here I thought there was a difference between:

  int foo:1,
      bar:1;

and

  int foo:1;
  int bar:1;

That made all the difference in this particular case. I guess I should
go read the language spec more carefully next time.
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