linux-kernel - Re: [tip:perf/core] perf: Fix broken union in ' struct perf_event_mmap

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Date:	Tue, 17 Sep 2013 16:35:30 -0400 (EDT)
From:	Vince Weaver <vince@...ter.net>
To:	mingo@...nel.org, hpa@...or.com, linux-kernel@...r.kernel.org,
	a.p.zijlstra@...llo.nl, adrian.hunter@...el.com, tglx@...utronix.de
cc:	linux-tip-commits@...r.kernel.org
Subject: Re: [tip:perf/core] perf: Fix broken union in ' struct
 perf_event_mmap_page'

On Tue, 17 Sep 2013, Vince Weaver wrote:

> 
> This patch somehow breaks the perf-ABI.
> 
> If I take a program that reads "mmap->cap_usr_rdpmc" and compile it
> against the new header with this change (say from 3.12-rc1)
> and then run it on an old kernel (say 3.11) then I get "0" for
> cap_usr_rdpmc.
> 
> If I take the same program and recompile against the old (without this 
> patch) header and run it on 3.11, I get the expected "1" value.

To follow up, the original case:

        union {
                __u64   capabilities;
                __u64   cap_usr_time  : 1,
                        cap_usr_rdpmc : 1,
                        cap_____res   : 62;
        };

        Then mmap->usr_rdpmc=1; gets assembled as

        400420:       80 0d 11 05 20 00 02    orb    $0x2,0x200511(%rip)        

The new version

       union {
                __u64   capabilities;
                struct {
                        __u64   cap_usr_time            : 1,
                                cap_usr_rdpmc           : 1,
                                cap_usr_time_zero       : 1,
                                cap_____res             : 61;
                };
        };

        mmap->usr_rdpmc=1; gets assembled as


       400427:       80 0d 02 05 20 00 01    orb    $0x1,0x200502(%rip)        

note the difference in the or value.

Vince
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