Re: [tip:perf/core] perf: Fix broken union in ' struct
perf_event_mmap_page'
[Posted September 24, 2013 by corbet]
| From: |
| Vince Weaver <vince-AT-deater.net> |
| To: |
| mingo-AT-kernel.org, hpa-AT-zytor.com, linux-kernel-AT-vger.kernel.org, a.p.zijlstra-AT-chello.nl, adrian.hunter-AT-intel.com, tglx-AT-linutronix.de |
| Subject: |
| Re: [tip:perf/core] perf: Fix broken union in ' struct perf_event_mmap_page' |
| Date: |
| Tue, 17 Sep 2013 16:23:25 -0400 (EDT) |
| Message-ID: |
| <alpine.DEB.2.02.1309171617400.13303@pianoman.cluster.toy> |
| Cc: |
| linux-tip-commits-AT-vger.kernel.org |
| Archive‑link: | |
Article |
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.
So something about this changed the bit pattern in an incompatible
fashion.
Vince
On Tue, 23 Jul 2013, tip-bot for Adrian Hunter wrote:
> Commit-ID: 860f085b74e9f0075de8140ed3a1e5b5e3e39aa8
> Gitweb: http://git.kernel.org/tip/860f085b74e9f0075de8140ed3a1e5b...
> Author: Adrian Hunter <adrian.hunter@intel.com>
> AuthorDate: Fri, 28 Jun 2013 16:22:17 +0300
> Committer: Ingo Molnar <mingo@kernel.org>
> CommitDate: Tue, 23 Jul 2013 12:17:10 +0200
>
> perf: Fix broken union in 'struct perf_event_mmap_page'
>
> The capabilities bits must not be "union'ed" together.
> Put them in a separate struct.
>
> Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
> Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
> Link: http://lkml.kernel.org/r/1372425741-1676-2-git-send-email...
> Signed-off-by: Ingo Molnar <mingo@kernel.org>
> ---
> 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 00d8274..0041aed 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;
> + };
> };
>
> /*
> --
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k
