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SubjectRe: [tip:perf/core] perf: Fix broken union in ' struct perf_event_mmap_page'

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/860f085b74e9f0075de8140ed3a1e5b5e3e39aa8
> 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-adrian.hunter@intel.com
> 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


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