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    Subject[tip:perf/urgent] perf mmap: Recalculate size for overwrite mode
    Commit-ID:  dc6c35c679e96987dc83a003f30bc2cc33c84c00
    Gitweb: https://git.kernel.org/tip/dc6c35c679e96987dc83a003f30bc2cc33c84c00
    Author: Kan Liang <kan.liang@intel.com>
    AuthorDate: Thu, 18 Jan 2018 13:26:17 -0800
    Committer: Arnaldo Carvalho de Melo <acme@redhat.com>
    CommitDate: Thu, 15 Feb 2018 09:51:57 -0300

    perf mmap: Recalculate size for overwrite mode

    In perf_mmap__push(), the 'size' need to be recalculated, otherwise the
    invalid data might be pushed to the record in overwrite mode.

    The issue is introduced by commit 7fb4b407a124 ("perf mmap: Don't
    discard prev in backward mode").

    When the ring buffer is full in overwrite mode, backward_rb_find_range()
    will be called to recalculate the 'start' and 'end'. The 'size' needs to
    be recalculated accordingly.

    Unconditionally recalculate the 'size', not just for full ring buffer in
    overwrite mode. Because:

    - There is no harmful to recalculate the 'size' for other cases.
    - The code of calculating 'start' and 'end' will be factored out later.
    The new function does not need to return 'size'.

    Signed-off-by: Kan Liang <kan.liang@intel.com>
    Acked-by: Jiri Olsa <jolsa@kernel.org>
    Cc: Andi Kleen <ak@linux.intel.com>
    Cc: Jin Yao <yao.jin@linux.intel.com>
    Cc: Namhyung Kim <namhyung@kernel.org>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Wang Nan <wangnan0@huawei.com>
    Fixes: 7fb4b407a124 ("perf mmap: Don't discard prev in backward mode")
    Link: http://lkml.kernel.org/r/1516310792-208685-3-git-send-email-kan.liang@intel.com
    Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
    ---
    tools/perf/util/mmap.c | 2 ++
    1 file changed, 2 insertions(+)

    diff --git a/tools/perf/util/mmap.c b/tools/perf/util/mmap.c
    index 05076e6..97cf4fa 100644
    --- a/tools/perf/util/mmap.c
    +++ b/tools/perf/util/mmap.c
    @@ -302,6 +302,8 @@ int perf_mmap__push(struct perf_mmap *md, bool overwrite,
    return -1;
    }

    + size = end - start;
    +
    if ((start & md->mask) + size != (end & md->mask)) {
    buf = &data[start & md->mask];
    size = md->mask + 1 - (start & md->mask);
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