__perf_session__process_pipe_events reuses the same memory buffer to
process all events in the pipe.

When reordering is needed (e.g. -b option), events are not immediately
flushed, but kept around until reordering is possible, causing
memory corruption.

The problem is usually observed by a "Unknown sample error" output. It
can easily be reproduced by:

  perf record -o - noploop | perf inject -b > output

Committer testing:

Before:

  $ perf record -o - stress -t 2 -c 2 | perf inject -b > /dev/null
  stress: info: [8297] dispatching hogs: 2 cpu, 0 io, 0 vm, 0 hdd
  stress: info: [8297] successful run completed in 2s
  [ perf record: Woken up 3 times to write data ]
  [ perf record: Captured and wrote 0.000 MB - ]
  Warning:
  Found 1 unknown events!

  Is this an older tool processing a perf.data file generated by a more recent tool?

  If that is not the case, consider reporting to linux-kernel@vger.kernel.org.

  $

After:

  $ perf record -o - stress -t 2 -c 2 | perf inject -b > /dev/null
  stress: info: [9027] dispatching hogs: 2 cpu, 0 io, 0 vm, 0 hdd
  stress: info: [9027] successful run completed in 2s
  [ perf record: Woken up 3 times to write data ]
  [ perf record: Captured and wrote 0.000 MB - ]
  no symbols found in /usr/bin/stress, maybe install a debug package?
  no symbols found in /usr/bin/stress, maybe install a debug package?
  $
Signed-off-by: David Carrillo-Cisneros <davidcc@google.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: He Kuang <hekuang@huawei.com>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Paul Turner <pjt@google.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Simon Que <sque@chromium.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/r/20170410201432.24807-3-davidcc@google.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>