perf session: Fix decompression of PERF_RECORD_COMPRESSED records

perf session: Fix decompression of PERF_RECORD_COMPRESSED records

Avoid termination of trace loading in case the last record in the
decompressed buffer partly resides in the following mmaped
PERF_RECORD_COMPRESSED record.

In this case NULL value returned by fetch_mmaped_event() means to
proceed to the next mmaped record then decompress it and load compressed
events.

The issue can be reproduced like this:

  $ perf record -z -- some_long_running_workload
  $ perf report --stdio -vv
  decomp (B): 44519 to 163000
  decomp (B): 48119 to 174800
  decomp (B): 65527 to 131072
  fetch_mmaped_event: head=0x1ffe0 event->header_size=0x28, mmap_size=0x20000: fuzzed perf.data?
  Error:
  failed to process sample
  ...

Testing:

  71: Zstd perf.data compression/decompression              : Ok

  $ tools/perf/perf report -vv --stdio
  decomp (B): 59593 to 262160
  decomp (B): 4438 to 16512
  decomp (B): 285 to 880
  Looking at the vmlinux_path (8 entries long)
  Using vmlinux for symbols
  decomp (B): 57474 to 261248
  prefetch_event: head=0x3fc78 event->header_size=0x28, mmap_size=0x3fc80: fuzzed or compressed perf.data?
  decomp (B): 25 to 32
  decomp (B): 52 to 120
  ...

Fixes: 57fc032ad643 ("perf session: Avoid infinite loop when seeing invalid header.size")
Link: https://marc.info/?l=linux-kernel&m=156580812427554&w=2
Co-developed-by: Jiri Olsa <jolsa@kernel.org>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Alexey Budankov <alexey.budankov@linux.intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lore.kernel.org/lkml/cf782c34-f3f8-2f9f-d6ab-145cee0d5322@linux.intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>