Re: [PATCH] perf diff: Don't crash on freeing errno-session

[Arnaldo Carvalho de Melo]

Em Tue, Mar 02, 2021 at 01:47:55PM +0900, Namhyung Kim escreveu:
> Hello,
> 
> On Tue, Mar 2, 2021 at 11:35 AM Dmitry Safonov <dima@xxxxxxxxxx> wrote:
> >
> > __cmd_diff() sets result of perf_session__new() to d->session.
> > In case of failure, it's errno and perf-diff may crash with:
> > failed to open perf.data: Permission denied
> > Failed to open perf.data
> > Segmentation fault (core dumped)
> >
> > From the coredump:
> > 0  0x00005569a62b5955 in auxtrace__free (session=0xffffffffffffffff)
> >     at util/auxtrace.c:2681
> > 1  0x00005569a626b37d in perf_session__delete (session=0xffffffffffffffff)
> >     at util/session.c:295
> > 2  perf_session__delete (session=0xffffffffffffffff) at util/session.c:291
> > 3  0x00005569a618008a in __cmd_diff () at builtin-diff.c:1239
> > 4  cmd_diff (argc=<optimized out>, argv=<optimized out>) at builtin-diff.c:2011
> > [..]
> >
> > Funny enough, it won't always crash. For me it crashes only if failed
> > file is second in cmd-line: the reason is that cmd_diff() check files for
> > branch-stacks [in check_file_brstack()] and if the first file doesn't
> > have brstacks, it doesn't proceed to try open other files from cmd-line.
> >
> > Check d->session before calling perf_session__delete().
> >
> > Another solution would be assigning to temporary variable, checking it,
> > but I find it easier to follow with IS_ERR() check in the same function.
> > After some time it's still obvious why the check is needed, and with
> > temp variable it's possible to make the same mistake.
> >
> > Cc: Alexander Shishkin <alexander.shishkin@xxxxxxxxxxxxxxx>
> > Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
> > Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> > Cc: Jiri Olsa <jolsa@xxxxxxxxxx>
> > Cc: Mark Rutland <mark.rutland@xxxxxxx>
> > Cc: Namhyung Kim <namhyung@xxxxxxxxxx>
> > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > Signed-off-by: Dmitry Safonov <dima@xxxxxxxxxx>
> 
> Acked-by: Namhyung Kim <namhyung@xxxxxxxxxx>

Thanks, tested, added a complete set of steps for a problem to be
reproduced and applied.

- Arnaldo

    Committer testing:
    
      $ perf record sleep 1
      [ perf record: Woken up 1 times to write data ]
      [ perf record: Captured and wrote 0.001 MB perf.data (8 samples) ]
      $ perf diff
      failed to open perf.data.old: No such file or directory
      Failed to open perf.data.old
      $ perf record sleep 1
      [ perf record: Woken up 1 times to write data ]
      [ perf record: Captured and wrote 0.001 MB perf.data (8 samples) ]
      $ perf diff
      # Event 'cycles:u'
      #
      # Baseline  Delta Abs  Shared Object     Symbol
      # ........  .........  ................  ..........................
      #
           0.92%    +87.66%  [unknown]         [k] 0xffffffff8825de16
          11.39%     +0.04%  ld-2.32.so        [.] __GI___tunables_init
          87.70%             ld-2.32.so        [.] _dl_check_map_versions
      $ sudo chown root:root perf.data
      [sudo] password for acme:
      $ perf diff
      failed to open perf.data: Permission denied
      Failed to open perf.data
      Segmentation fault (core dumped)
      $
    
    After the patch:
    
      $ perf diff
      failed to open perf.data: Permission denied
      Failed to open perf.data
      $
    
    Signed-off-by: Dmitry Safonov <dima@xxxxxxxxxx>
    Acked-by: Namhyung Kim <namhyung@xxxxxxxxxx>
    Tested-by: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>