Re: [PATCH 1/2] tracing: Simplify & fix saved_tgids logic

[Paul Burton <paulburton@google.com>]

Hi Joel,

On Wed, Jun 30, 2021 at 06:29:55PM -0400, Joel Fernandes wrote:
> On Tue, Jun 29, 2021 at 8:34 PM Paul Burton <paulburton@google.com> wrote:
> >
> > The tgid_map array records a mapping from pid to tgid, where the index
> > of an entry within the array is the pid & the value stored at that index
> > is the tgid.
> >
> > The saved_tgids_next() function iterates over pointers into the tgid_map
> > array & dereferences the pointers which results in the tgid, but then it
> > passes that dereferenced value to trace_find_tgid() which treats it as a
> > pid & does a further lookup within the tgid_map array. It seems likely
> > that the intent here was to skip over entries in tgid_map for which the
> > recorded tgid is zero, but instead we end up skipping over entries for
> > which the thread group leader hasn't yet had its own tgid recorded in
> > tgid_map.
> >
> > A minimal fix would be to remove the call to trace_find_tgid, turning:
> >
> >   if (trace_find_tgid(*ptr))
> >
> > into:
> >
> >   if (*ptr)
> >
> > ..but it seems like this logic can be much simpler if we simply let
> > seq_read() iterate over the whole tgid_map array & filter out empty
> > entries by returning SEQ_SKIP from saved_tgids_show(). Here we take that
> > approach, removing the incorrect logic here entirely.
> 
> Looks reasonable except for one nit:
> 
> > Signed-off-by: Paul Burton <paulburton@google.com>
> > Fixes: d914ba37d714 ("tracing: Add support for recording tgid of tasks")
> > Cc: Steven Rostedt <rostedt@goodmis.org>
> > Cc: Ingo Molnar <mingo@redhat.com>
> > Cc: Joel Fernandes <joelaf@google.com>
> > Cc: <stable@vger.kernel.org>
> > ---
> >  kernel/trace/trace.c | 38 +++++++++++++-------------------------
> >  1 file changed, 13 insertions(+), 25 deletions(-)
> >
> > diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> > index d23a09d3eb37b..9570667310bcc 100644
> > --- a/kernel/trace/trace.c
> > +++ b/kernel/trace/trace.c
> > @@ -5608,37 +5608,20 @@ static const struct file_operations tracing_readme_fops = {
> >
> >  static void *saved_tgids_next(struct seq_file *m, void *v, loff_t *pos)
> >  {
> > -       int *ptr = v;
> > +       int pid = ++(*pos);
> >
> > -       if (*pos || m->count)
> > -               ptr++;
> > -
> > -       (*pos)++;
> > -
> > -       for (; ptr <= &tgid_map[PID_MAX_DEFAULT]; ptr++) {
> > -               if (trace_find_tgid(*ptr))
> > -                       return ptr;
> 
> It would be great if you can add back the check for !tgid_map to both
> next() and show() as well, for added robustness (since the old code
> previously did it).

That condition cannot happen, because both next() & show() are called to
iterate through the content of the seq_file & by definition their v
argument is non-NULL (else seq_file would have finished iterating
already). That argument came from either start() or an earlier call to
next(), which would only have returned a non-NULL pointer into tgid_map
if tgid_map is non-NULL.

I've added comments to this effect in v2, though the second patch in v2
does wind up effectively adding back the check in next() anyway in order
to reuse some code.

I was tempted to just add the redundant checks anyway (pick your battles
and all) but for show() in particular it wound up making things seem
non-sensical to me ("display the value describing this non-NULL pointer
into tgid_map only if tgid_map is not NULL?").

Thanks,
    Paul

> With that change:
> Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> 
> thanks,
> 
> -Joel
> 
> 
> > -       }
> > +       if (pid > PID_MAX_DEFAULT)
> > +               return NULL;
> >
> > -       return NULL;
> > +       return &tgid_map[pid];
> >  }
> >
> >  static void *saved_tgids_start(struct seq_file *m, loff_t *pos)
> >  {
> > -       void *v;
> > -       loff_t l = 0;
> > -
> > -       if (!tgid_map)
> > +       if (!tgid_map || *pos > PID_MAX_DEFAULT)
> >                 return NULL;
> >
> > -       v = &tgid_map[0];
> > -       while (l <= *pos) {
> > -               v = saved_tgids_next(m, v, &l);
> > -               if (!v)
> > -                       return NULL;
> > -       }
> > -
> > -       return v;
> > +       return &tgid_map[*pos];
> >  }
> >
> >  static void saved_tgids_stop(struct seq_file *m, void *v)
> > @@ -5647,9 +5630,14 @@ static void saved_tgids_stop(struct seq_file *m, void *v)
> >
> >  static int saved_tgids_show(struct seq_file *m, void *v)
> >  {
> > -       int pid = (int *)v - tgid_map;
> > +       int *entry = (int *)v;
> > +       int pid = entry - tgid_map;
> > +       int tgid = *entry;
> > +
> > +       if (tgid == 0)
> > +               return SEQ_SKIP;
> >
> > -       seq_printf(m, "%d %d\n", pid, trace_find_tgid(pid));
> > +       seq_printf(m, "%d %d\n", pid, tgid);
> >         return 0;
> >  }
> >
> >
> > base-commit: 62fb9874f5da54fdb243003b386128037319b219
> > --
> > 2.32.0.93.g670b81a890-goog
> >