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

[Joel Fernandes]

On Tue, Jun 29, 2021 at 8:34 PM Paul Burton <paulburton@xxxxxxxxxx> 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@xxxxxxxxxx>
> Fixes: d914ba37d714 ("tracing: Add support for recording tgid of tasks")
> Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Joel Fernandes <joelaf@xxxxxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx>
> ---
>  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).

With that change:
Reviewed-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>

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
>