Re: [PATCH] mm: kfence: print the age time for alloacted objectes to trace memleak

[Marco Elver <elver@google.com>]

On Mon, 5 Aug 2024 at 05:35, chenqiwu <qiwuchen55@gmail.com> wrote:
>
> On Sun, Aug 04, 2024 at 10:37:43AM +0200, Marco Elver wrote:
> >
> > Well, what I'm saying, having this info also for FREED objects on the
> > free stack can be useful in some debugging scenarios when you get a
> > use-after-free, and you want to know the elapsed time since the free
> > happened. I have done this calculation manually before, which is why I
> > suggested it. Maybe it's not useful for you for finding leaks, but
> > that's just one usecase.
> >
> Agreed with your concern scenarios.
> How about the following change with additonal object state info?
>
> +       u64 interval_nsec = local_clock() - meta->alloc_track.ts_nsec;
> +       unsigned long rem_interval_nsec = do_div(interval_nsec, NSEC_PER_SEC);
>
>         /* Timestamp matches printk timestamp format. */
> -       seq_con_printf(seq, "%s by task %d on cpu %d at %lu.%06lus:\n",
> +       seq_con_printf(seq, "%s by task %d on cpu %d at %lu.%06lus (%lu.%06lus ago) for %s object:\n",
>                        show_alloc ? "allocated" : "freed", track->pid,
> -                      track->cpu, (unsigned long)ts_sec, rem_nsec / 1000);
> +                      track->cpu, (unsigned long)ts_sec, rem_nsec / 1000,
> +                      (unsigned long)interval_nsec, rem_interval_nsec / 1000,
> +                      meta->state == KFENCE_OBJECT_ALLOCATED? "allocated" : "freed");
>
> In this way, we can find leaks by grep "allocated object" and inspect the elapsed time of
> use-after-free by grep "freed object".

The "allocated/freed" info is superfluous, as freed objects will have
a free stack.

Consider a slightly better script vs. just using grep.
/sys/kernel/debug/kfence/objects is of secondary concern and was added
primarily as a debugging aid for KFENCE developers. We never thought
it could be used to look for leaks, but good you found another use for
it. ;-)
The priority is to keep regular error reports generated by KFENCE
readable. Adding this "allocated/freed" info just makes the line
longer and is not useful.

I'm happy with the "(%lu.%06lus ago)" part alone.