Re: [PATCH v6 12/12] kernel/kthread: show a warning if kthread's comm is truncated

[Yafang Shao]

On Tue, Oct 26, 2021 at 5:35 AM Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>
> On Mon, Oct 25, 2021 at 08:33:15AM +0000, Yafang Shao wrote:
> > Show a warning if task comm is truncated. Below is the result
> > of my test case:
> >
> > truncated kthread comm:I-am-a-kthread-with-lon, pid:14 by 6 characters
> >
> > Suggested-by: Petr Mladek <pmladek@xxxxxxxx>
> > Signed-off-by: Yafang Shao <laoar.shao@xxxxxxxxx>
> > Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
> > Cc: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> > Cc: Arnaldo Carvalho de Melo <arnaldo.melo@xxxxxxxxx>
> > Cc: Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx>
> > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
> > Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
> > Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> > Cc: Petr Mladek <pmladek@xxxxxxxx>
> > ---
> >  kernel/kthread.c | 7 ++++++-
> >  1 file changed, 6 insertions(+), 1 deletion(-)
> >
> > diff --git a/kernel/kthread.c b/kernel/kthread.c
> > index 5b37a8567168..46b924c92078 100644
> > --- a/kernel/kthread.c
> > +++ b/kernel/kthread.c
> > @@ -399,12 +399,17 @@ struct task_struct *__kthread_create_on_node(int (*threadfn)(void *data),
> >       if (!IS_ERR(task)) {
> >               static const struct sched_param param = { .sched_priority = 0 };
> >               char name[TASK_COMM_LEN];
> > +             int len;
> >
> >               /*
> >                * task is already visible to other tasks, so updating
> >                * COMM must be protected.
> >                */
> > -             vsnprintf(name, sizeof(name), namefmt, args);
> > +             len = vsnprintf(name, sizeof(name), namefmt, args);
> > +             if (len >= TASK_COMM_LEN) {
>
> And since this failure case is slow-path, we could improve the warning
> as other had kind of suggested earlier with something like this instead:
>

It Makes sense to me.  I will do it as you suggested.

>                         char *full_comm;
>
>                         full_comm = kvasprintf(GFP_KERNEL, namefmt, args);
>                         pr_warn("truncated kthread comm '%s' to '%s' (pid:%d)\n",
>                                 full_comm, name);
>
>                         kfree(full_comm);
>                 }
> >               set_task_comm(task, name);
> >               /*
> >                * root may have changed our (kthreadd's) priority or CPU mask.
> > --
> > 2.17.1
> >
>
> --
> Kees Cook



-- 
Thanks
Yafang