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

[Yafang Shao <laoar.shao@gmail.com>]

On Thu, Oct 28, 2021 at 4:10 AM Petr Mladek <pmladek@suse.com> wrote:
>
> On Mon 2021-10-25 14:35:42, Kees Cook 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@suse.com>
> > > Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
> > > Reviewed-by: Kees Cook <keescook@chromium.org>
> > > Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> > > Cc: Arnaldo Carvalho de Melo <arnaldo.melo@gmail.com>
> > > Cc: Andrii Nakryiko <andrii.nakryiko@gmail.com>
> > > Cc: Peter Zijlstra <peterz@infradead.org>
> > > Cc: Steven Rostedt <rostedt@goodmis.org>
> > > Cc: Al Viro <viro@zeniv.linux.org.uk>
> > > Cc: Kees Cook <keescook@chromium.org>
> > > Cc: Petr Mladek <pmladek@suse.com>
> > > ---
> > >  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:
> >
> >                       char *full_comm;
> >
> >                       full_comm = kvasprintf(GFP_KERNEL, namefmt, args);
>
> You need to use va_copy()/va_end() if you want to use the same va_args
> twice.
>
> For example, see how kvasprintf() is implemented. It calls
> vsnprintf() twice and it uses va_copy()/va_end() around the the first call.
>

Does it mean that if we want to call vsnprintf() three times, we must
use va_copy()/va_end() around the first call and the second call ?
IOW, if we call vsnprintf() multiple times, all the calls except the
last call should be protected by va_copy()/va_end().
Actually I don't quite understand why we should do it like this. I
will try to understand it, and appreciate it if you could explain it
in detail.

BTW,  can we use va_copy()/va_end() in vsnprintf(), then the caller
doesn't need to care how many times it will call vsnprintf().

> kvasprintf() could also return NULL if there is not enough memory.

Right. We need to do the NULL check.

>
> >                       pr_warn("truncated kthread comm '%s' to '%s' (pid:%d)\n",
> >                               full_comm, name);
>
> BTW: Is this message printed during normal boot? I did not tried the
> patchset myself.
>

Yes, it will be printed at boot time.

> We should add this warning only if there is a good solution how to
> avoid the truncated names. And we should me sure that the most common
> kthreads/workqueues do not trigger it. It would be ugly to print many
> warnings during boot if people could not get rid of them easily.
>

As we have extended task comm to 24, there's no such warning printed
for the existing kthreads/workqueues.
IOW, it will only print for the newly introduced one if it has a long name.
That means this printing is under control.

> >                       kfree(full_comm);
> >               }
> > >             set_task_comm(task, name);
> > >             /*
> > >              * root may have changed our (kthreadd's) priority or CPU mask.
>
> Best Regards,
> Petr



-- 
Thanks
Yafang