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

From: Yafang Shao <laoar.shao@gmail.com>
To: Kees Cook <keescook@chromium.org>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	Arnaldo Carvalho de Melo <arnaldo.melo@gmail.com>,
	Petr Mladek <pmladek@suse.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Al Viro <viro@zeniv.linux.org.uk>,
	Valentin Schneider <valentin.schneider@arm.com>,
	Qiang Zhang <qiang.zhang@windriver.com>,
	robdclark <robdclark@chromium.org>,
	christian <christian@brauner.io>,
	Dietmar Eggemann <dietmar.eggemann@arm.com>,
	Ingo Molnar <mingo@redhat.com>,
	Juri Lelli <juri.lelli@redhat.com>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	David Miller <davem@davemloft.net>,
	Jakub Kicinski <kuba@kernel.org>,
	Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Andrii Nakryiko <andrii@kernel.org>, Martin Lau <kafai@fb.com>,
	Song Liu <songliubraving@fb.com>, Yonghong Song <yhs@fb.com>,
	john fastabend <john.fastabend@gmail.com>,
	KP Singh <kpsingh@kernel.org>,
	dennis.dalessandro@cornelisnetworks.com,
	mike.marciniszyn@cornelisnetworks.com, dledford@redhat.com,
	jgg@ziepe.ca, linux-rdma@vger.kernel.org,
	netdev <netdev@vger.kernel.org>, bpf <bpf@vger.kernel.org>,
	"linux-perf-use." <linux-perf-users@vger.kernel.org>,
	linux-fsdevel@vger.kernel.org, Linux MM <linux-mm@kvack.org>,
	LKML <linux-kernel@vger.kernel.org>,
	kernel test robot <oliver.sang@intel.com>,
	kbuild test robot <lkp@intel.com>,
	Andrii Nakryiko <andrii.nakryiko@gmail.com>
Subject: Re: [PATCH v6 12/12] kernel/kthread: show a warning if kthread's comm is truncated
Date: Tue, 26 Oct 2021 10:23:54 +0800	[thread overview]
Message-ID: <CALOAHbA40qJZjdiM6+_e+t-qrE3kdHROPSTE4ttv4JT-HKCG-g@mail.gmail.com> (raw)
In-Reply-To: <202110251431.F594652F@keescook>

On Tue, Oct 26, 2021 at 5:35 AM Kees Cook <keescook@chromium.org> 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:
>

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