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

From: Petr Mladek <pmladek@suse.com>
To: Kees Cook <keescook@chromium.org>
Cc: Yafang Shao <laoar.shao@gmail.com>,
	akpm@linux-foundation.org, rostedt@goodmis.org,
	mathieu.desnoyers@efficios.com, arnaldo.melo@gmail.com,
	peterz@infradead.org, viro@zeniv.linux.org.uk,
	valentin.schneider@arm.com, qiang.zhang@windriver.com,
	robdclark@chromium.org, christian@brauner.io,
	dietmar.eggemann@arm.com, mingo@redhat.com,
	juri.lelli@redhat.com, vincent.guittot@linaro.org,
	davem@davemloft.net, kuba@kernel.org, ast@kernel.org,
	daniel@iogearbox.net, andrii@kernel.org, kafai@fb.com,
	songliubraving@fb.com, yhs@fb.com, john.fastabend@gmail.com,
	kpsingh@kernel.org, dennis.dalessandro@cornelisnetworks.com,
	mike.marciniszyn@cornelisnetworks.com, dledford@redhat.com,
	jgg@ziepe.ca, linux-rdma@vger.kernel.org, netdev@vger.kernel.org,
	bpf@vger.kernel.org, linux-perf-users@vger.kernel.org,
	linux-fsdevel@vger.kernel.org, linux-mm@kvack.org,
	linux-kernel@vger.kernel.org, oliver.sang@intel.com,
	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: Wed, 27 Oct 2021 22:10:49 +0200	[thread overview]
Message-ID: <YXmySeDsxxbA7hcq@alley> (raw)
In-Reply-To: <202110251431.F594652F@keescook>

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.

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

> 			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.

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.

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

Best Regards,
Petr