linux-kernel - Re: [PATCH] tty: Add MOXA NPort Real TTY Driver

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Date:   Wed, 22 Jul 2020 09:11:40 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Johnson CH Chen (陳昭勳) 
        <JohnsonCH.Chen@...a.com>
Cc:     Jiri Slaby <jirislaby@...il.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-serial@...r.kernel.org" <linux-serial@...r.kernel.org>,
        Victor Yu (游勝義) <victor.yu@...a.com>,
        Danny Lin (林政易) <danny.lin@...a.com>
Subject: Re: [PATCH] tty: Add MOXA NPort Real TTY Driver

On Wed, Jul 22, 2020 at 07:04:00AM +0000, Johnson CH Chen (陳昭勳) wrote:
> Hi Greg,
> 
> Thanks for your response!
> 
> > > > > +	unsigned long flag;
> > > > > +	unsigned char cmd_buffer[84];
> > > > > +	unsigned char rsp_buffer[84];
> > > >
> > > > You seem to have two "static" buffers here, for your device, that 
> > > > you semi-randomly write to all over the place, but I can't find 
> > > > any locking or coordination between things that prevents multiple 
> > > > commands from not just overwritting each other.
> > > >
> > > For cmd_buffer[], we use npreal_wait_and_set_command() to make sure 
> > > cmd_buffer[] is safe to be written by checking "cmd_buffer[0] == 0".
> > 
> > And what locks are protecting you there?
> > 
> > > For rsp_buffer[], we use npreal_wait_command_completed() to make 
> > > sure rsp_buffer[] is desired by checking rsp_buffer[0] and rsp_buffer[1].
> > > Command_set and command should be checked. Besides, rsp_buffer[] is 
> > > got from user space by "NPREAL_NET_CMD_RESPONSE" in 
> > > npreal_net_ioctl().
> > 
> > Again, what locking is really handling this?
> > 
> 
> It's better to protect cmd_buffer[84] and rsp_buffer[84] by locking completely. They are safe because NPort driver should be worked with NPort daemon before, and NPort daemon is designed to be simple.

I'm sorry, but I do not understand this answer at all.  Something can be
"simple" and still be totally wrong :)

Without locking, this code is broken.

thanks,

greg k-h