linux-kernel - Re: [PATCH net-next v2] misc: Add Renesas Synchronization Management Unit (SMU) support

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Date:   Fri, 12 Feb 2021 17:22:37 +0100
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Min Li <min.li.xe@...esas.com>
Cc:     "derek.kiernan@...inx.com" <derek.kiernan@...inx.com>,
        "dragan.cvetic@...inx.com" <dragan.cvetic@...inx.com>,
        "arnd@...db.de" <arnd@...db.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net-next v2] misc: Add Renesas Synchronization Management
 Unit (SMU) support

On Fri, Feb 12, 2021 at 04:02:14PM +0000, Min Li wrote:
> > 
> > xilinx_sdfec.c has:
> > 
> > 	static int xsdfec_dev_open(struct inode *iptr, struct file *fptr)
> > 	{
> > 	        return 0;
> > 	}
> > 
> > Which isn't even needed at all, but it is NOT trying to keep people from
> > calling open multiple times.
> > 
> > As for why the above logic does not work in your driver, think of what
> > happens if someone opens the character device node, and then calls
> > dup(2) on it and passes that file descriptor off to another program.  Or just
> > calls it multiple times from different threads in the same program.
> > The kernel does not know what is happening here, and so, "do not allow to
> > be opened multiple times" does not do anything to keep userspace from
> > actually writing to the device node from multiple processes or threads.
> > 
> > So don't even try, it's not worth it.
> > 
> > > I mean if an application failed at opening the device, how can it
> > > proceed to talk the device without a file descriptor?
> > 
> > See above for how to do this.
> > 
> > thanks,
> > 
> > greg k-h
> 
> Hi Greg
> 
> Thanks for your insight for this. Now I can see this change doesn't prevent deliberate hacker from opening the driver multiple times.
> 
> What I had in mind is that it does prevent some unintentional mistake like some one accidentally opens the application twice. The second
> one would fail due to the change here.

Do not add kernel code that does not work for it's intended purpose :)

thanks,

greg k-h