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

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Date:   Fri, 12 Feb 2021 16:02:14 +0000
From:   Min Li <min.li.xe@...esas.com>
To:     Greg KH <gregkh@...uxfoundation.org>
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

> 
> 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.