linux-kernel - Re: nbd: add locking to nbd

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Date:	Mon, 19 Jan 2009 09:56:59 -0500
From:	Paul Clements <paul.clements@...eleye.com>
To:	Pavel Machek <pavel@...e.cz>
CC:	kernel list <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...l.org>
Subject: Re: nbd: add locking to nbd_ioctl

Pavel Machek wrote:
>> Pavel Machek wrote:
>>> On Fri 2009-01-16 10:24:06, Paul Clements wrote:

>> lo->sock is only modified under tx_lock (except for SET_SOCK, where the 
>> device is being initialized, in which case it's impossible for any other 
>> thread to be accessing the device)
> 
> Well, unless the user is evil or confused? :-).

Even in that case, you're just going to get EBUSY. Nothing bad will 
happen. SET_SOCK checks for lo->file, so it cannot be called on an 
active nbd device.


>> As for other fields, I assume you're talking about blksize, et al. 
>> Taking tx_lock doesn't prevent you from screwing yourself if you modify 
>> those while the device is active. You'd need to disallow those ioctls 
>> when the device is active (check lo->file). Again, this is only going to 
>> happen if you really misuse the ioctls.
> 
> Ok, I'll take a look at the missing checks. I'd really like to make
> this "stable" -- no amount of misuse should crash the kernel.

Just to summarize, I don't think we need to hold tx_lock around the 
entirety of nbd_ioctl. We do need one extra tx_lock around xmit_timeout 
and we do need to check for lo->file and return EBUSY in all of the 
SET_*SIZE* ioctls.

Thanks,
Paul

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