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

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Message-ID: <20090126164959.GB4145@atrey.karlin.mff.cuni.cz>
Date:	Mon, 26 Jan 2009 17:49:59 +0100
From:	Pavel Machek <pavel@...e.cz>
To:	Paul Clements <paul.clements@...eleye.com>
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.

I could do that but it would be a bit too complex, and still rely on
big kernel lock. Would you agree to patch that added tx_lock around
all of it, and moved ioctl to unlocked ioctl?

You are right that errors were much less severe than I thought in the
beggining.

									Pavel
-- 
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