Re: nbd: add locking to nbd_ioctl

[Paul Clements]

Pavel Machek wrote:
> > 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?

OK, I can buy the complexity argument. Your patch sounds fine to me.

--
Paul
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