Re: nbd: add locking to nbd_ioctl

From: Paul Clements
Date: Mon Jan 19 2009 - 09:57:28 EST

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.


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