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SubjectRe: 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
--
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html


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