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    SubjectRe: nbd: add locking to nbd_ioctl
    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.


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