linux-kernel - Re: broken device locking, sg vs. sg

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Message-Id: <E1HXptY-00017P-FP@flower>
Date:	Sun, 1 Apr 2007 04:34:16 +0200
From:	Oleg Verych <olecom@...wer.upol.cz>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
Cc:	Eduard Bloch <edi@....de>, linux-kernel@...r.kernel.org,
	debburn-devel@...ts.alioth.debian.org
Subject: Re: broken device locking, sg vs. sg_io on block devices

> From: Alan Cox
> Newsgroups: gmane.linux.kernel
> Subject: Re: broken device locking, sg vs. sg_io on block devices
> Date: Sun, 1 Apr 2007 01:14:52 +0100
>
[]
>> Again, it doesn't have to. It can pass the locking operations to the
>> related block device driver.
>
> No it can't. The driver has no idea what the locking rules are for
> arbitary command blocks send to arbitary devices. /dev/sg is a *raw*
> interface. You can send anything to anyone, and the locking rules for
> that are far too complex for a giant morass of kernel code to get added.
>
> The mess begins because you use /dev/sg and put it in a cdrom group
> instead of using SG_IO on the /dev/sr device.

(offtop: 'cdrom' is as ugly as 'floppy' for anything like usb,
firewire connected storage, why not use 'optics' and 'external' or
something?)

> The mess continues because of the user of O_EXCL locking thus forcing
> re-open/close by HAL

Manpage states something bad about it also...

> instead of fcntl based co-operative locking.

> > > getty/modem/uucp/terminal emulator/slip/ppp/..

Programs you've mentioned may have co-operative locking, but 'dd' or
'cat' have no knowledge of it for sure. Yet nothing prevents allowed user
program to use this tools on /dev/tty*.

AFAIK kernel developers are always ready for very broken userspace, yet
co-operative locking is a job of the userspace programmers of very
different tools.

> The job of the kernel is not and never has been to anticipate and correct
> everything stupid someone tries to do in user space.

> As I said before the people wanting to arbitrate serial ports got this
> right in the mid 1970's your situation is not much more complicated,

Do you mean co-operative locking or carrier detection as a pre-hotplug
thing (:?

Tell me, please, somebody, why non-exclusive co-operative locking (if it
was implemented anyways), racy and already used in userspace applications
O_EXCL are better than _mandatory locking_? I've found this helpful against
any broken userspace, trying hijack my device and read or write bytes
to it.
____
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