linux-kernel - Re: usbfs, claiming entire usb devices

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Message-ID: <ac3eb2510905091023l42747289h56d59f3b0a7b5b90@mail.gmail.com>
Date:	Sat, 9 May 2009 19:23:30 +0200
From:	Kay Sievers <kay.sievers@...y.org>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
Cc:	Alan Stern <stern@...land.harvard.edu>,
	Kernel development list <linux-kernel@...r.kernel.org>,
	Pantelis Koukousoulas <pktoss@...il.com>,
	USB list <linux-usb@...r.kernel.org>
Subject: Re: usbfs, claiming entire usb devices

On Sat, May 9, 2009 at 19:15, Alan Cox <alan@...rguk.ukuu.org.uk> wrote:
>> > Yes.  That is, a process shouldn't be allowed to access a locked device
>> > unless that process is the lock holder.
>>
>> You think the pid or the uid would make more sense?
>
> How about neither ?
>
> The standard Unix behaviour is to open the file O_EXCL if you want
> exclusivity. Neither uid or pid are helpful or work in the many
> environments where you want security - in particular where (as is very
> common with user space driver type code) you want parts of your code
> running setuid and parts not, as two processes with different pid and uid
> values.
>
> If O_EXCL is interpeted as exclusive access (versus kernel and re-open of
> the same node) then you can implement the rest of the sematics in user
> space.

Hmm, but his is a lock for "future" device, which did not even show up
at that point the "lock" will be taken. The files would not be opened
a second time.

The lock-file and the device file are two different files, both would
be opened only by one process - and we are playing around here with
the idea how to connect these both processes.

Thanks,
Kay
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