Messages in this thread |  | | | From | Kay Sievers <> | | Date | Thu, 21 May 2009 23:07:29 +0200 | | Subject | Re: How to tell whether a struct file is held by a process? |
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On Thu, May 21, 2009 at 16:06, Alan Stern <stern@rowland.harvard.edu> wrote: > On Thu, 21 May 2009, Al Viro wrote: >> On Tue, May 19, 2009 at 12:57:21PM -0400, Alan Stern wrote: >> > What's the best way to tell whether the current process has a >> > particular struct file among its open files? Is there any better way >> > to find out than blindly calling fget() for each possible fd? >> > >> > Is this a totally insane thing to do? >> >> It is insane. You might lock fdtable and scan it, but as soon as you >> drop the spinlock your return value is worthless. > > In this case, I believe that changes after the check has been made > won't hurt -- everything relevant to my work will be serialized by a > separate lock. > >> What are you trying to do? If the process is cooperating, you don't really >> need that in the kernel, if it's not, the check is not usable... > > I'm trying to work out a good way to reserve access rights to a device > -- even if that device doesn't exist yet. > > Here's the story. People have requested that the kernel add a > mechanism whereby a user program can get more-or-less exclusive access > rights to a USB device. In fact, they'd like to reserve these rights > for any device plugged into a particular USB port. So even if no > device is plugged into that port at the moment, the program should get > exclusive access as soon as a new device is detected there. > > In order to prevent programs from dying without releasing their > exclusive rights, it seems natural to implement these rights as open > files. Thus, opening file A(P) will give a program exclusive access > rights to any device plugged into USB port P. Closing A(P) releases > the rights. (The A(P) files would be implemented as single-open files, > probably in sysfs.) > > The problem is this. Let D be the device plugged into port P. When > some program opens D's device file, it's necessary to check whether > that same program has an open file reference for A(P), i.e., has opened > A(P), or has inherited a descriptor for A(P) from its parent, or has > been passed such a descriptor over a Unix socket, etc. If not, and if > A(P) is open (owned by someone else), then access to D is denied. > > If you can think up a better way to implement these exclusive access > rights, I'd be glad to hear it.
Not sure, if we already discussed that with a conclusion: this "prevent access to a future device" interface needs to work at the pid level, or would a uid/gid check be sufficient?
Root can do all that stuff anyway, even with the locking in place. Uid/gid file permissions need to be applied to the "lock file", so specified non-root users can use that interface.
Maybe it would be good enough, to check that the one that opened the "lock file" has the same uid/gid as the on that tries to open the device when it has shown up?
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