Re: [PATCH] sr: Ensure disk is revalidated when media changes

From: Amit Shah
Date: Tue Apr 12 2011 - 00:52:06 EST

On (Fri) 08 Apr 2011 [09:52:07], Tejun Heo wrote:
> Hello,
> On Fri, Apr 08, 2011 at 05:43:16PM +0100, Stefan Hajnoczi wrote:
> > >> I think it would make sense to refresh the inode size on media change
> > >> so that even open file descriptors see the new size and a single
> > >> process cannot force a stale value for all other userspace processes
> > >> on the system.
> > >
> > > Hmmm... I don't know.  Maybe we can but I'm not sure whether there's a
> > > good reason for it.  cdrom is locked while opened after all.  Are
> > > there actual problems?
> >
> > Yeah, sorry I didn't explain what the use case was. With QEMU you can
> > pass through the physical CD-ROM into the virtual machine.
> >
> > QEMU opens /dev/cdrom with O_NONBLOCK | O_RDONLY. The guest can test
> > if the medium is present and QEMU will do ioctl(fd,
> > CDROM_DRIVE_STATUS, CDSL_CURRENT). The guest can also lock the tray
> > and eject, again using the respective ioctls. Read operations are
> > serviced by performing a read on the file descriptor in QEMU. And
> > finally the medium size is queried by QEMU using lseek(fd, 0,
> > SEEK_END).
> >
> > Today QEMU cannot keep /dev/cdrom open across media change because it
> > will have an outdated inode size returned from lseek(fd, 0, SEEK_END).
> > But if the cdrom driver (or sr) refresh the inode size on media
> > change then there is no need to work around this from userspace.
> Hmmm... ISTR there was some discussion about changing inode size on
> the fly quite a while ago. I didn't follow the discussion but it
> seemed to have rather nasty/delicate implications.

I don't necessarily agree with having to modify inode sizes on the
fly, but the main bug here is that the inode doesn't get invalidated
if a CDROM is ejected while a process has an fd to the CDROM device
opened. So as in the original case, if a CD is swapped with one
having more data, lseek continues to report the original media's size
in the process that keeps the fd open across eject/insert.

I haven't tried this on physical systems, so can't count out it being
a qemu bug as well.

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