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

From: Stefan Hajnoczi
Date: Tue Apr 12 2011 - 04:16:51 EST

On Tue, Apr 12, 2011 at 5:51 AM, Amit Shah <amit.shah@xxxxxxxxxx> wrote:
> 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 will have an outdated inode size returned from lseek(fd, 0, SEEK_END).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.

This sounds exactly like the bug that I described and it happens on
bare metal without virtualization.

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