On Wed, Jan 19, 2022 at 09:52:47AM +0100, Jan Kara wrote: > Ping? Any opinion? Sorry for the delay. > On Thu 13-01-22 16:47:35, Jan Kara wrote: > > Hello, > > > > Tetsuo has been doing some changes to the loop device shutdown in the > > kernel and that broke LTP that is doing essentially the following loop: > > > > while :; do mount -o loop,ro isofs.iso isofs/; umount isofs/; done > > > > And this loop is broken because of a subtle interaction with systemd-udev > > that also opens the loop device. The race seems to be in mount(8) handling > > itself and the altered kernel timing makes it happen. It look like: > > > > bash systemd-udev > > mount -o loop,ro isofs.iso isofs/ > > /dev/loop0 is created and bound to isofs.iso, autoclear is set for > > loop0 > > opens /dev/loop0 > > umount isofs/ > > loop0 still lives because systemd-udev still has device open > > mount -o loop,ro isofs.iso isofs/ > > gets to mnt_context_setup_loopdev() > > loopcxt_find_overlap() > > sees loop0 is still valid and with proper parameters > > reuse = true; > > close /dev/loop0 > > last fd closed => loop0 is > > cleaned up > > loopcxt_get_fd() > > opens loop0 but it is no longer the device we wanted! > > calls mount(2) which fails because we cannot read from the loop device > > > > It seems to me that mnt_context_setup_loopdev() should actually recheck > > that loop device parameters still match what we need after opening > > /dev/loop0 (if LOOP_GET_STATUS ioctl succeeds on the fd, you are guaranteed > > the loop device is in that state and will not be torn down under your > > hands). What do you think? Seems like elegant solution. Please, send a patch. (It would be possible to be care about autoclear in loopcxt_find_overlap(), but it sounds complicated and probably still racy. > > > > Honza > > -- > > Jan Kara <jack@xxxxxxxx> > > SUSE Labs, CR > -- > Jan Kara <jack@xxxxxxxx> > SUSE Labs, CR > -- Karel Zak <kzak@xxxxxxxxxx> http://karelzak.blogspot.com
