[SRU][B][PATCH 0/1] loop: fix I/O error on fsync() in detached loop devices

[Jorge Merlino]

BugLink: https://bugs.launchpad.net/bugs/1856871

SRU Justification:

[Impact]

* There's an I/O error on fsync() in a detached loop device if it has
been previously attached. The issue is that write cache is enabled in
the attach path in loop_configure() but it isn't disabled in the detach
path; thus it remains enabled in the block device regardless of whether
it is attached or not.

* fsync() on detached loop devices can be called by partition tools and
commands run by sosreport, so the unexpected kernel error message might
surprise users or even distract from the actual issue being
investigatedr. It might also trigger alerts in
logging/monitoring/alerting stacks

[Fix]

* Disable write cache in the detach path

[Test Plan]

* Attach and detach an image to a loop device and test fsync return
value aterwards

# DEV=/dev/loop7

# IMG=/tmp/image
# truncate --size 1M $IMG

# losetup $DEV $IMG
# losetup -d $DEV

Before:
    # strace -e fsync parted -s $DEV print 2>&1 | grep fsync
    fsync(3)                                = -1 EIO (Input/output error)
    Warning: Error fsyncing/closing /dev/loop7: Input/output error
    [  982.529929] blk_update_request: I/O error, dev loop7, sector 0 op 0x1:(WRITE) flags 0x800 phys_seg 0 prio class 0

After:
    # strace -e fsync parted -s $DEV print 2>&1 | grep fsync
    fsync(3)                                = 0

[Where problems could occur]

* The detach path for block devices is modified. Worst case scenario
would be an error when detaching loop devices.

Mauricio Faria de Oliveira (1):
  loop: fix I/O error on fsync() in detached loop devices

 drivers/block/loop.c | 3 +++
 1 file changed, 3 insertions(+)

-- 
2.17.1