loopdev: sync capacity after setting it

I recently tried to mount an hfsplus file system from an image file with
a partition table by using the loop offset and sizelimit options to specify
the location of the file system.

hfsplus stores some metadata at a set offset from the end of the partition,
so it's sensitive to the device size reported by the kernel.

It worked with this:

But failed with this:

/dev/loop0: [0089]:2 (<imagefile>), offset 32768, sizelimit 102400000
/dev/loop1: [0089]:2 (<imagefile>), offset 32768, sizelimit 102400000

/proc/partitions shows the correct number of blocks to match the sizelimit.

But if I set a breakpoint in mount before the mount syscall, I could see:
102400000
102432768

The kernel loop driver will set the gendisk capacity of the device at
LOOP_SET_STATUS64 but won't sync it to the block device until one of two
conditions are met: All open file descriptors referring to the device are
closed (and it will sync when re-opened) or if the LOOP_SET_CAPACITY ioctl
is called to sync it. Since mount opens the device and passes it directly
to the mount syscall after LOOP_SET_STATUS64 without closing and reopening
it, the sizelimit argument is effectively ignroed. The capacity needs to
be synced immediately for it to work as expected.

This patch adds the LOOP_SET_CAPACITY call to loopctx_setup_device since
the device isn't yet released to the user, so it's safe to sync the capacity
immediately.

[kzak@redhat.com: - port to the current git HEAD,
                  - use uint64_t]

Signed-off-by: Jeff Mahoney <jeffm@suse.com>
Signed-off-by: Karel Zak <kzak@redhat.com>

1 files changed, 16 insertions, 0 deletions

diff --git a/sys-utils/mount.8 b/sys-utils/mount.8
index 72f0f0b45..626d86169 100644
--- a/sys-utils/mount.8
+++ b/sys-utils/mount.8
@@ -2976,6 +2976,22 @@ and
 .BR ioctl
 families of functions) may lead to inconsistent result due to the lack of
 consistency check in kernel even if noac is used.
+.PP
+The
+.B loop
+option with the
+.B offset
+or
+.B sizelimit
+options used may fail when using older kernels if the
+.B mount
+command can't confirm that the size of the block device has been configured
+as requested. This situation can be worked around by using
+the
+.B losetup
+command manually before calling
+.B mount
+with the configured loop device.
 .SH HISTORY
 A
 .B mount