Re: [PATCH] loopdev: sync capacity after setting it

Jeff Mahoney
2013-03-17T14:18:41Z

On 03/15/2013 05:04 PM, Jeff Mahoney wrote:
> 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:
> # losetup -r -o 32k --sizelimit=102400000 <loopdev> <imagefile>
> # mount -r -t hfsplus <loopdev> <mountpoint>
>
> But failed with this:
> # mount -r -oloop,offset=32k,sizelimit=102400000 <imagefile> <mountpoint>
>
> # losetup -a
> /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:
> # blockdev --getsize64 /dev/loop[01]
> 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.

It turns out this ioctl wasn't introduced until 2.6.30. I'll fix that up
and resend tomorrow.

-Jeff

> Signed-off-by: Jeff Mahoney <jeffm@suse.com>
> ---
> lib/loopdev.c | 12 ++++++++++++
> 1 file changed, 12 insertions(+)
>
> --- a/lib/loopdev.c
> +++ b/lib/loopdev.c
> @@ -1105,6 +1105,18 @@ int loopcxt_setup_device(struct loopdev_
> goto err;
> }
>
> + /*
> + * mount passes the file descriptor without closing it, so
> + * the capacity isn't updated automatically. File systems
> + * won't see the new size if it's not the full size of
> + * the backing file.
> + */
> + if ((lc->info.lo_offset || lc->info.lo_sizelimit) &&
> + ioctl(dev_fd, LOOP_SET_CAPACITY, 0)) {
> + DBG(lc, loopdev_debug("LOOP_SET_CAPACITY failed: %m"));
> + goto err;
> + }
> +
> DBG(lc, loopdev_debug("setup: LOOP_SET_STATUS64: OK"));
>
> memset(&lc->info, 0, sizeof(lc->info));
>

--
Jeff Mahoney
SUSE Labs