On 3/18/13 5:27 AM, Karel Zak wrote: > On Sun, Mar 17, 2013 at 10:18:28AM -0400, Jeff Mahoney wrote: >> 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. > > Do you mean #ifdef LOOP_SET_CAPACITY? I can fix the patch manually. > > (It seem that we already use this ioctl in code without any extra > care about old kernel and nobody complains :-) Yeah, but that's in losetup --set-capacity where it's an explicit operation. This change will add the ioctl into every loopdev_setup_device call when the offset or sizelimit options are used. If it isn't supported by the kernel, the ioctl will fail silently and *maybe* the mount will fail, but that's totally dependent on the the file system. If the mount succeeds, it will be done outside of the parameters the user requested. So, all I really want to do is dump an error message when the ioctl fails with -ENOTTY || -EINVAL about there being a lack of kernel support. We shouldn't allow the device configuration to proceed. The part that makes it more "fun" is that a few patches in 3.9-rc1 fixed this in the kernel, so it won't actually be needed for new kernels. -Jeff -- Jeff Mahoney SUSE Labs
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