Messages in this thread | | | Subject | Re: Commit d5fd456c88aba4fcf77d35fe38024a8d5c814686 - "loopdev: use LOOP_CONFIG ioctl" broke loop on x86-64 w/ 32 bit userspace | From | Krzysztof Olędzki <> | Date | Tue, 27 Jul 2021 22:46:06 -0700 |
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On 2021-07-27 at 18:24, Jens Axboe wrote: > On 7/27/21 4:56 PM, Krzysztof Olędzki wrote: >> On 2021-07-27 at 15:39, Krzysztof Olędzki wrote: >>> On 2021-07-27 at 14:53, Krzysztof Olędzki wrote: >>>> Hi, >>>> >>>> I have a number of (older) systems that are still based on 32 bit >>>> userspace but are running a relatively modern 64 bit kernel - >>>> 5.4-stable, where BTW - LOOP_CONFIGURE is not yet available. >>>> >>>> I noticed that starting with util-linux-2.37 it is no longer possible to >>>> mount images using loop: >>>> >>>> # mount /usr/install/iso/systemrescue-8.04-amd64.iso /mnt/cdrom >>>> mount: /mnt/cdrom: failed to setup loop device for >>>> /usr/install/iso/systemrescue-8.04-amd64.iso. >>>> >>>> Reverting d5fd456c88aba4fcf77d35fe38024a8d5c814686 fixes the problem: >>>> >>>> /tmp/util-linux-2.37# ./mount >>>> /usr/install/iso/systemrescue-8.04-amd64.iso /mnt/cdrom >>>> mount: /mnt/cdrom: WARNING: source write-protected, mounted read-only. >>>> >>>> I have not tested if 32 bit kernel + 32 bit userspace is also affected, >>>> but 64 bit kernel + 64 bit userspace works. >>> >>> Some debugging data: >>> >>> 30399: loopdev: CXT: [0xff8d0f98]: using loop-control >>> 30399: loopdev: CXT: [0xff8d0f98]: loop0 name assigned >>> 30399: loopdev: CXT: [0xff8d0f98]: find_unused by loop-control [rc=0] >>> 30399: libmount: LOOP: [0x57cbbcb0]: trying to use /dev/loop0 >>> 30399: loopdev: CXT: [0xff8d0f98]: set backing file=/usr/install/iso/systemrescue-8.04-amd64.iso >>> 30399: loopdev: CXT: [0xff8d0f98]: set flags=4 >>> 30399: loopdev: SETUP: [0xff8d0f98]: device setup requested >>> 30399: loopdev: SETUP: [0xff8d0f98]: backing file open: OK >>> 30399: loopdev: CXT: [0xff8d0f98]: open /dev/loop0 [rw]: Success >>> 30399: loopdev: SETUP: [0xff8d0f98]: device open: OK >>> 30399: loopdev: SETUP: [0xff8d0f98]: LOOP_CONFIGURE failed: Inappropriate ioctl for device >>> 30399: loopdev: SETUP: [0xff8d0f98]: failed [rc=-25] >>> 30399: libmount: LOOP: [0x57cbbcb0]: failed to setup device >>> 30399: loopdev: CXT: [0xff8d0f98]: de-initialize >>> 30399: loopdev: CXT: [0xff8d0f98]: closing old open fd >>> 30399: loopdev: ITER: [0xff8d1168]: de-initialize >>> 30399: libmount: CXT: [0x57cbbcb0]: mount: preparing failed >>> 30399: libmount: CXT: [0x57cbbcb0]: excode: rc=32 message="failed to setup loop device for /usr/install/iso/systemrescue-8.04-amd64.iso" >>> mount: /mnt/cdrom: failed to setup loop device for /usr/install/iso/systemrescue-8.04-amd64.iso. >>> 30399: libmount: CXT: [0x57cbbcb0]: <---- reset [status=0] ----> >>> >>> Seems like the code expects EINVAL (-22) but gets ENOTTY (-25), confirmed with strace: >>> ioctl(4, LOOP_CONFIGURE, {fd=3, block_size=0, info={lo_offset=0, lo_number=0, lo_flags=LO_FLAGS_AUTOCLEAR, lo_file_name="/usr/install/iso/systemrescue-8.04-amd64.iso", ...}}) = -1 ENOTTY (Inappropriate ioctl for device) >>> >>> Indeed, changing the code from: >>> if (errno != EINVAL) >>> to: >>> if (errno != EINVAL && errno != ENOTTY) >>> allows it to work. >>> >>> Not that with 64-bit userspace, kernel returns EINVAL: >>> >>> ioctl(4, LOOP_CONFIGURE, {fd=3, block_size=0, info={lo_offset=0, lo_number=0, lo_flags=LO_FLAGS_AUTOCLEAR, lo_file_name="/usr/src/PACKAGES/systemrescue-8.04-amd64.iso", ...}}) = -1 EINVAL (Invalid argument) >> >> ... which is because lo_compat_ioctl returns -ENOIOCTLCMD for >> unsupported cmds, while lo_ioctl returns -EINVAL via lo_simple_ioctl. >> >> And vfs_ioctl returns -ENOTTY for -ENOIOCTLCMD. >> >> Now the question is if this inconsistency is intended? :) > > That's unfortunate, but probably not something that can get corrected at > this time. The correct return value for an unknown ioctl is -ENOTTY > (ENOIOCTLCMD isn't user visible, should get turned into that).
Yes, it does - as I said, vfs_ioctl handles this properly. However, this only works for .compat_ioctl (via mentioned lo_compat_ioctl which returns -ENOIOCTLCMD) but not for .ioctl (via lo_ioctl, which returns -EINVAL).
> But > current behavior is set in stone at this point, even if it is > technically incorrect.
Agreed. And even if this could be somehow fixed in further kernels, I believe we still need to fix the userspace to support and properly handle all the existing kernels.
So, to confirm - checking for both EINVAL and ENOTTY after LOOP_CONFIGURE is the proper way of taking care this?
https://git.kernel.org/pub/scm/utils/util-linux/util-linux.git/tree/lib/loopdev.c?id=d4423cce9b9001c9de7ebc6f64f6cc2bb854944c#n1362
Thanks, Krzysztof
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