Re: Commit d5fd456c88aba4fcf77d35fe38024a8d5c814686 - "loopdev: use LOOP_CONFIG ioctl" broke loop on x86-64 w/ 32 bit userspace

[Jens Axboe <axboe@kernel.dk>]

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). But
current behavior is set in stone at this point, even if it is
technically incorrect.

-- 
Jens Axboe