Re: [BUG] FLOPPY DRIVER since 5.10.20

[Denis Efremov <efremov@linux.com>]

On 7/26/21 3:23 PM, Mark Hounschell wrote:
> On 7/26/21 7:37 AM, Denis Efremov wrote:
>>
>>
>> On 7/26/21 2:17 PM, Mark Hounschell wrote:
>>> On 7/26/21 3:57 AM, Denis Efremov wrote:
>>>> Hi,
>>>>
>>>> On 7/23/21 9:47 PM, Mark Hounschell wrote:
>>>>>
>>>>> These 2 incremental patches, patch-5.10.19-20 and patch-5.11.2-3 have broken the user land fd = open("/dev/fd0", (O_RDWR | O_NDELAY)); functionality.
>>>>
>>>> Thank you for the report, I'm looking into this.
>>>>
>>>>> Since FOREVER before the patch, when using O_NDELAY, one could open the floppy device with no media inserted or even with write protected media without error. "Read-only file system" status is returned only when we actually tried to write to it. We have software still in use today that relies on this functionality.
>>>>
>>>> If it's a project with open sources could you please give a link?
>>>>
>>>> Regards,
>>>> Denis
>>>>
>>> This is immaterial but fdutils and libdsk both use rely on this flag. Who can know who else does. The point is it should NOT have been changed.
>>
>> Yes, I asked this only to add utils and this behavior to the tests.
>> And be more specific about why we should preserve this behavior in
>> next commit messages.
>>
> 
> Well, first thing is now you can't open a floppy with a write protected floppy installed. I don't think that was intended but that is now how it is.
> 
> Next there are commands that can be sent to the floppy via "ioctl(fd, FDRAWCMD,  &raw_cmd);" that do NOT require a floppy diskette to be installed.
> 
> All commands issued to the device that require a floppy diskette without a diskette installed fail with the proper status letting you know the device is not ready / no diskette installed. That goes for write protected floppies too.
> 
> There is no reason to force a user to only be able to operate on Linux fdformat formatted floppies.
> 

It appears that the story behind the issue is long enough.
I'll try to sum up the things:
[1] 09954bad4487 floppy: refactor open() flags handling
[2] ff06db1efb2a floppy: fix open(O_ACCMODE) for ioctl-only open
[3] 468c298ad3ed Revert "floppy: fix open(O_ACCMODE) for ioctl-only open"
[4] f2791e7eadf4 Revert "floppy: refactor open() flags handling"
[5] 8a0c014cd205 floppy: reintroduce O_NDELAY fix

In [1] we tried to fix O_NDELAY behavior because it's hard to define
proper non-blocking behavior for floppies. We also added
"!(mode & (FMODE_READ|FMODE_WRITE))" sanity check for open in that patch.
Motivation for the changes was that it's easy to livelock the system with
floppy's O_NDELAY and syzkaller spotted it. Just for the record, /dev/fd0
is only accessible by the root user in recent distros. 

Patch [1] broke ioctl-only opens in fdutils because:
$ grep -nre open ./setfdprm.c 
60:     if ((fd = open(argv[0],3)) < 0) { /* 3 == no access at all */
Patch [2] reverted "!(mode & (FMODE_READ|FMODE_WRITE))" to fix ioctls.
I guess [2] was not enough and Jens completely reverted [1] with [3] [4].

The last [5] patch restores the open function to the [2] state (it's possible
to use ioctl with open O_ACCMODE). [5] was added because libblkid use O_NONBLOCK
for probing devices, and floppy driver prints many I/O errors to the kernel log.
There are also problems with mounts after. I'm afraid simple revert for [5] is
not enough, otherwise we will face libblkid issues once again.I'll try to test the things and find a more elegant solution.

Denis