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SubjectRe: Resend: Another 4.4 to 4.5 floppy issue
On Mon, 11 Jul 2016, Mark Hounschell wrote:

> Well, all that was specified in my original post. I can no longer open the
> floppy drive with no floppy media inserted. Worse, I can also no longer open a
> floppy with media inserted that is not a "linux" recognized format. A floppy
> drive is a removable media device and should be treated as such. The original
> implementation of the O_NDELAY flag allowed it to be.
>
> Any removable media device should be capable of being opened with no, or even
> unrecognizable media installed. The kernel and its utilities should not
> "assume" to much when it comes to removable media. Consider a SCSI tape drive
> or even a removable media SCSI disk drive. How would you explain an open
> failure to someone trying to open a SCSI tape drive that had no tape or even a
> "non-tar" formatted tape media in it???
> Or better yet, trying to open a removable media device the was write protected
> but didn't include O_RDONLY in the open?

Alright, so you are basically supplementing O_NDELAY flag in order to
avoid check_disk_change() being called. It's rather a coincidence that it
has worked this way, but I agree with you that we can't ignore the fact
that there is userspace relying on this behavior.

> The original behavior of the floppy driver was correct. I have no idea
> what BUG these changes were supposed to fix but the "fix" obviously
> broke user land. Was this bug reported by some new ROBOT test or
> something? The kernel floppy driver has been stable for years now

That's not really true; the code is a racy mess, and this is being
uncovered only when virtualized floppy devices started to exist (because
they are much faster than a real hardware, and the different timing
reveals bugs that were not visible before).

This particular fix was because syzkaller found a way how easily corrupt
kernel memory using O_NDELAY to floppy driver; see

https://lkml.org/lkml/2016/2/2/848

> so I am really confused as to why these changes were induced.

The floppy driver is in an orphan mode; no new "features" are added "just
because". Everything that's happening there is to fix real bugs in the
kernel.

I'll look into ways how to fix this, but I am afraid this is going to be
really tricky. Therefore we'd have to very likely proceed asap with revert
of 09954bad448 and coming up with a workaround that'd still avoid the bug
reported by syzkaller.

--
Jiri Kosina
SUSE Labs

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