Re: [RFC PATCH 0/7] Allow race-free block device handling

Demi Marie Obenour
2023-02-02T20:58:09Z

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On Thu, Feb 02, 2023 at 02:56:34PM -0500, Mike Snitzer wrote:
> On Thu, Feb 02 2023 at 1:41P -0500,
> Demi Marie Obenour <demi@invisiblethingslab.com> wrote:
>
> > On Thu, Feb 02, 2023 at 11:50:37AM -0500, Mike Snitzer wrote:
> > > On Wed, Jan 25 2023 at 10:33P -0500,
> > > Demi Marie Obenour <demi@invisiblethingslab.com> wrote:
> > >
> > > > This work aims to allow userspace to create and destroy block devices
> > > > in a race-free and leak-free way,
> > >
> > > "race-free and leak-free way" implies there both races and leaks in
> > > existing code. You're making claims that are likely very specific to
> > > your Xen use-case. Please explain more carefully.
> >
> > Will do in v2.
> >
> > > > and to allow them to be exposed to
> > > > other Xen VMs via blkback without leaks or races. It’s marked as RFC
> > > > for a few reasons:
> > > >
> > > > - The code has been only lightly tested. It might be unstable or
> > > > insecure.
> > > >
> > > > - The DM_DEV_CREATE ioctl gains a new flag. Unknown flags were
> > > > previously ignored, so this could theoretically break buggy userspace
> > > > tools.
> > >
> > > Not seeing a reason that type of DM change is needed. If you feel
> > > strongly about it send a separate patch and we can discuss it.
> >
> > Patch 2/7 is the diskseq change. v2 will contain a revised and tested
> > version with a greatly expanded commit message.
>
> I'm aware that 2/7 is where you make the DM change to disallow unknown
> flags, what I'm saying is I don't see a reason for that change.

Thanks for the clarification.

> Certainly doesn't look to be a requirement for everything else in that
> patch.

Indeed it is not. I will make it a separate patch.

> So send a separate patch, but I'm inclined to _not_ accept it because
> it does potentially break some userspace.

Is it okay to add DM_FILE_DESCRIPTOR_FLAG (with the same meaning as in
2/7) _without_ rejecting unknown flags? The same patch would bump the
minor version number, so userspace would still be able to tell if the
kernel supported DM_FILE_DESCRIPTOR_FLAG. If you wanted, I could ignore
DM_FILE_DESCRIPTOR_FLAG unless the minor number passed by userspace is
sufficiently recent.

Another option would be to make userspace opt-in to strict parameter
checking by passing 5 as the major version instead of 4. Userspace
programs that passed 4 would get the old behavior, while userspace
programs that passed 5 would get strict parameter checking and be able
to use new features such as DM_FILE_DESCRIPTOR_FLAG.

> > > > - I have no idea if I got the block device reference counting and
> > > > locking correct.
> > >
> > > Your headers and justifcation for this line of work are really way too
> > > terse. Please take the time to clearly make the case for your changes
> > > in both the patch headers and code.
> >
> > I will expand the commit message in v2, but I am not sure what you want
> > me to add to the code comments. Would you mind explaining?
>
> Nothing specific about code, was just a general reminder (based on how
> terse the 2/7 header was).
>
> Mike

Thanks for the feedback!
--
Sincerely,
Demi Marie Obenour (she/her/hers)
Invisible Things Lab

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