Re: [dm-devel] [PATCH v2 3/4] dm ioctl: Allow userspace to suppress uevent generation

From: Demi Marie Obenour <demi@invisiblethingslab.com>
To: Milan Broz <gmazyland@gmail.com>,
	Alasdair Kergon <agk@redhat.com>,
	Mike Snitzer <snitzer@kernel.org>,
	dm-devel@redhat.com
Cc: linux-kernel@vger.kernel.org
Subject: Re: [dm-devel] [PATCH v2 3/4] dm ioctl: Allow userspace to suppress uevent generation
Date: Sun, 25 Jun 2023 12:02:29 -0400	[thread overview]
Message-ID: <ZJhlGCP0gTz7T3gG@itl-email> (raw)
In-Reply-To: <0b22e328-40e1-54d7-367c-96059a3fef7c@gmail.com>

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On Sun, Jun 25, 2023 at 03:25:38PM +0200, Milan Broz wrote:
> On 6/25/23 01:09, Demi Marie Obenour wrote:
> > Userspace can use this to avoid spamming udev with events that udev
> > should ignore.
> 
> Well, does it also mean that udev will not create /dev/disk/by-* symlinks
> (as response to the change udev event followed by internal udev blkid scan)?

In the use-case I have for this feature (block devices for Qubes VMs)
the blkid scan is unwanted and there are udev rules to prevent this.

> If it is a private device, that is ok. But for a visible device I think
> that it breaks some assumptions in userspace (presence of symlinks mentioned
> above etc).

The devices I am considering are implementation details of a userspace
process.  Nobody else should be opening them.  Ideally, no other
userspace process would even know they exist, at least without mucking
around in /proc or using ptrace.

> So, what is the exact use for this patch?

Ephemeral devices that are created, opened, marked for deferred removal,
assigned to a Xen VM (needs another patch currently being worked on),
and then closed.  udev has no business scanning these devices, and
indeed for it to scan them at all would be a security vulnerability
since their contents are under guest control.  There are udev rules to
ignore these devices, but for udev to even process the event wastes CPU
time and delays processing of other events that actually matter.  The
only symlink that possibly ought to be created is /dev/disk/by-diskseq
and I can just do that myself.
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)
Invisible Things Lab

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