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

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Message-ID: <81c94a1e-b316-c195-402c-01776f3200dc@gmail.com>
Date:   Sun, 25 Jun 2023 18:33:33 +0200
From:   Milan Broz <gmazyland@...il.com>
To:     Demi Marie Obenour <demi@...isiblethingslab.com>,
        Alasdair Kergon <agk@...hat.com>,
        Mike Snitzer <snitzer@...nel.org>, dm-devel@...hat.com
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [dm-devel] [PATCH v2 3/4] dm ioctl: Allow userspace to suppress
 uevent generation

On 6/25/23 18:02, Demi Marie Obenour wrote:
> 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.
But this is not clear from the patch header. I guess you also need
to disable udev inotify on close on write, which will trigger device scan too.

BTW we use exactly this scenario in cryptsetup for years with existing flags
(DM_UDEV_DISABLE_SUBSYSTEM_RULES_FLAG | DM_UDEV_DISABLE_DISK_RULES_FLAG
DM_UDEV_DISABLE_OTHER_RULES_FLAG) - just rules are ignored while uevent is still
sent.
Anyway, not sure we need another way to disable it; I just asked do you need it.

Milan