linux-kernel - Re: [PATCH v2 7/7] drm/virtio: implement userptr: add interval tree

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Message-ID: <6fb21077-c254-49a7-97fd-64c87322ea43@gmail.com>
Date: Wed, 2 Apr 2025 14:45:35 -0400
From: Demi Marie Obenour <demiobenour@...il.com>
To: Dmitry Osipenko <dmitry.osipenko@...labora.com>,
 "Huang, Honglei1" <Honglei1.Huang@....com>, David Airlie
 <airlied@...hat.com>, Gerd Hoffmann <kraxel@...hat.com>,
 Gurchetan Singh <gurchetansingh@...omium.org>, Chia-I Wu
 <olvaffe@...il.com>, Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
 Maxime Ripard <mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>,
 Simona Vetter <simona@...ll.ch>, Rob Clark <robdclark@...il.com>,
 Huang Rui <ray.huang@....com>
Cc: dri-devel@...ts.freedesktop.org, virtualization@...ts.linux.dev,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 7/7] drm/virtio: implement userptr: add interval tree

On 4/2/25 8:34 AM, Dmitry Osipenko wrote:
> On 4/2/25 04:53, Huang, Honglei1 wrote:
>>
>> On 2025/3/30 19:57, Dmitry Osipenko wrote:
>>> If the purpose of this feature is to dedup usrptr BOs of a the single
>>> process/application, can this can be done in userspace? 
> 
> I assume it can be done in userspace, don't see why it needs to be in
> kernel.

The kernel definitely does not need to be responsible for deduplication,
but is it safe to allow userspace to create overlapping BOs, especially
ones that are partially but not entirely overlapping?  If the userspace
libraries ~everyone will be using refuse to create such BOs, then
overlapping BOs will be tested by ~nobody, and untested kernel code is
a good place for security vulnerabilities to linger.

If there are no legitimate use-cases for overlapping BOs, I would treat
attempts to create them as an errors and return -EINVAL, indicating that
the userspace code attempting to create them is buggy.  Userspace can
deduplicate the BOs itself if necessary.  Of course, there need to be
tests for userspace attempting to create overlapping BOs, including
attempting to do so concurrently from multiple threads.

That said, probably the most important part is consistency with userptr
in other (non-virtio) drivers, such as Intel and AMD.  If they allow
overlapping userptr BOs, then virtio should too; if they do not, then
virtio should also forbid them.
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)