linux-kernel - Re: [PATCH v4 2/7] iommufd: Add iommufd_object_tombstone

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Message-ID: <20250711174402.GG1951027@nvidia.com>
Date: Fri, 11 Jul 2025 14:44:02 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: Xu Yilun <yilun.xu@...ux.intel.com>
Cc: kevin.tian@...el.com, will@...nel.org, aneesh.kumar@...nel.org,
	iommu@...ts.linux.dev, linux-kernel@...r.kernel.org,
	joro@...tes.org, robin.murphy@....com, shuah@...nel.org,
	nicolinc@...dia.com, aik@....com, dan.j.williams@...el.com,
	baolu.lu@...ux.intel.com, yilun.xu@...el.com
Subject: Re: [PATCH v4 2/7] iommufd: Add iommufd_object_tombstone_user()
 helper

On Wed, Jul 09, 2025 at 12:02:29PM +0800, Xu Yilun wrote:
> Add the iommufd_object_tombstone_user() helper, which allows the caller
> to destroy an iommufd object created by userspace.
> 
> This is useful on some destroy paths when the kernel caller finds the
> object should have been removed by userspace but is still alive. With
> this helper, the caller destroys the object but leave the object ID
> reserved (so called tombstone). The tombstone prevents repurposing the
> object ID without awareness of the original user.
> 
> Since this happens for abnormal userspace behavior, for simplicity, the
> tombstoned object ID would be permanently leaked until
> iommufd_fops_release(). I.e. the original user gets an error when
> calling ioctl(IOMMU_DESTROY) on that ID.
> 
> The first use case would be to ensure the iommufd_vdevice can't outlive
> the associated iommufd_device.
> 
> Suggested-by: Jason Gunthorpe <jgg@...dia.com>
> Reviewed-by: Lu Baolu <baolu.lu@...ux.intel.com>
> Co-developed-by: Aneesh Kumar K.V (Arm) <aneesh.kumar@...nel.org>
> Signed-off-by: Aneesh Kumar K.V (Arm) <aneesh.kumar@...nel.org>
> Signed-off-by: Xu Yilun <yilun.xu@...ux.intel.com>
> ---
>  drivers/iommu/iommufd/iommufd_private.h | 23 ++++++++++++++++++++++-
>  drivers/iommu/iommufd/main.c            | 24 ++++++++++++++++++++++--
>  2 files changed, 44 insertions(+), 3 deletions(-)

Reviewed-by: Jason Gunthorpe <jgg@...dia.com>

Jason