linux-kernel - Re: [PATCH v5 03/13] iommufd: Add iommufd_verify_unfinalized

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Message-ID: <20241029144907.GB209124@nvidia.com>
Date: Tue, 29 Oct 2024 11:49:07 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: Nicolin Chen <nicolinc@...dia.com>
Cc: kevin.tian@...el.com, will@...nel.org, joro@...tes.org,
	suravee.suthikulpanit@....com, robin.murphy@....com,
	dwmw2@...radead.org, baolu.lu@...ux.intel.com, shuah@...nel.org,
	linux-kernel@...r.kernel.org, iommu@...ts.linux.dev,
	linux-arm-kernel@...ts.infradead.org,
	linux-kselftest@...r.kernel.org, eric.auger@...hat.com,
	jean-philippe@...aro.org, mdf@...nel.org, mshavit@...gle.com,
	shameerali.kolothum.thodi@...wei.com, smostafa@...gle.com,
	yi.l.liu@...el.com, aik@....com, zhangfei.gao@...aro.org,
	patches@...ts.linux.dev
Subject: Re: [PATCH v5 03/13] iommufd: Add iommufd_verify_unfinalized_object

On Fri, Oct 25, 2024 at 04:49:43PM -0700, Nicolin Chen wrote:
> To support driver-allocated vIOMMU objects, it's suggested to call the
> allocator helper in IOMMU dirvers. However, there is no guarantee that
> drivers will all use it and allocate objects properly.
> 
> Add a helper for iommufd core to verify if an unfinalized object is at
> least reserved in the ictx.

I don't think we need this..

iommufd_object_finalize() already does:

	old = xa_store(&ictx->objects, obj->id, obj, GFP_KERNEL);
	/* obj->id was returned from xa_alloc() so the xa_store() cannot fail */
	WARN_ON(old);

So, we could enhance it to be more robust, like this patch is doing:

void iommufd_object_finalize(struct iommufd_ctx *ictx,
			     struct iommufd_object *obj)
{
	void *old;

	old = xa_cmpxchg(&ictx->objects, obj->id, XA_ZERO_ENTRY, obj,
			 GFP_KERNEL);
	/* obj->id was returned from xa_alloc() so the xa_store() cannot fail */
	WARN_ON(old != XA_ZERO_ENTRY);

It is always a driver bug to not initialize that object, so WARN_ON is
OK.

Jason