RE: [PATCH v2] iommu: Check dev->iommu in iommu_dev_xxx functions

[Shameerali Kolothum Thodi <shameerali.kolothum.thodi@huawei.com>]

> -----Original Message-----
> From: Shameerali Kolothum Thodi
> Sent: 12 February 2021 16:45
> To: 'Robin Murphy' <robin.murphy@arm.com>; linux-kernel@vger.kernel.org;
> iommu@lists.linux-foundation.org
> Cc: joro@8bytes.org; jean-philippe@linaro.org; will@kernel.org; Zengtao (B)
> <prime.zeng@hisilicon.com>; linuxarm@openeuler.org
> Subject: RE: [PATCH v2] iommu: Check dev->iommu in iommu_dev_xxx functions
> 
> 
> 
> > -----Original Message-----
> > From: Robin Murphy [mailto:robin.murphy@arm.com]
> > Sent: 12 February 2021 16:39
> > To: Shameerali Kolothum Thodi <shameerali.kolothum.thodi@huawei.com>;
> > linux-kernel@vger.kernel.org; iommu@lists.linux-foundation.org
> > Cc: joro@8bytes.org; jean-philippe@linaro.org; will@kernel.org; Zengtao (B)
> > <prime.zeng@hisilicon.com>; linuxarm@openeuler.org
> > Subject: Re: [PATCH v2] iommu: Check dev->iommu in iommu_dev_xxx
> functions
> >
> > On 2021-02-12 14:54, Shameerali Kolothum Thodi wrote:
> > > Hi Robin/Joerg,
> > >
> > >> -----Original Message-----
> > >> From: Shameer Kolothum
> [mailto:shameerali.kolothum.thodi@huawei.com]
> > >> Sent: 01 February 2021 12:41
> > >> To: linux-kernel@vger.kernel.org; iommu@lists.linux-foundation.org
> > >> Cc: joro@8bytes.org; robin.murphy@arm.com; jean-philippe@linaro.org;
> > >> will@kernel.org; Zengtao (B) <prime.zeng@hisilicon.com>;
> > >> linuxarm@openeuler.org
> > >> Subject: [Linuxarm] [PATCH v2] iommu: Check dev->iommu in
> > iommu_dev_xxx
> > >> functions
> > >>
> > >> The device iommu probe/attach might have failed leaving dev->iommu
> > >> to NULL and device drivers may still invoke these functions resulting
> > >> in a crash in iommu vendor driver code. Hence make sure we check that.
> > >>
> > >> Also added iommu_ops to the "struct dev_iommu" and set it if the dev
> > >> is successfully associated with an iommu.
> > >>
> > >> Fixes: a3a195929d40 ("iommu: Add APIs for multiple domains per
> device")
> > >> Signed-off-by: Shameer Kolothum
> > <shameerali.kolothum.thodi@huawei.com>
> > >> ---
> > >> v1 --> v2:
> > >>   -Added iommu_ops to struct dev_iommu based on the discussion with
> > Robin.
> > >>   -Rebased against iommu-tree core branch.
> > >
> > > A gentle ping on this...
> >
> > Is there a convincing justification for maintaining yet another copy of
> > the ops pointer rather than simply dereferencing iommu_dev->ops at point
> > of use?
> >
> 
> TBH, nothing I can think of now. That was mainly the way I interpreted your
> suggestion
> from the v1.  Now it looks like you didn’t mean it :). I am Ok to rework it to
> dereference
> it from iommu_dev. Please let me know.

So we can do something like this,

index fd76e2f579fe..5fd31a3cec18 100644
--- a/drivers/iommu/iommu.c
+++ b/drivers/iommu/iommu.c
@@ -2865,10 +2865,12 @@ EXPORT_SYMBOL_GPL(iommu_fwspec_add_ids);
  */
 int iommu_dev_enable_feature(struct device *dev, enum iommu_dev_features feat)
 {
-       const struct iommu_ops *ops = dev->bus->iommu_ops;
+       if (dev->iommu && dev->iommu->iommu_dev && dev->iommu->iommu_dev->ops)
+               struct iommu_ops  *ops = dev->iommu->iommu_dev->ops;
 
-       if (ops && ops->dev_enable_feat)
-               return ops->dev_enable_feat(dev, feat);
+               if (ops->dev_enable_feat)
+                       return ops->dev_enable_feat(dev, feat);
+       }
 
        return -ENODEV;
 }

Again, not sure we need to do the checking for iommu->dev and ops here. If the
dev->iommu is set, is it safe to assume that we have a valid iommu->iommu_dev
and ops always? (May be it is safer to do the checking in case something
else breaks this assumption in future). Please let me know your thoughts.

Thanks,
Shameer