Re: [PATCH 3/3] iommu/uapi: Add helper function for size lookup

[Alex Williamson <alex.williamson@redhat.com>]

On Wed, 29 Jan 2020 14:40:46 -0700
Alex Williamson <alex.williamson@redhat.com> wrote:

> On Tue, 28 Jan 2020 22:02:04 -0800
> Jacob Pan <jacob.jun.pan@linux.intel.com> wrote:
> 
> > IOMMU UAPI can be extended in the future by adding new
> > fields at the end of each user data structure. Since we use
> > a unified UAPI version for compatibility checking, a lookup
> > function is needed to find the correct user data size to copy
> > from user.
> > 
> > This patch adds a helper function based on a 2D lookup with
> > version and type as input arguments.
> > 
> > Signed-off-by: Liu Yi L <yi.l.liu@intel.com>
> > Signed-off-by: Jacob Pan <jacob.jun.pan@linux.intel.com>
> > ---
> >  drivers/iommu/iommu.c | 22 ++++++++++++++++++++++
> >  include/linux/iommu.h |  6 ++++++
> >  2 files changed, 28 insertions(+)
> > 
> > diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
> > index 7dd51c5d2ba1..9e5de9abebdf 100644
> > --- a/drivers/iommu/iommu.c
> > +++ b/drivers/iommu/iommu.c
> > @@ -1696,6 +1696,28 @@ int iommu_sva_unbind_gpasid(struct iommu_domain *domain, struct device *dev,
> >  }
> >  EXPORT_SYMBOL_GPL(iommu_sva_unbind_gpasid);
> >  
> > +
> > +/**
> > + * Maintain a UAPI version to user data structure size lookup for each
> > + * API function types we support. e.g. bind guest pasid, cache invalidation.
> > + * As data structures being extended with new members, the offsetofend()
> > + * will identify the new sizes.
> > + */
> > +const static int iommu_uapi_data_size[NR_IOMMU_UAPI_TYPE][IOMMU_UAPI_VERSION] = {
> > +	/* IOMMU_UAPI_BIND_GPASID */
> > +	{offsetofend(struct iommu_gpasid_bind_data, vtd)},
> > +	/* IOMMU_UAPI_CACHE_INVAL */
> > +	{offsetofend(struct iommu_cache_invalidate_info, addr_info)},

This seems prone to errors in future revisions.  Both of the above
reference the end of fields within an anonymous union.  When a new
field is added, it's not necessarily the newest field that needs to be
listed here, but the largest at the time.  So should the current
version always use sizeof instead (or name the union so we can
reference it)?  I'm not sure of an error proof way to make sure we keep
the N-1 version consistent when we add a new version though.  More
comments?

Also, is the 12-bytes of padding in struct iommu_gpasid_bind_data
excessive with this new versioning scheme?  Per rule #2 I'm not sure if
we're allowed to repurpose those padding bytes, but if we add fields to
the end of the structure as the scheme suggests, we're stuck with not
being able to expand the union for new fields.

Thanks,
Alex

> > +	/* IOMMU_UAPI_PAGE_RESP */
> > +	{offsetofend(struct iommu_page_response, code)},
> > +};
> > +
> > +int iommu_uapi_get_data_size(int type, int version)
> > +{  
> 
> Seems like this is asking for a bounds check,
> 
>   if (type >= NR_IOMMU_UAPI_TYPE || version > IOMMU_UAPI_VERSION)
>   	return -EINVAL;
> 
> If we add new types in future versions, I assume we'd back fill the
> table with -EINVAL as well (rather than zero).  Thanks,
> 
> Alex
> 
> > +	return iommu_uapi_data_size[type][version - 1];
> > +}
> > +EXPORT_SYMBOL_GPL(iommu_uapi_get_data_size);
> > +
> >  static void __iommu_detach_device(struct iommu_domain *domain,
> >  				  struct device *dev)
> >  {
> > diff --git a/include/linux/iommu.h b/include/linux/iommu.h
> > index 9718c109ea0a..416fe02160ba 100644
> > --- a/include/linux/iommu.h
> > +++ b/include/linux/iommu.h
> > @@ -500,6 +500,7 @@ extern int iommu_report_device_fault(struct device *dev,
> >  				     struct iommu_fault_event *evt);
> >  extern int iommu_page_response(struct device *dev,
> >  			       struct iommu_page_response *msg);
> > +extern int iommu_uapi_get_data_size(int type, int version);
> >  
> >  extern int iommu_group_id(struct iommu_group *group);
> >  extern struct iommu_group *iommu_group_get_for_dev(struct device *dev);
> > @@ -885,6 +886,11 @@ static inline int iommu_page_response(struct device *dev,
> >  	return -ENODEV;
> >  }
> >  
> > +static int iommu_uapi_get_data_size(int type, int version)
> > +{
> > +	return -ENODEV;
> > +}
> > +
> >  static inline int iommu_group_id(struct iommu_group *group)
> >  {
> >  	return -ENODEV;  
>