Re: [PATCH] iommu/of: Fix of_iommu_configure() for disabled IOMMUs

[Robin Murphy]

On 04/08/17 12:16, Robin Murphy wrote:
> Sudeep reports that the logic is slightly broken when a PCI iommu-map
> entry targets an IOMMU marked as disabled in DT, since of_pci_map_rid()
> succeeds in following a phandle, and of_iommu_xlate() doesn't return an
> error value, but we miss checking whether ops was actually non-NULL.
> Whilst this could be solved with a point fix in of_pci_iommu_init(), it
> suggests that all the juggling of ERR_PTR values through the ops pointer
> is proving rather too complicated for its own good, so let's instead
> simplify the whole flow (with a side-effect of eliminating the cause of
> the bug).
> 
> The fact that we now rely on iommu_fwspec means that we no longer need
> to pass around an iommu_ops pointer at all - we can simply propagate a
> regular int return value until we know whether we have a viable IOMMU,
> then retrieve the ops from the fwspec if and when we actually need them.
> This makes everything a bit more uniform and certainly easier to follow.
> 
> Reported-by: Sudeep Holla <sudeep.holla@xxxxxxx>
> Signed-off-by: Robin Murphy <robin.murphy@xxxxxxx>
> ---
> 
> This could be squashed into the currently-queued offender, but if not,
> Fixes: d87beb749281 ("iommu/of: Handle PCI aliases properly")
> 
> Robin.
> 
>  drivers/iommu/of_iommu.c | 63 ++++++++++++++++++++++++------------------------
>  1 file changed, 31 insertions(+), 32 deletions(-)
> 
> diff --git a/drivers/iommu/of_iommu.c b/drivers/iommu/of_iommu.c
> index be8ac1ddec06..198ef9a49f64 100644
> --- a/drivers/iommu/of_iommu.c
> +++ b/drivers/iommu/of_iommu.c
> @@ -25,6 +25,8 @@
>  #include <linux/of_pci.h>
>  #include <linux/slab.h>
>  
> +#define NO_IOMMU	1
> +
>  static const struct of_device_id __iommu_of_table_sentinel
>  	__used __section(__iommu_of_table_end);
>  
> @@ -109,8 +111,8 @@ static bool of_iommu_driver_present(struct device_node *np)
>  	return of_match_node(&__iommu_of_table, np);
>  }
>  
> -static const struct iommu_ops
> -*of_iommu_xlate(struct device *dev, struct of_phandle_args *iommu_spec)
> +static int of_iommu_xlate(struct device *dev,
> +			  struct of_phandle_args *iommu_spec)
>  {
>  	const struct iommu_ops *ops;
>  	struct fwnode_handle *fwnode = &iommu_spec->np->fwnode;
> @@ -120,24 +122,20 @@ static const struct iommu_ops
>  	if ((ops && !ops->of_xlate) ||
>  	    !of_device_is_available(iommu_spec->np) ||
>  	    (!ops && !of_iommu_driver_present(iommu_spec->np)))
> -		return NULL;
> +		return NO_IOMMU;
>  
>  	err = iommu_fwspec_init(dev, &iommu_spec->np->fwnode, ops);
>  	if (err)
> -		return ERR_PTR(err);
> +		return err;
>  	/*
>  	 * The otherwise-empty fwspec handily serves to indicate the specific
>  	 * IOMMU device we're waiting for, which will be useful if we ever get
>  	 * a proper probe-ordering dependency mechanism in future.
>  	 */
>  	if (!ops)
> -		return ERR_PTR(-EPROBE_DEFER);
> +		return -EPROBE_DEFER;
>  
> -	err = ops->of_xlate(dev, iommu_spec);
> -	if (err)
> -		return ERR_PTR(err);
> -
> -	return ops;
> +	return ops->of_xlate(dev, iommu_spec);
>  }
>  
>  struct of_pci_iommu_alias_info {
> @@ -148,7 +146,6 @@ struct of_pci_iommu_alias_info {
>  static int of_pci_iommu_init(struct pci_dev *pdev, u16 alias, void *data)
>  {
>  	struct of_pci_iommu_alias_info *info = data;
> -	const struct iommu_ops *ops;
>  	struct of_phandle_args iommu_spec = { .args_count = 1 };
>  	int err;
>  
> @@ -156,13 +153,12 @@ static int of_pci_iommu_init(struct pci_dev *pdev, u16 alias, void *data)
>  			     "iommu-map-mask", &iommu_spec.np,
>  			     iommu_spec.args);
>  	if (err)
> -		return err == -ENODEV ? 1 : err;
> +		return err == -ENODEV ? NO_IOMMU : err;
>  
> -	ops = of_iommu_xlate(info->dev, &iommu_spec);
> +	err = of_iommu_xlate(info->dev, &iommu_spec);
>  	of_node_put(iommu_spec.np);
> -
> -	if (IS_ERR(ops))
> -		return PTR_ERR(ops);
> +	if (err)
> +		return err;
>  
>  	return info->np == pdev->bus->dev.of_node;
>  }
> @@ -170,9 +166,9 @@ static int of_pci_iommu_init(struct pci_dev *pdev, u16 alias, void *data)
>  const struct iommu_ops *of_iommu_configure(struct device *dev,
>  					   struct device_node *master_np)
>  {
> -	const struct iommu_ops *ops = NULL;
> +	const struct iommu_ops *ops;
>  	struct iommu_fwspec *fwspec = dev->iommu_fwspec;
> -	int err;
> +	int err = NO_IOMMU;
>  
>  	if (!master_np)
>  		return NULL;
> @@ -198,10 +194,6 @@ const struct iommu_ops *of_iommu_configure(struct device *dev,
>  
>  		err = pci_for_each_dma_alias(to_pci_dev(dev),
>  					     of_pci_iommu_init, &info);
> -		if (err) /* err > 0 means the walk stopped, but non-fatally */
> -			ops = ERR_PTR(min(err, 0));
> -		else /* success implies both fwspec and ops are now valid */
> -			ops = dev->iommu_fwspec->ops;
>  	} else {
>  		struct of_phandle_args iommu_spec;
>  		int idx = 0;
> @@ -209,28 +201,35 @@ const struct iommu_ops *of_iommu_configure(struct device *dev,
>  		while (!of_parse_phandle_with_args(master_np, "iommus",
>  						   "#iommu-cells",
>  						   idx, &iommu_spec)) {
> -			ops = of_iommu_xlate(dev, &iommu_spec);
> +			err = of_iommu_xlate(dev, &iommu_spec);
>  			of_node_put(iommu_spec.np);
>  			idx++;
> -			if (IS_ERR_OR_NULL(ops))
> +			if (err)
>  				break;
>  		}
>  	}
> +
> +	/*
> +	 * Two success conditions are represented by non-negative err here:
> +	 * >0 : there is no IOMMU, or one was unavailable for non-fatal reasons
> +	 *  0 : we found an IOMMU, and dev->fwspec is initialised appropriately
> +	 * <0 : any actual error
> +	 */
> +	if (err > 0)
> +		return NULL;
> +
>  	/*
>  	 * If we have reason to believe the IOMMU driver missed the initial
>  	 * add_device callback for dev, replay it to get things in order.
>  	 */
> -	if (!IS_ERR_OR_NULL(ops) && ops->add_device &&
> -	    dev->bus && !dev->iommu_group) {
> +	ops = dev->iommu_fwspec->ops;
> +	if (!err && ops->add_device && dev->bus && !dev->iommu_group)
>  		err = ops->add_device(dev);
> -		if (err)
> -			ops = ERR_PTR(err);
> -	}
>  
>  	/* Ignore all other errors apart from EPROBE_DEFER */
> -	if (IS_ERR(ops) && (PTR_ERR(ops) != -EPROBE_DEFER)) {
> -		dev_dbg(dev, "Adding to IOMMU failed: %ld\n", PTR_ERR(ops));
> -		ops = NULL;
> +	if (err && err != -EPROBE_DEFER) {
> +		dev_dbg(dev, "Adding to IOMMU failed: %d\n", err);
> +		return NULL;
>  	}

Ugh, no, now we fail to pass -EPROBE_DEFER properly (turns out my test
device was also deferring for other reasons and getting lucky). I really
am starting to hate ERR_PTRs...

v2 coming shortly, sorry.

Robin.

>  
>  	return ops;
>