linux-kernel - Re: [PATCH v7 3/4] iommufd: Add IOMMU_GET_HW

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Date:   Tue, 15 Aug 2023 15:29:21 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Nicolin Chen <nicolinc@...dia.com>
Cc:     Yi Liu <yi.l.liu@...el.com>, joro@...tes.org,
        alex.williamson@...hat.com, kevin.tian@...el.com,
        robin.murphy@....com, baolu.lu@...ux.intel.com, cohuck@...hat.com,
        eric.auger@...hat.com, kvm@...r.kernel.org, mjrosato@...ux.ibm.com,
        chao.p.peng@...ux.intel.com, yi.y.sun@...ux.intel.com,
        peterx@...hat.com, jasowang@...hat.com,
        shameerali.kolothum.thodi@...wei.com, lulu@...hat.com,
        suravee.suthikulpanit@....com, iommu@...ts.linux.dev,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
        zhenzhong.duan@...el.com
Subject: Re: [PATCH v7 3/4] iommufd: Add IOMMU_GET_HW_INFO

On Tue, Aug 15, 2023 at 10:31:09AM -0700, Nicolin Chen wrote:
> On Tue, Aug 15, 2023 at 01:32:01PM -0300, Jason Gunthorpe wrote:
> > On Fri, Aug 11, 2023 at 12:15:00AM -0700, Yi Liu wrote:
> > 
> > > +static int iommufd_get_hw_info(struct iommufd_ucmd *ucmd)
> > > +{
> > > +	struct iommu_hw_info *cmd = ucmd->cmd;
> > > +	unsigned int length = cmd->data_len;
> > > +	struct iommufd_device *idev;
> > > +	void __user *user_ptr;
> > > +	u32 hw_info_type;
> > > +	int rc = 0;
> > > +
> > > +	if (cmd->flags || cmd->__reserved || !cmd->data_len)
> > > +		return -EOPNOTSUPP;
> > 
> > Is there a reason to block 0 data_len? I think this should work. The
> > code looks OK?
> 
> I did a quick test passing !data_len and !data_ptr. And it works
> by returning the type only.
> 
> Yet, in that case, should we mention this in the uAPI kdoc? It
> feels to me that the uAPI always expects user space to read out
> a length of data.

Well the way it ought to work is that userspace can pass in 0 length
and the kernel will return the correct length

So maybe this does need resending with this removed:

	*length = min(*length, data_len);

Also I see clear_user is called wrong, it doesn't return errno.

Please check and repost it ASAP I will update the branch. Probably
needs some doc adjusting too.

I came up with this:

int iommufd_get_hw_info(struct iommufd_ucmd *ucmd)
{
	struct iommu_hw_info *cmd = ucmd->cmd;
	void __user *user_ptr = u64_to_user_ptr(cmd->data_ptr);
	const struct iommu_ops *ops;
	struct iommufd_device *idev;
	unsigned int data_len;
	unsigned int copy_len;
	void *data = NULL;
	int rc;

	if (cmd->flags || cmd->__reserved)
		return -EOPNOTSUPP;

	idev = iommufd_get_device(ucmd, cmd->dev_id);
	if (IS_ERR(idev))
		return PTR_ERR(idev);

	ops = dev_iommu_ops(idev->dev);
	if (!ops->hw_info) {
		data = ops->hw_info(idev->dev, &data_len, &cmd->out_data_type);
		if (IS_ERR(data)) {
			rc = PTR_ERR(data);
			goto err_put;
		}

		/*
		 * drivers that have hw_info callback should have a unique
		 * iommu_hw_info_type.
		 */
		if (WARN_ON_ONCE(cmd->out_data_type ==
				 IOMMU_HW_INFO_TYPE_NONE)) {
			rc = -ENODEV;
			goto out;
		}
	} else {
		cmd->out_data_type = IOMMU_HW_INFO_TYPE_NONE;
		data_len = 0;
		data = NULL;
	}

	copy_len = min(cmd->data_len, data_len);
	if (copy_to_user(user_ptr, data, copy_len)) {
		rc = -EFAULT;
		goto out;
	}

	/*
	 * Zero the trailing bytes if the user buffer is bigger than the
	 * data size kernel actually has.
	 */
	if (copy_len < cmd->data_len) {
		if (clear_user(user_ptr + copy_len, cmd->data_len - copy_len)) {
			rc = -EFAULT;
			goto out;
		}
	}

	/*
	 * We return the length the kernel supports so userspace may know what
	 * the kernel capability is. It could be larger than the input buffer.
	 */
	cmd->data_len = data_len;

	rc = iommufd_ucmd_respond(ucmd, sizeof(*cmd));
out:
	kfree(data);
err_put:
	iommufd_put_object(&idev->obj);
	return rc;
}