linux-kernel - Re: [PATCH v2 4/6] iommufd: Deliver fault messages to user space

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Date:   Fri, 1 Dec 2023 11:24:59 -0400
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Lu Baolu <baolu.lu@...ux.intel.com>
Cc:     Kevin Tian <kevin.tian@...el.com>, Joerg Roedel <joro@...tes.org>,
        Will Deacon <will@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>,
        Nicolin Chen <nicolinc@...dia.com>,
        Yi Liu <yi.l.liu@...el.com>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        iommu@...ts.linux.dev, linux-kselftest@...r.kernel.org,
        virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 4/6] iommufd: Deliver fault messages to user space

On Thu, Oct 26, 2023 at 10:49:28AM +0800, Lu Baolu wrote:

> +static ssize_t hwpt_fault_fops_write(struct file *filep,
> +				     const char __user *buf,
> +				     size_t count, loff_t *ppos)
> +{
> +	size_t response_size = sizeof(struct iommu_hwpt_page_response);
> +	struct hw_pgtable_fault *fault = filep->private_data;
> +	struct iommu_hwpt_page_response response;
> +	struct iommufd_hw_pagetable *hwpt;
> +	struct iopf_group *iter, *group;
> +	struct iommufd_device *idev;
> +	size_t done = 0;
> +	int rc = 0;
> +
> +	if (*ppos || count % response_size)
> +		return -ESPIPE;
> +
> +	mutex_lock(&fault->mutex);
> +	while (!list_empty(&fault->response) && count > done) {
> +		rc = copy_from_user(&response, buf + done, response_size);
> +		if (rc)
> +			break;
> +
> +		/* Get the device that this response targets at. */
> +		idev = container_of(iommufd_get_object(fault->ictx,
> +						       response.dev_id,
> +						       IOMMUFD_OBJ_DEVICE),
> +				    struct iommufd_device, obj);
> +		if (IS_ERR(idev)) {
> +			rc = PTR_ERR(idev);
> +			break;
> +		}

See here it might be better to have a per-fd list of outstanding
faults per-fd and then the cookie would just index that list, then you
get everything in one shot instead of having to do a xarray looking
and then a linear list search

> +static const struct file_operations hwpt_fault_fops = {
> +	.owner		= THIS_MODULE,
> +	.read		= hwpt_fault_fops_read,
> +	.write		= hwpt_fault_fops_write,
> +};

nonseekable_open() behavior should be integrated into this

> +static int hw_pagetable_get_fault_fd(struct hw_pgtable_fault *fault)
> +{
> +	struct file *filep;
> +	int fdno;
> +
> +	fdno = get_unused_fd_flags(O_CLOEXEC);
> +	if (fdno < 0)
> +		return fdno;
> +
> +	filep = anon_inode_getfile("[iommufd-pgfault]", &hwpt_fault_fops,
> +				   fault, O_RDWR);
> +	if (IS_ERR(filep)) {
> +		put_unused_fd(fdno);
> +		return PTR_ERR(filep);
> +	}
> +
> +	fd_install(fdno, filep);
> +	fault->fault_file = filep;
> +	fault->fault_fd = fdno;

fd_install must be the very last thing before returning success from a
system call because we cannot undo it.

There are other failure paths before here and the final return

Jason