linux-kernel - Re: [PATCH v6 05/14] iommufd: Add IOMMUFD_OBJ_VEVENTQ and IOMMUFD_CMD_VEVENTQ

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Message-ID: <20250218152959.GB4099685@nvidia.com>
Date: Tue, 18 Feb 2025 11:29:59 -0400
From: Jason Gunthorpe <jgg@...dia.com>
To: Nicolin Chen <nicolinc@...dia.com>
Cc: kevin.tian@...el.com, corbet@....net, will@...nel.org, joro@...tes.org,
	suravee.suthikulpanit@....com, robin.murphy@....com,
	dwmw2@...radead.org, baolu.lu@...ux.intel.com, shuah@...nel.org,
	linux-kernel@...r.kernel.org, iommu@...ts.linux.dev,
	linux-arm-kernel@...ts.infradead.org,
	linux-kselftest@...r.kernel.org, linux-doc@...r.kernel.org,
	eric.auger@...hat.com, jean-philippe@...aro.org, mdf@...nel.org,
	mshavit@...gle.com, shameerali.kolothum.thodi@...wei.com,
	smostafa@...gle.com, ddutile@...hat.com, yi.l.liu@...el.com,
	patches@...ts.linux.dev
Subject: Re: [PATCH v6 05/14] iommufd: Add IOMMUFD_OBJ_VEVENTQ and
 IOMMUFD_CMD_VEVENTQ_ALLOC

On Fri, Jan 24, 2025 at 04:30:34PM -0800, Nicolin Chen wrote:
> +	list_add_tail(&vevent->node, &eventq->deliver);
> +	vevent->on_list = true;
> +	vevent->header.sequence = atomic_read(&veventq->sequence);
> +	if (atomic_read(&veventq->sequence) == INT_MAX)
> +		atomic_set(&veventq->sequence, 0);
> +	else
> +		atomic_inc(&veventq->sequence);
> +	spin_unlock(&eventq->lock);

This is all locked, we don't need veventq->sequence to be an atomic?

The bounding can be done with some simple math:

  veventq->sequence = (veventq->sequence + 1) & INT_MAX;

> +static struct iommufd_vevent *
> +iommufd_veventq_deliver_fetch(struct iommufd_veventq *veventq)
> +{
> +	struct iommufd_eventq *eventq = &veventq->common;
> +	struct list_head *list = &eventq->deliver;
> +	struct iommufd_vevent *vevent = NULL;
> +
> +	spin_lock(&eventq->lock);
> +	if (!list_empty(list)) {
> +		vevent = list_first_entry(list, struct iommufd_vevent, node);
> +		list_del(&vevent->node);
> +		vevent->on_list = false;
> +	}
> +	/* Make a copy of the overflow node for copy_to_user */
> +	if (vevent == &veventq->overflow) {
> +		vevent = kzalloc(sizeof(*vevent), GFP_ATOMIC);
> +		if (vevent)
> +			memcpy(vevent, &veventq->overflow, sizeof(*vevent));
> +	}

This error handling is wonky, if we can't allocate then we shouldn't
have done the list_del. Just return NULL which will cause
iommufd_veventq_fops_read() to exist and userspace will try again.

> @@ -403,6 +531,10 @@ static int iommufd_eventq_fops_release(struct inode *inode, struct file *filep)
>  {
>  	struct iommufd_eventq *eventq = filep->private_data;
>  
> +	if (eventq->obj.type == IOMMUFD_OBJ_VEVENTQ) {
> +		atomic_set(&eventq_to_veventq(eventq)->sequence, 0);
> +		atomic_set(&eventq_to_veventq(eventq)->num_events, 0);
> +	}

Why? We are about to free the memory?

> +int iommufd_veventq_alloc(struct iommufd_ucmd *ucmd)
> +{
> +	struct iommu_veventq_alloc *cmd = ucmd->cmd;
> +	struct iommufd_veventq *veventq;
> +	struct iommufd_viommu *viommu;
> +	int fdno;
> +	int rc;
> +
> +	if (cmd->flags || cmd->type == IOMMU_VEVENTQ_TYPE_DEFAULT)
> +		return -EOPNOTSUPP;
> +	if (!cmd->veventq_depth)
> +		return -EINVAL;

Check __reserved for 0 too

Jason