Re: [PATCH v6 06/10] iommufd: Add iommufd fault object

[Baolu Lu <baolu.lu@linux.intel.com>]

On 6/7/24 5:17 PM, Tian, Kevin wrote:
>> From: Lu Baolu <baolu.lu@linux.intel.com>
>> Sent: Monday, May 27, 2024 12:05 PM
>>
>> +static ssize_t iommufd_fault_fops_read(struct file *filep, char __user *buf,
>> +				       size_t count, loff_t *ppos)
>> +{
>> +	size_t fault_size = sizeof(struct iommu_hwpt_pgfault);
>> +	struct iommufd_fault *fault = filep->private_data;
>> +	struct iommu_hwpt_pgfault data;
>> +	struct iommufd_device *idev;
>> +	struct iopf_group *group;
>> +	struct iopf_fault *iopf;
>> +	size_t done = 0;
>> +	int rc = 0;
>> +
>> +	if (*ppos || count % fault_size)
>> +		return -ESPIPE;
> 
> the man page says:
> 
> "If count is zero, read() returns zero and has no  other  results."

My understanding is that reading zero bytes is likely to check if a file
descriptor is valid and ready for reading without actually taking any
data from it.

In this code, it just returns 0 and it's compatible with the man page.
Or, I overlooked anything?

> 
>> +
>> +	mutex_lock(&fault->mutex);
>> +	while (!list_empty(&fault->deliver) && count > done) {
>> +		group = list_first_entry(&fault->deliver,
>> +					 struct iopf_group, node);
>> +
>> +		if (group->fault_count * fault_size > count - done)
>> +			break;
>> +
>> +		rc = xa_alloc(&fault->response, &group->cookie, group,
>> +			      xa_limit_32b, GFP_KERNEL);
>> +		if (rc)
>> +			break;
>> +
>> +		idev = to_iommufd_handle(group->attach_handle)->idev;
>> +		list_for_each_entry(iopf, &group->faults, list) {
>> +			iommufd_compose_fault_message(&iopf->fault,
>> +						      &data, idev,
>> +						      group->cookie);
>> +			rc = copy_to_user(buf + done, &data, fault_size);
>> +			if (rc) {
> 
> 'rc' should be converted to -EFAULT.

Yes. I will make it like this:

        if (copy_to_user(buf + done, &data, fault_size)) {
                xa_erase(&fault->response, group->cookie);
                rc = -EFAULT;
                break;
        }

> 
>> +				xa_erase(&fault->response, group->cookie);
>> +				break;
>> +			}
>> +			done += fault_size;
>> +		}
>> +
>> +		list_del(&group->node);
>> +	}
>> +	mutex_unlock(&fault->mutex);
>> +
>> +	return done == 0 ? rc : done;
> 
> again this doesn't match the manual:
> 
> "On error, -1 is returned, and errno is set appropriately."
> 
> it doesn't matter whether 'done' is 0.

I don't quite follow here. The code is doing the following:

- If done == 0, it means nothing has been copied to user space. This
   could be due to two reasons:

   1) the user read with a count set to 0, or
   2) a failure case.

   The code returns 0 for the first case and an error number for the
   second.

- If done != 0, some data has been copied to user space. In this case,
   the code returns the number of data copied regardless of the value of
   rc.

> 
>> +
>> +static int iommufd_fault_fops_release(struct inode *inode, struct file *filep)
>> +{
>> +	struct iommufd_fault *fault = filep->private_data;
>> +
>> +	iommufd_ctx_put(fault->ictx);
>> +	refcount_dec(&fault->obj.users);
>> +	return 0;
>> +}
> 
> hmm this doesn't sound correct. the context and refcount are
> acquired in iommufd_fault_alloc() but here they are reverted when
> the fd is closed...

These two refcounts were requested when the fault object was installed
to the fault FD.

        filep = anon_inode_getfile("[iommufd-pgfault]", &iommufd_fault_fops,
                                    fault, O_RDWR);
         if (IS_ERR(filep)) {
                 rc = PTR_ERR(filep);
                 goto out_abort;
         }

         refcount_inc(&fault->obj.users);
         iommufd_ctx_get(fault->ictx);
         fault->filep = filep;

These refcounts must then be released when the FD is released.

>> +
>> +	filep = anon_inode_getfile("[iommufd-pgfault]",
>> &iommufd_fault_fops,
>> +				   fault, O_RDWR);
>> +	if (IS_ERR(filep)) {
>> +		rc = PTR_ERR(filep);
>> +		goto out_abort;
>> +	}
>> +
>> +	refcount_inc(&fault->obj.users);
>> +	iommufd_ctx_get(fault->ictx);
>> +	fault->filep = filep;
> 
> those 3 lines can be moved after below fdno get. It's reads slightly
> clearer to put file related work together before getting to the last piece
> of intiailzation.

The filep is allocated and initialized together.

>> +
>> +	fdno = get_unused_fd_flags(O_CLOEXEC);
>> +	if (fdno < 0) {
>> +		rc = fdno;
>> +		goto out_fput;
>> +	}
>> +
>> @@ -332,6 +332,7 @@ union ucmd_buffer {
>>   	struct iommu_ioas_unmap unmap;
>>   	struct iommu_option option;
>>   	struct iommu_vfio_ioas vfio_ioas;
>> +	struct iommu_fault_alloc fault;
> 
> alphabetic
> 
>> @@ -381,6 +382,8 @@ static const struct iommufd_ioctl_op
>> iommufd_ioctl_ops[] = {
>>   		 val64),
>>   	IOCTL_OP(IOMMU_VFIO_IOAS, iommufd_vfio_ioas, struct
>> iommu_vfio_ioas,
>>   		 __reserved),
>> +	IOCTL_OP(IOMMU_FAULT_QUEUE_ALLOC, iommufd_fault_alloc,
>> struct iommu_fault_alloc,
>> +		 out_fault_fd),
> 
> ditto

Yes, sure. I wasn't aware of the order.

Best regards,
baolu