linux-kernel - Re: [PATCH] vfio: Reuse file f

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Date: Fri, 28 Jun 2024 13:21:26 +0800
From: Yan Zhao <yan.y.zhao@...el.com>
To: Jason Gunthorpe <jgg@...dia.com>
CC: "Tian, Kevin" <kevin.tian@...el.com>, "kvm@...r.kernel.org"
	<kvm@...r.kernel.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "alex.williamson@...hat.com"
	<alex.williamson@...hat.com>, "peterx@...hat.com" <peterx@...hat.com>,
	"ajones@...tanamicro.com" <ajones@...tanamicro.com>
Subject: Re: [PATCH] vfio: Reuse file f_inode as vfio device inode

On Thu, Jun 27, 2024 at 09:42:09AM -0300, Jason Gunthorpe wrote:
> On Thu, Jun 27, 2024 at 05:51:01PM +0800, Yan Zhao wrote:
> 
> > > > > This doesn't seem right.. There is only one device but multiple file
> > > > > can be opened on that device.
> > Maybe we can put this assignment to vfio_df_ioctl_bind_iommufd() after
> > vfio_df_open() makes sure device->open_count is 1.
> 
> Yeah, that seems better.
> 
> Logically it would be best if all places set the inode once the
> inode/FD has been made to be the one and only way to access it.
For group path, I'm afraid there's no such a place ensuring only one active fd
in kernel.
I tried modifying QEMU to allow two openings and two assignments of the same
device. It works and appears to guest that there were 2 devices, though this
ultimately leads to device malfunctions in guest.

> > BTW, in group path, what's the benefit of allowing multiple open of device?
> 
> I don't know, the thing that opened the first FD can just dup it, no
> idea why two different FDs would be useful. It is something we removed
> in the cdev flow
>
Thanks. However, from the code, it reads like a drawback of the cdev flow :)
I don't understand why the group path is secure though.

        /*
         * Only the group path allows the device to be opened multiple
         * times.  The device cdev path doesn't have a secure way for it.
         */
        if (device->open_count != 0 && !df->group)
                return -EINVAL;