linux-kernel - Re: [PATCH] udevadm-info: Don't access sysfs 'resource<N>' files

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Date:	Mon, 18 Mar 2013 18:20:46 +0100
From:	Bjørn Mork <bjorn@...k.no>
To:	Alex Williamson <alex.williamson@...hat.com>
Cc:	Greg KH <gregkh@...uxfoundation.org>, Kay Sievers <kay@...y.org>,
	Myron Stowe <mstowe@...hat.com>,
	Myron Stowe <myron.stowe@...hat.com>,
	linux-hotplug@...r.kernel.org, linux-pci@...r.kernel.org,
	yuxiangl@...vell.com, yxlraid@...il.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] udevadm-info: Don't access sysfs 'resource<N>' files

Alex Williamson <alex.williamson@...hat.com> writes:

> At least for KVM the kernel fix is the addition of the vfio driver which
> gives us a non-sysfs way to do this.  If this problem was found a few
> years later and we were ready to make the switch I'd support just
> removing these resource files.  In the meantime we have userspace that
> depends on this interface, so I'm open to suggestions how to fix it.

I am puzzled by a couple of things in this discussion:

1) do you seriously mean that a userspace application (any, not just
   udevadm or qemu or whatever) should be able to read and write these
   registers while the device is owned by a driver?  How is that ever
   going to work?

2) is it really so that a device can be so fundamentally screwed up by
   reading some registers, that a later driver probe cannot properly
   reinitialize it?

I would have thought that the solution to all this was to return -EINVAL
on any attemt to read or write these files while a driver is bound to
the device.  If userspace is going to use the API, then the application
better unbind any driver first.

Or? Am I missing something here?

> If we want to blacklist this specific device, that's fine, but as others
> have pointed out it's really a class problem.  Perhaps we report 1 byte
> extra for the file length where EOF-1 is an enable byte?  Is there
> anything else in file ops that we could use to make it slightly more
> complicated than open(), read() to access the device?  Thanks,

If there really are devices which cannot handle reading at all, and
cannot be reset to a sane state by later driver initialization, then a
blacklist could be added for those devices.  This should not be a common
problem.

Bjørn
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