Re: [PATCH v10 4/8] PCI/sysfs: Allow userspace to query and set device reset mechanism

[Bjorn Helgaas <helgaas@kernel.org>]

On Wed, Jul 28, 2021 at 11:29:50PM +0530, Amey Narkhede wrote:
> On 21/07/27 06:28PM, Bjorn Helgaas wrote:
> > On Fri, Jul 09, 2021 at 06:08:09PM +0530, Amey Narkhede wrote:
> > > Add reset_method sysfs attribute to enable user to query and set user
> > > preferred device reset methods and their ordering.
> > >
> > > Co-developed-by: Alex Williamson <alex.williamson@redhat.com>
> > > Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
> > > Signed-off-by: Amey Narkhede <ameynarkhede03@gmail.com>
> > > ---
> > >  Documentation/ABI/testing/sysfs-bus-pci |  19 +++++
> > >  drivers/pci/pci-sysfs.c                 | 103 ++++++++++++++++++++++++
> > >  2 files changed, 122 insertions(+)
> > >
> > > diff --git a/Documentation/ABI/testing/sysfs-bus-pci b/Documentation/ABI/testing/sysfs-bus-pci
> > > index ef00fada2..43f4e33c7 100644
> > > --- a/Documentation/ABI/testing/sysfs-bus-pci
> > > +++ b/Documentation/ABI/testing/sysfs-bus-pci
> > > @@ -121,6 +121,25 @@ Description:
> > >  		child buses, and re-discover devices removed earlier
> > >  		from this part of the device tree.
> > >
> > > +What:		/sys/bus/pci/devices/.../reset_method
> > > +Date:		March 2021
> > > +Contact:	Amey Narkhede <ameynarkhede03@gmail.com>
> > > +Description:
> > > +		Some devices allow an individual function to be reset
> > > +		without affecting other functions in the same slot.
> > > +
> > > +		For devices that have this support, a file named
> > > +		reset_method will be present in sysfs. Initially reading
> > > +		this file will give names of the device supported reset
> > > +		methods and their ordering. After write, this file will
> > > +		give names and ordering of currently enabled reset methods.
> > > +		Writing the name or comma separated list of names of any of
> > > +		the device supported reset methods to this file will set
> > > +		the reset methods and their ordering to be used when
> > > +		resetting the device. Writing empty string to this file
> > > +		will disable ability to reset the device and writing
> > > +		"default" will return to the original value.
> > > +
> > >  What:		/sys/bus/pci/devices/.../reset
> > >  Date:		July 2009
> > >  Contact:	Michael S. Tsirkin <mst@redhat.com>
> >
> [...]
> 
> > > +		int i;
> > > +
> > > +		if (sysfs_streq(name, ""))
> > > +			continue;
> > > +
> > > +		name = strim(name);
> > > +
> > > +		for (i = 1; i < PCI_NUM_RESET_METHODS; i++) {
> > > +			if (sysfs_streq(name, pci_reset_fn_methods[i].name) &&
> > > +			    !pci_reset_fn_methods[i].reset_fn(pdev, 1)) {
> > > +				reset_methods[n++] = i;
> >
> > Can we build this directly in pdev->reset_methods[] so we don't need
> > the memcpy() below?
> >
> This is to avoid writing partial values directly to dev->reset_methods.
> So for example if user writes flr,unsupported_value then
> dev->reset_methods should not be modified even though flr is valid reset
> method in this example to avoid partial writes. Either operation(in this
> case writing supported reset methods to reset_method attr) succeeds
> completely or it fails othewise.

I guess the question is, if somebody writes

  flr junk bus

and we set the supported methods to "flr bus", is that OK?  It seems
OK to me; obviously we have to protect ourselves from attack, but
over-zealous checking can make things unnecessarily complicated.