linux-kernel - Re: mmap/munmap in sysfs

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Date:   Wed, 26 Jun 2019 13:06:52 +0200
From:   "Rafael J. Wysocki" <rjw@...ysocki.net>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Bjorn Helgaas <helgaas@...nel.org>,
        Myron Stowe <myron.stowe@...hat.com>,
        linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
        Mika Westerberg <mika.westerberg@...ux.intel.com>
Subject: Re: mmap/munmap in sysfs

On Wednesday, June 26, 2019 3:07:46 AM CEST Greg Kroah-Hartman wrote:
> On Tue, Jun 25, 2019 at 05:36:08PM -0500, Bjorn Helgaas wrote:
> > Hi Greg, et al,
> > 
> > Userspace can mmap PCI device memory via the resourceN files in sysfs,
> > which use pci_mmap_resource().  I think this path is unaware of power
> > management, so the device may be runtime-suspended, e.g., it may be in
> > D1, D2, or D3, where it will not respond to memory accesses.
> > 
> > Userspace accesses while the device is suspended will cause PCI
> > errors, so I think we need something like the patch below.  But this
> > isn't sufficient by itself because we would need a corresponding
> > pm_runtime_put() when the mapping goes away.  Where should that go?
> > Or is there a better way to do this?
> > 
> > 
> > diff --git a/drivers/pci/pci-sysfs.c b/drivers/pci/pci-sysfs.c
> > index 6d27475e39b2..aab7a47679a7 100644
> > --- a/drivers/pci/pci-sysfs.c
> > +++ b/drivers/pci/pci-sysfs.c
> > @@ -1173,6 +1173,7 @@ static int pci_mmap_resource(struct kobject *kobj, struct bin_attribute *attr,
> >  
> >  	mmap_type = res->flags & IORESOURCE_MEM ? pci_mmap_mem : pci_mmap_io;
> >  
> > +	pm_runtime_get_sync(pdev);
> >  	return pci_mmap_resource_range(pdev, bar, vma, mmap_type, write_combine);
> >  }
> >  
> 
> Ugh, we never thought about this when adding the mmap sysfs interface
> all those years ago :(
> 
> I think you are right, this will not properly solve the issue, but I
> don't know off the top of my head where to solve this.  Maybe Rafael has
> a better idea as he knows the pm paths much better than I do?

Well, let me think about this a bit.