linux-kernel - Re: [PATCH] pci: iov: use device lock to protect IOV sysfs accesses

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Message-ID: <20170530230718.GG14896@bhelgaas-glaptop.roam.corp.google.com>
Date:   Tue, 30 May 2017 18:07:18 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Jakub Kicinski <jakub.kicinski@...ronome.com>
Cc:     Bjorn Helgaas <bhelgaas@...gle.com>, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Emil Tantilov <emil.s.tantilov@...el.com>,
        Alexander Duyck <alexander.h.duyck@...el.com>,
        oss-drivers@...ronome.com
Subject: Re: [PATCH] pci: iov: use device lock to protect IOV sysfs accesses

On Fri, May 26, 2017 at 04:58:20PM -0700, Jakub Kicinski wrote:
> On Fri, 26 May 2017 18:47:26 -0500, Bjorn Helgaas wrote:
> > On Mon, May 22, 2017 at 03:50:23PM -0700, Jakub Kicinski wrote:
> > > PCI core sets the driver pointer before calling ->probe() and only
> > > clears it after ->remove().  This means driver's ->sriov_configure()
> > > callback will happily race with probe() and remove(), most likely
> > > leading to BUGs, since drivers don't expect this.  
> > 
> > I guess you're referring to the pci_dev->driver pointer set by
> > local_pci_probe(), and this is important because sriov_numvfs_store()
> > checks that pointer, right?
> 
> Yes, exactly.  I initially thought this is how the safety of sriov
> callback may have been ensured, but since the order of
> local_pci_probe() and the assignment is what it is, it can't.

Right.  I was hoping other subsystems would establish a convention
about whether we set the ->driver pointer before or after calling the
driver probe() method, but if there is one, I don't see it.
local_pci_probe() and really_probe() set ->driver first, but
pnp_device_probe() calls the probe() method first.

Can you expand on how you reproduce this problem?  The only real way I
see to call ->sriov_configure() is via the sysfs entry point, and I
would think user-space code would typically not touch that until after
it knows the driver has claimed a device.  But I can certainly imagine
targeted test code that could hit this problem.

Bjorn