Subject: Re: [PATCH v4 3/3] IB/ipoib: Log sysfs 'dev_id' accesses from userspace

From: Arseny Maslennikov <[email-masked]>
Date: Sun, 9 Sep 2018 21:11:46 +0300

On Fri, Sep 07, 2018 at 09:43:59AM -0600, Jason Gunthorpe wrote:
> On Thu, Sep 06, 2018 at 05:51:12PM +0300, Arseny Maslennikov wrote:
> > Some tools may currently be using only the deprecated attribute;
> > let's print an elaborate and clear deprecation notice to kmsg.
> > 
> > To do that, we have to replace the whole sysfs file, since we inherit
> > the original one from netdev.
> > 
> > Signed-off-by: Arseny Maslennikov <[email-masked]>
> >  drivers/infiniband/ulp/ipoib/ipoib_main.c | 31 +++++++++++++++++++++++
> >  1 file changed, 31 insertions(+)
> > 
> > diff --git a/drivers/infiniband/ulp/ipoib/ipoib_main.c b/drivers/infiniband/ulp/ipoib/ipoib_main.c
> > index 30f840f874b3..74732726ec6f 100644
> > +++ b/drivers/infiniband/ulp/ipoib/ipoib_main.c
> > @@ -2386,6 +2386,35 @@ int ipoib_add_pkey_attr(struct net_device *dev)
> >  	return device_create_file(&dev->dev, &dev_attr_pkey);
> >  }
> >  
> > +/*
> > + * We erroneously exposed the iface's port number in the dev_id
> > + * sysfs field long after dev_port was introduced for that purpose[1],
> > + * and we need to stop everyone from relying on that.
> > + * Let's overload the shower routine for the dev_id file here
> > + * to gently bring the issue up.
> > + *
> > + * [1] https://www.spinics.net/lists/netdev/msg272123.html
> > + */
> > +static ssize_t dev_id_show(struct device *dev,
> > +			   struct device_attribute *attr, char *buf)
> > +{
> > +	struct net_device *ndev = to_net_dev(dev);
> > +
> > +	if (ndev->dev_id == ndev->dev_port)
> > +		netdev_info_once(ndev,
> > +			"\"%s\" wants to know my dev_id. Should it look at dev_port instead? See Documentation/ABI/testing/sysfs-class-net for more info.\n",
> > +			current->comm);
> > +
> > +	return sprintf(buf, "%#x\n", ndev->dev_id);
> > +}
> > +static DEVICE_ATTR_RO(dev_id);
> > +
> > +int ipoib_intercept_dev_id_attr(struct net_device *dev)
> > +{
> > +	device_remove_file(&dev->dev, &dev_attr_dev_id);
> > +	return device_create_file(&dev->dev, &dev_attr_dev_id);
> > +}
> 
> Isn't this racey with userspace? Ie what happens if udev is querying
> the dev_id right here?

udev in particular does not use dev_id at all since 2014, because "why
would we keep using dev_id if it is not the right thing to use?".

> 
> Do we know there is no userspace doing this?
> 

Not for sure.

If we move all the sysfs handling stuff we introduce in _add_port():
 - pkey
 - umcast
 - {create,delete}_child
 - connected/datagram mode
to _ndo_init(), which is called by register_netdev before it sends
the netlink message, would that suffice to eliminate the race?
(Sysfs files for {create,delete}_child go to _parent_init() then).

> >  static struct net_device *ipoib_add_port(const char *format,
> >  					 struct ib_device *hca, u8 port)
> >  {
> > @@ -2427,6 +2456,8 @@ static struct net_device *ipoib_add_port(const char *format,
> >  	 */
> >  	ndev->priv_destructor = ipoib_intf_free;
> >  
> > +	if (ipoib_intercept_dev_id_attr(ndev))
> > +		goto sysfs_failed;
> 
> No device_remove_file needed?
> 

As far as I understand, unregister_netdevice takes care of it
while destroying the related kobject.

> Jason