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 23:55:12 +0300

On Sun, Sep 09, 2018 at 09:11:46PM +0300, Arseny Maslennikov wrote:
> 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).
> 

No, we can't, sorry for the noise. ndo_init() runs before the kobject
becomes available.

Anyway, our sysfs attributes being racy is unrelated to the patch series
subject, and I can't come up with any other ideas what to do with them
that do not involve adjustments to register_netdev.

> > >  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