Re: [PATCH net v2 2/3] net: core: add netdev_class_has_file_ns() helper function

[Taehee Yoo]

On Sun, 5 Apr 2020 at 16:32, Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
>

Hi Greg,
Thank you for the review!

> On Sun, Apr 05, 2020 at 02:18:22AM +0900, Taehee Yoo wrote:
> > On Sun, 5 Apr 2020 at 00:51, Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
> > >
> >
> > Hi Greg,
> > Thank you for your review!
> >
> > > On Sat, Apr 04, 2020 at 02:19:09PM +0000, Taehee Yoo wrote:
> > > > This helper function is to check whether the class file "/sys/class/net/*"
> > > > is existing or not.
> > > > In the next patch, this helper function will be used.
> > > >
> > > > Reported-by: syzbot+830c6dbfc71edc4f0b8f@xxxxxxxxxxxxxxxxxxxxxxxxx
> > > > Fixes: b76cdba9cdb2 ("[PATCH] bonding: add sysfs functionality to bonding (large)")
> > > > Signed-off-by: Taehee Yoo <ap420073@xxxxxxxxx>
> > > > ---
> > > >
> > > > v1 -> v2:
> > > >  - use class_has_file_ns(), which is introduced by the first patch.
> > > >
> > > >  include/linux/netdevice.h | 2 +-
> > > >  net/core/net-sysfs.c      | 6 ++++++
> > > >  2 files changed, 7 insertions(+), 1 deletion(-)
> > > >
> > > > diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
> > > > index 130a668049ab..a04c487c0975 100644
> > > > --- a/include/linux/netdevice.h
> > > > +++ b/include/linux/netdevice.h
> > > > @@ -4555,7 +4555,7 @@ int netdev_class_create_file_ns(const struct class_attribute *class_attr,
> > > >                               const void *ns);
> > > >  void netdev_class_remove_file_ns(const struct class_attribute *class_attr,
> > > >                                const void *ns);
> > > > -
> > > > +bool netdev_class_has_file_ns(const char *name, const void *ns);
> > > >  static inline int netdev_class_create_file(const struct class_attribute *class_attr)
> > > >  {
> > > >       return netdev_class_create_file_ns(class_attr, NULL);
> > > > diff --git a/net/core/net-sysfs.c b/net/core/net-sysfs.c
> > > > index cf0215734ceb..8a20d658eff0 100644
> > > > --- a/net/core/net-sysfs.c
> > > > +++ b/net/core/net-sysfs.c
> > > > @@ -1914,6 +1914,12 @@ void netdev_class_remove_file_ns(const struct class_attribute *class_attr,
> > > >  }
> > > >  EXPORT_SYMBOL(netdev_class_remove_file_ns);
> > > >
> > > > +bool netdev_class_has_file_ns(const char *name, const void *ns)
> > > > +{
> > > > +     return class_has_file_ns(&net_class, name, ns);
> > > > +}
> > > > +EXPORT_SYMBOL(netdev_class_has_file_ns);
> > >
> > > Again, this feels broken, it can not solve a race condition.
> > >
> >
> > This function is considered to be used under rtnl mutex and
> > I assume that no one could use "/sys/class/net/*" outside of rtnl mutex.
> > So, I think it returns the correct information under rtnl mutex.
>
> But you are creating a globally exported function that can be called
> from anywhere, and as such, is not useful because it has no locking or
> hints of how to use it correctly at all.
>

Yes, I agree with that.

> Again, don't push this "solution" down to sysfs to solve, you know if
> you have a device that is not cleaned up yet, so don't try to
> rename/create a device of the same name before that is finished.
>

Okay, Thank you for that.
I will find another way to fix it.

Thanks a lot!
Taehee Yoo