Re: [PATCH net-next v3 2/9] sysfs: add sysfs_link_change_owner()

[Christian Brauner]

On Thu, Feb 20, 2020 at 12:14:43PM +0100, Greg Kroah-Hartman wrote:
> On Tue, Feb 18, 2020 at 05:29:36PM +0100, Christian Brauner wrote:
> > Add a helper to change the owner of a sysfs link.
> > This function will be used to correctly account for kobject ownership
> > changes, e.g. when moving network devices between network namespaces.
> > 
> > Signed-off-by: Christian Brauner <christian.brauner@xxxxxxxxxx>
> > ---
> > /* v2 */
> > -  Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>:
> >    - Add comment how ownership of sysfs object is changed.
> > 
> > /* v3 */
> > -  Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>:
> >    - Add explicit uid/gid parameters.
> > ---
> >  fs/sysfs/file.c       | 40 ++++++++++++++++++++++++++++++++++++++++
> >  include/linux/sysfs.h | 10 ++++++++++
> >  2 files changed, 50 insertions(+)
> > 
> > diff --git a/fs/sysfs/file.c b/fs/sysfs/file.c
> > index 32bb04b4d9d9..df5107d7b3fd 100644
> > --- a/fs/sysfs/file.c
> > +++ b/fs/sysfs/file.c
> > @@ -570,6 +570,46 @@ static int internal_change_owner(struct kernfs_node *kn, struct kobject *kobj,
> >  	return kernfs_setattr(kn, &newattrs);
> >  }
> >  
> > +/**
> > + *	sysfs_link_change_owner - change owner of a link.
> > + *	@kobj:	object of the kernfs_node the symlink is located in.
> > + *	@targ:	object of the kernfs_node the symlink points to.
> > + *	@name:	name of the link.
> > + *	@kuid:	new owner's kuid
> > + *	@kgid:	new owner's kgid
> > + */
> > +int sysfs_link_change_owner(struct kobject *kobj, struct kobject *targ,
> > +			    const char *name, kuid_t kuid, kgid_t kgid)
> > +{
> > +	struct kernfs_node *parent, *kn = NULL;
> > +	int error;
> > +
> > +	if (!kobj)
> > +		parent = sysfs_root_kn;
> > +	else
> > +		parent = kobj->sd;
> 
> I don't understand this, why would (!kobj) ever be a valid situation?

Yeah, a caller could just pass in "sysfs_root_kn" itself if they for
some reason needed to.

> 
> > +	if (!targ->state_in_sysfs)
> > +		return -EINVAL;
> 
> Should you also check kobj->state_in_sysfs as well?

Probably. I'll take a closer look now.

Thanks!
Christian