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

[Greg Kroah-Hartman]

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?

> +	if (!targ->state_in_sysfs)
> +		return -EINVAL;

Should you also check kobj->state_in_sysfs as well?

thanks,

greg k-h