Re: [PATCH net-next 05/10] sysfs: add sysfs_change_owner()

[Greg Kroah-Hartman]

On Wed, Feb 12, 2020 at 11:43:16AM +0100, Christian Brauner wrote:
> Add a helper to change the owner of sysfs objects.

Seems sane, but:

> The ownership of a sysfs object is determined based on the ownership of
> the corresponding kobject, i.e. only if the ownership of a kobject is
> changed will this function change the ownership of the corresponding
> sysfs entry.

A "sysfs object" is a kobject.  So I don't understand this sentance,
sorry.

> 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>
> ---
>  fs/sysfs/file.c       | 35 +++++++++++++++++++++++++++++++++++
>  include/linux/sysfs.h |  6 ++++++
>  2 files changed, 41 insertions(+)
> 
> diff --git a/fs/sysfs/file.c b/fs/sysfs/file.c
> index 6239d9584f0b..6a0fe88061fd 100644
> --- a/fs/sysfs/file.c
> +++ b/fs/sysfs/file.c
> @@ -642,3 +642,38 @@ int sysfs_file_change_owner(struct kobject *kobj, const char *name)
>  	return error;
>  }
>  EXPORT_SYMBOL_GPL(sysfs_file_change_owner);
> +
> +/**
> + *	sysfs_change_owner - change owner of the given object.
> + *	@kobj:	object.
> + */
> +int sysfs_change_owner(struct kobject *kobj)

What does this change the owner of the given object _to_?

> +{
> +	int error;
> +	const struct kobj_type *ktype;
> +
> +	if (!kobj->state_in_sysfs)
> +		return -EINVAL;
> +
> +	error = sysfs_file_change_owner(kobj, NULL);

It passes NULL?


> +	if (error)
> +		return error;
> +
> +	ktype = get_ktype(kobj);
> +	if (ktype) {
> +		struct attribute **kattr;
> +
> +		for (kattr = ktype->default_attrs; kattr && *kattr; kattr++) {
> +			error = sysfs_file_change_owner(kobj, (*kattr)->name);
> +			if (error)
> +				return error;
> +		}
> +
> +		error = sysfs_groups_change_owner(kobj, ktype->default_groups);
> +		if (error)
> +			return error;
> +	}
> +
> +	return 0;
> +}
> +EXPORT_SYMBOL_GPL(sysfs_change_owner);

I can understand wanting to change owners/groups/whatever of existing
sysfs objects and their files, but I can't figure out how to call this
function to set the attribute I want to change.

With only one parameter, how does this work?  It guesses?  :)

thanks,

greg k-h