Re: [PATCH net-next v2 01/10] sysfs: add sysfs_file_change_owner{_by_name}()

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Mon, Feb 17, 2020 at 05:14:27PM +0100, Christian Brauner wrote:
> Add helpers to change owner of a sysfs files.
> 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.
> 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@ubuntu.com>
> ---
> /* v2 */
> -  Greg Kroah-Hartman <gregkh@linuxfoundation.org>:
>    - Better naming for sysfs_file_change_owner() to reflect the fact that it
>      can be used to change the owner of the kobject itself by passing NULL as
>      argument.
> - Christian Brauner <christian.brauner@ubuntu.com>:
>   - Split sysfs_file_change_owner() into two helpers sysfs_change_owner() and
>     sysfs_change_owner_by_name(). The former changes the owner of the kobject
>     itself, the latter the owner of the kobject looked up via the name
>     argument.
> ---
>  fs/sysfs/file.c       | 82 +++++++++++++++++++++++++++++++++++++++++++
>  include/linux/sysfs.h | 14 ++++++++
>  2 files changed, 96 insertions(+)
> 
> diff --git a/fs/sysfs/file.c b/fs/sysfs/file.c
> index 130fc6fbcc03..8f2607de2456 100644
> --- a/fs/sysfs/file.c
> +++ b/fs/sysfs/file.c
> @@ -558,3 +558,85 @@ void sysfs_remove_bin_file(struct kobject *kobj,
>  	kernfs_remove_by_name(kobj->sd, attr->attr.name);
>  }
>  EXPORT_SYMBOL_GPL(sysfs_remove_bin_file);
> +
> +static int internal_change_owner(struct kernfs_node *kn, struct kobject *kobj)
> +{
> +	kuid_t uid;
> +	kgid_t gid;
> +	struct iattr newattrs = {
> +		.ia_valid = ATTR_UID | ATTR_GID,
> +	};
> +
> +	kobject_get_ownership(kobj, &uid, &gid);
> +	newattrs.ia_uid = uid;
> +	newattrs.ia_gid = gid;
> +
> +	return kernfs_setattr(kn, &newattrs);
> +}
> +
> +/**
> + *	sysfs_file_change_owner_by_name - change owner of a file.
> + *	@kobj:	object.
> + *	@name:	name of the file to change.
> + *
> + * To change the ownership of a sysfs object, the caller must first change the
> + * uid/gid of the kobject and then call this function.

Why have the caller do this?  Why not pass the uid/gid as a parameter
here?  That would make it totally obvious as to what is happening here,
right?

Otherwise this function is depending on someone doing something before
calling it, and that's going to be a very very hard thing to always
ensure/audit.

thanks,

greg k-h