Re: [PATCH v4 2/6] sysfs: wrap __compat_only_sysfs_link_entry_to_kobj function to change the symlink name

[Greg KH <gregkh@linuxfoundation.org>]

On Fri, Dec 06, 2019 at 05:54:30PM +0530, Sourabh Jain wrote:
> The __compat_only_sysfs_link_entry_to_kobj function creates a symlink to a
> kobject but doesn't provide an option to change the symlink file name.
> 
> This patch adds a wrapper function create_sysfs_symlink_entry_to_kobj that
> extends the __compat_only_sysfs_link_entry_to_kobj functionality which
> allows function caller to customize the symlink name.
> 
> Signed-off-by: Sourabh Jain <sourabhjain@linux.ibm.com>
> ---
>  fs/sysfs/group.c      | 28 +++++++++++++++++++++++++---
>  include/linux/sysfs.h | 12 ++++++++++++
>  2 files changed, 37 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/sysfs/group.c b/fs/sysfs/group.c
> index d41c21fef138..5eb38145b957 100644
> --- a/fs/sysfs/group.c
> +++ b/fs/sysfs/group.c
> @@ -424,6 +424,25 @@ EXPORT_SYMBOL_GPL(sysfs_remove_link_from_group);
>  int __compat_only_sysfs_link_entry_to_kobj(struct kobject *kobj,
>  				      struct kobject *target_kobj,
>  				      const char *target_name)
> +{
> +	return create_sysfs_symlink_entry_to_kobj(kobj, target_kobj,
> +						target_name, NULL);
> +}
> +EXPORT_SYMBOL_GPL(__compat_only_sysfs_link_entry_to_kobj);
> +
> +/**
> + * create_sysfs_symlink_entry_to_kobj - add a symlink to a kobject pointing
> + * to a group or an attribute
> + * @kobj:		The kobject containing the group.
> + * @target_kobj:	The target kobject.
> + * @target_name:	The name of the target group or attribute.
> + * @symlink_name:	The name of the symlink file (target_name will be
> + *			considered if symlink_name is NULL).
> + */
> +int create_sysfs_symlink_entry_to_kobj(struct kobject *kobj,
> +				       struct kobject *target_kobj,
> +				       const char *target_name,
> +				       const char *symlink_name)
>  {
>  	struct kernfs_node *target;
>  	struct kernfs_node *entry;
> @@ -448,12 +467,15 @@ int __compat_only_sysfs_link_entry_to_kobj(struct kobject *kobj,
>  		return -ENOENT;
>  	}
>  
> -	link = kernfs_create_link(kobj->sd, target_name, entry);
> +	if (!symlink_name)
> +		symlink_name = target_name;
> +
> +	link = kernfs_create_link(kobj->sd, symlink_name, entry);
>  	if (IS_ERR(link) && PTR_ERR(link) == -EEXIST)
> -		sysfs_warn_dup(kobj->sd, target_name);
> +		sysfs_warn_dup(kobj->sd, symlink_name);
>  
>  	kernfs_put(entry);
>  	kernfs_put(target);
>  	return PTR_ERR_OR_ZERO(link);
>  }
> -EXPORT_SYMBOL_GPL(__compat_only_sysfs_link_entry_to_kobj);
> +EXPORT_SYMBOL_GPL(create_sysfs_symlink_entry_to_kobj);
> diff --git a/include/linux/sysfs.h b/include/linux/sysfs.h
> index 5420817ed317..123c6f10333a 100644
> --- a/include/linux/sysfs.h
> +++ b/include/linux/sysfs.h
> @@ -300,6 +300,10 @@ void sysfs_remove_link_from_group(struct kobject *kobj, const char *group_name,
>  int __compat_only_sysfs_link_entry_to_kobj(struct kobject *kobj,
>  				      struct kobject *target_kobj,
>  				      const char *target_name);
> +int create_sysfs_symlink_entry_to_kobj(struct kobject *kobj,
> +				       struct kobject *target_kobj,
> +				       const char *target_name,
> +				       const char *symlink_name);

sysfs_create_symlink_entry_to_kobj()?

I can't remember why we put __compat_only there, perhaps because we do
not want people to really use this unless you really really have to?

So then keep compat_only here as well?

What breaks if you remove those undocumented sysfs files?  What
userspace tool do you have that will even notice?

thanks,

greg k-h