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

From: Greg KH <[email-masked]>
Date: Fri, 6 Dec 2019 20:14:20 +0100

On Fri, Dec 06, 2019 at 11:57:53PM +0530, Sourabh Jain wrote:
> 
> 
> On 12/6/19 6:16 PM, Greg KH wrote:
> > 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 <[email-masked]>
> >> ---
> >>  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?
> 
> We don't have much option here. I tried replicating the sysfs files
> in older patch series but creating symlink at old location is much
> better approach.
> 
> The __compat_only_sysfs_link_entry_to_kobj function is pretty generic,
> unable to understand the reason behind restricting its usage.
> 
> > 
> > So then keep compat_only here as well?
> 
> Sure, I will rename the wrapper function.
> 
> But how about changing the function signature instead of creating
> a wrapper function?
> 
> Considering the fact that there are only two places this function
> has called.
> 
> > 
> > What breaks if you remove those undocumented sysfs files?  What
> > userspace tool do you have that will even notice?
> 
> The scripts used in kdump service need those sysfs files to control
> the dump collection. So we can't just move the sysfs files to the
> new location.

If you can not change them, then just document them and live with it.
Why do this extra work to create a symlink for something you will never
use?

greg k-h