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

Greg KH
2019-12-06T19:14:26Z

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 <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?
>
> 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