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Subject: Re: [PATCH v4 2/6] sysfs: wrap __compat_only_sysfs_link_entry_to_kobj
 function to change the symlink name
To:     Greg KH <gregkh@linuxfoundation.org>
Cc:     corbet@lwn.net, linux-doc@vger.kernel.org,
        linux-kernel@vger.kernel.org, linuxppc-dev@ozlabs.org,
        mahesh@linux.vnet.ibm.com, hbathini@linux.ibm.com
References: <20191206122434.29587-1-sourabhjain@linux.ibm.com>
 <20191206122434.29587-3-sourabhjain@linux.ibm.com>
 <20191206124642.GB1360047@kroah.com>
 <3aabdf19-ccbf-e99a-c560-2b110e8b536a@linux.ibm.com>
 <20191206191420.GA192422@kroah.com>
From:   Sourabh Jain <sourabhjain@linux.ibm.com>
Date:   Sat, 7 Dec 2019 08:51:58 +0530
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On 12/7/19 12:44 AM, Greg KH wrote:
> 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?

Eventually the scripts will change but I think it is better to have some
overlap time to avoid breaking those scripts.


Thanks,
Sourabh Jain