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

Sourabh Jain
2019-12-07T03:22:18Z

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