Re: [PATCH 1/1] drm/amdgpu: ignore -EPERM error from debugfs

["Lazar, Lijo" <lijo.lazar@amd.com>]

On 10/6/2021 11:49 AM, Christian König wrote:
> Am 06.10.21 um 06:51 schrieb Lazar, Lijo:
>>
>>
>> On 10/5/2021 10:15 PM, Christian König wrote:
>>> Am 05.10.21 um 15:49 schrieb Das, Nirmoy:
>>>>
>>>> On 10/5/2021 3:22 PM, Christian König wrote:
>>>>>
>>>>>
>>>>> Am 05.10.21 um 15:11 schrieb Nirmoy Das:
>>>>>> Debugfs core APIs will throw -EPERM when user disables debugfs
>>>>>> using CONFIG_DEBUG_FS_ALLOW_NONE or with kernel param. We shouldn't
>>>>>> see that as an error. Also validate drm root dentry before creating
>>>>>> amdgpu debugfs files.
>>>>>>
>>>>>> Signed-off-by: Nirmoy Das <nirmoy.das@amd.com>
>>>>>> ---
>>>>>>   drivers/gpu/drm/amd/amdgpu/amdgpu_debugfs.c | 10 ++++++++++
>>>>>>   1 file changed, 10 insertions(+)
>>>>>>
>>>>>> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_debugfs.c 
>>>>>> b/drivers/gpu/drm/amd/amdgpu/amdgpu_debugfs.c
>>>>>> index 6611b3c7c149..d786072e918b 100644
>>>>>> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_debugfs.c
>>>>>> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_debugfs.c
>>>>>> @@ -1617,6 +1617,16 @@ int amdgpu_debugfs_init(struct 
>>>>>> amdgpu_device *adev)
>>>>>>       struct dentry *ent;
>>>>>>       int r, i;
>>>>>>   +    if (IS_ERR(root)) {
>>>>>> +        /* When debugfs is disabled we get -EPERM which is not an
>>>>>> +         * error as this is user controllable.
>>>>>> +         */
>>>>>
>>>>> Well setting primary->debugfs_root to an error code is probably not 
>>>>> a good idea to begin with.
>>>>>
>>>>> When debugfs is disabled that should most likely be NULL.
>>>>
>>>>
>>>> If we set primary->debugfs_root to  NULL then we need to add bunch 
>>>> of NULL checks everywhere before creating any debugfs files
>>>>
>>>> because debugfs_create_{file|dir}() with NULL root is still valid. I 
>>>> am assuming a hypothetical case when debugfs_root dir creation fails 
>>>> even with debugfs enabled
>>>>
>>>> but further calls are successful.  This wont be a problem if we 
>>>> propagate the error code.
>>>
>>> Yeah, but an error code in members is ugly like hell and potentially 
>>> causes crashes instead.
>>>
>>> I strongly suggest to fix this so that root is NULL when debugfs 
>>> isn't available and we add proper checks for that instead.
>>
>> This shouldn't be done. A NULL is a valid parent for debugfs API. An 
>> invalid parent is always checked like this
>>           if (IS_ERR(parent))
>>                 return parent;
>>
>> Instead of adding redundant work like NULL checks, let the API do its 
>> work and don't break the API contract. For ex: usage of sample client, 
>> you may look at the drm usage; it does the same.
> 
> Yeah, but that is horrible API design and should be avoided.
> 
> ERR_PTR(), PTR_ERR(), IS_ERR() and similar are supposed to be used as 
> alternative to signaling errors as return values from functions and 
> should *never* ever be used to signal errors in pointer members.
> 

One escape route may be - add another export from debugfs like 
debugfs_is_valid_node() which adheres to the current logic in debugfs 
API and use that in client code. Whenever debugfs changes to a different 
logic from IS_ERR, let that be changed.

Thanks,
Lijo

> Regards,
> Christian.
> 
>>
>> Thanks,
>> Lijo
>>
>>>
>>> Regards,
>>> Christian.
>>>
>>>>
>>>>
>>>> Regards,
>>>>
>>>> Nirmoy
>>>>
>>>>>
>>>>> Regards,
>>>>> Christian.
>>>>>
>>>>>> +        if (PTR_ERR(root) == -EPERM)
>>>>>> +            return 0;
>>>>>> +
>>>>>> +        return PTR_ERR(ent);
>>>>>> +    }
>>>>>> +
>>>>>>       ent = debugfs_create_file("amdgpu_preempt_ib", 0600, root, 
>>>>>> adev,
>>>>>>                     &fops_ib_preempt);
>>>>>>       if (IS_ERR(ent)) {
>>>>>
>>>
>