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

["Christian König" <christian.koenig@amd.com>]

Am 06.10.21 um 08:32 schrieb Lazar, Lijo:
>
>
> 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.

Well that would then rather be drm_is_debugfs_enabled(), because that we 
separate debugfs handling into a drm core and individual drivers is drm 
specific.

Christian.

>
> 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)) {
>>>>>>
>>>>
>>