Re: [PATCH bpf-next 1/2] bpf: enable the "open" operator on a pinned path of a struct_osp link.

[Martin KaFai Lau <martin.lau@linux.dev>]

On 4/23/24 10:16 AM, Kui-Feng Lee wrote:
> 
> 
> On 4/22/24 16:43, Martin KaFai Lau wrote:
>> On 4/22/24 10:30 AM, Kui-Feng Lee wrote:
>>>
>>>
>>> On 4/22/24 10:12, Kui-Feng Lee wrote:
>>>>
>>>>
>>>> On 4/19/24 17:05, Martin KaFai Lau wrote:
>>>>> On 4/16/24 5:25 PM, Kui-Feng Lee wrote:
>>>>>> +int bpffs_struct_ops_link_open(struct inode *inode, struct file *filp)
>>>>>> +{
>>>>>> +    struct bpf_struct_ops_link *link = inode->i_private;
>>>>>> +
>>>>>> +    /* Paired with bpf_link_put_direct() in bpf_link_release(). */
>>>>>> +    bpf_link_inc(&link->link);
>>>>>> +    filp->private_data = link;
>>>>>> +    return 0;
>>>>>> +}
>>>>>> diff --git a/kernel/bpf/inode.c b/kernel/bpf/inode.c
>>>>>> index af5d2ffadd70..b020d761ab0a 100644
>>>>>> --- a/kernel/bpf/inode.c
>>>>>> +++ b/kernel/bpf/inode.c
>>>>>> @@ -360,11 +360,16 @@ static int bpf_mkmap(struct dentry *dentry, umode_t 
>>>>>> mode, void *arg)
>>>>>>   static int bpf_mklink(struct dentry *dentry, umode_t mode, void *arg)
>>>>>>   {
>>>>>> +    const struct file_operations *fops;
>>>>>>       struct bpf_link *link = arg;
>>>>>> -    return bpf_mkobj_ops(dentry, mode, arg, &bpf_link_iops,
>>>>>> -                 bpf_link_is_iter(link) ?
>>>>>> -                 &bpf_iter_fops : &bpffs_obj_fops);
>>>>>> +    if (bpf_link_is_iter(link))
>>>>>> +        fops = &bpf_iter_fops;
>>>>>> +    else if (link->type == BPF_LINK_TYPE_STRUCT_OPS)
>>>>>
>>>>> Open a pinned link and then update should not be specific to struct_ops 
>>>>> link. e.g. should be useful to the cgroup link also?
>>>>
>>>> It could be. Here, I played safe in case it creates any unwanted side
>>>> effect for links of unknown types.
>>>
>>> By the way, may I put it in a follow up patch if we want cgroup links?
>>
>> This does not feel right. It is not struct_ops specific.
>>
>> Before we dive in further, there is BPF_OBJ_GET which can get a fd of a pinned 
>> bpf obj (prog, map, and link). Take a look at bpf_link__open() in libbpf. Does 
>> it work for the use case that needs to update the link?
>>
> It should work.
> So, this patch is not necessary. However, it is still a nice and
> intuitive feature. WDYT?

There is already BPF_OBJ_GET which works for all major bpf obj types (prog, map, 
and link). Having open only works for link and only works for one link type 
(struct_ops) is not very convincing.

Beside, I am not sure how the file flags (e.g. rdonly...etc) should be handled. 
I don't know enough in this area, so I will defer to others to comment in 
general the usefulness and the approach.