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

[Kui-Feng Lee <sinquersw@gmail.com>]

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?