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 4/19/24 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.

> 
> Andrii, wdyt about supporting other link types also?
> 
>> +        fops = &bpf_link_fops;
>> +    else
>> +        fops = &bpffs_obj_fops;
>> +    return bpf_mkobj_ops(dentry, mode, arg, &bpf_link_iops, fops);
>>   }
>>   static struct dentry *
>> diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
>> index 7d392ec83655..f66bc6215faa 100644
>> --- a/kernel/bpf/syscall.c
>> +++ b/kernel/bpf/syscall.c
>> @@ -3108,7 +3108,19 @@ static void bpf_link_show_fdinfo(struct 
>> seq_file *m, struct file *filp)
>>   }
>>   #endif
>> -static const struct file_operations bpf_link_fops = {
>> +/* Support opening pinned links */
>> +static int bpf_link_open(struct inode *inode, struct file *filp)
>> +{
>> +    struct bpf_link *link = inode->i_private;
>> +
>> +    if (link->type == BPF_LINK_TYPE_STRUCT_OPS)
>> +        return bpffs_struct_ops_link_open(inode, filp);
>> +
>> +    return -EOPNOTSUPP;
>> +}
>> +
>> +const struct file_operations bpf_link_fops = {
>> +    .open = bpf_link_open,
>>   #ifdef CONFIG_PROC_FS
>>       .show_fdinfo    = bpf_link_show_fdinfo,
>>   #endif
>