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 4/16/24 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?

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