[Question] Should `CAP_NET_ADMIN` be needed when opening `/dev/ppp`?

[Wang Shanker <shankerwangmiao@gmail.com>]

Hi, all.

                                                                               
I’ve recently met some problems when trying to create a pppoe network link 
inside a unprivileged container. There is a uid namespace which maps root 
inside to a normal user outside. There is also a separate net namespace in the 
container. I create a dev node inside the container and set right 
permission.    

However, `/dev/ppp` cannot get opened since the mapped normal user does not 
have `CAP_NET_ADMIN`. The related code is in `drivers/net/ppp/ppp_generic.c`: 
`int ppp_open()` 

```
static int ppp_open(struct inode *inode, struct file *file)
{
	/*
	 * This could (should?) be enforced by the permissions on /dev/ppp.
	 */
	if (!capable(CAP_NET_ADMIN))
		return -EPERM;
	return 0;
}
```

I wonder why CAP_NET_ADMIN is needed here, rather than leaving it to the
permission of the device node. If there is no need, I suggest that the 
CAP_NET_ADMIN check be removed.