Re: [patch 1/2] fork_connector: add a fork connector

[Evgeniy Polyakov]

On Tue, 22 Mar 2005 11:18:07 -0800
Ram <linuxram@xxxxxxxxxx> wrote:

> > I still do not see why it is needed.
> > Super-user can run ip command and turn network interface off
> > not waiting while apache or named exits or unbind.
> > 
> > In theory I can create some kind of userspace registration mechanism,
> > when userspace application reports it's pid to the connector, 
> > and then it sends data to the specified pids, but does not 
> > allow controlling from userspace.
> > But I really do not think it is a good idea to permit
> > non-priviledged userspace processes to know about deep
> > kernel internals through connector's messages.
> 
> Yes. non-priviledged userspace processes should not know
> any deep kernel internals through connector events.
> 
> I think what I am driving at is, an application that is critically
> dependent on the fork-notification, suddenly stops receiving such
> notification because some other application has switched off the 
> service without its notice. 
> 
> the reason I am concerned is I am planning to feed this fork-events
> to my in-kernel module. Side note: I would really like support for
> in-kernel listners through connector infrastructure.

fork connector can be extended to send to the rest of the world
information that it was turned off or on.

The easiest way to listen inside the kernel for connector's 
notifications is creating appropriate socket in userspace
and then obtain it through the following code which must be
run in context of the process that created above socket:

	struct file *file;
	struct inode *inode;
	int err = 0;
	
	file = fget(userspace_socket_number_in_the_current_process_context);
	if (!file) {
		return -ENODEV;
	}
	inode = file->f_dentry->d_inode;
	if (inode->i_sock) {
		sock = SOCKET_I(inode);
		err = 0;
	} else {
		fput(file);
		err = -ENODEV;
	}

sock is struct sock, which then can be used for read/write
operations using 
kernel_recvmsg(sock, &msg, &iov, 1, size, 0);
kernel_sendmsg(sock, &msg, &iov, 1, size);

with the following initialisation:

sock->sk->sk_allocation |= GFP_NOIO;
iov.iov_base = buf;
iov.iov_len = size;
msg.msg_name = NULL;
msg.msg_namelen = 0;
msg.msg_control = NULL;
msg.msg_controllen = 0;
msg.msg_namelen = 0;
msg.msg_flags = msg_flags | MSG_NOSIGNAL;


Of course it can be done directly from kernelspace without
touching process's structures, but this way is easier.

> RP

	Evgeniy Polyakov

Only failure makes us experts. -- Theo de Raadt
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