[CRIU] [PATCH v7 7/9] unix: Add support of ghost sockets

[Cyrill Gorcunov]

On Mon, Jun 04, 2018 at 12:52:20PM -0700, Andrey Vagin wrote:
> > +
> > +			snprintf(path, sizeof(path), UNIX_GHOST_FMT, ui->name);
> > +			if (rename(ui->name, path)) {
> > +				ret = -errno;
> > +				pr_perror("ghost: Can't rename id %#x ino %d addr %s -> %s\n",
> > +					  ui->ue->id, ui->ue->ino, ui->name, path);
> > +				return ret;
> > +			}
> > +			ui->flags |= USK_GHOST_RENAMED;
> 
> Why do we use a global flag for this? Why we can't rename this file
> back after bind()?

Because once we have it renamed we exit out of this routine and
start restoring file permissions on this socket, iow, there gonna
be more manipulations on this address later.

> > +	pr_debug("ghost: Resolving addresses\n");
> > +
> > +	list_for_each_entry(ui, &unix_ghost_addr, ghost_node) {
> > +		pr_debug("ghost: id %#x type %s state %s ino %d peer %d address %s\n",
> > +			 ui->ue->id, socket_type_name(ui->ue->type),
> > +			 tcp_state_name(ui->ue->state),
> > +			 ui->ue->ino, ui->peer ? ui->peer->ue->ino : 0,
> > +			 ui->name);
> > +
> > +		/*
> > +		 * Drop any existing trash on the FS and mark the
> > +		 * peer as a ghost one, so we will put it into
> > +		 * fdstore to be able to connect into it even
> > +		 * when the address is removed from the FS.
> > +		 */
> > +		unlink_sk(ui);
> 
> Hm. If a socket is a ghost one, we don't know an owner of a socket file
> on a file system, so we don't have rights to delete it, do we?

We always do that, been cleaning any existing socket appeared on
the file system after the checkpoint. Actually there should not
be any file. But to be fair I don't remember the details why
we start removing any existing trash in firs place, been too
long ago.