Re: [RFC 12/12] netlink: Rename tap interface when late binding discovers template name

[David Gibson <david@gibson.dropbear.id.au>]

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On Sun, Dec 14, 2025 at 08:54:41PM -0500, Jon Maloy wrote:
> When pasta starts without a template interface (-I), it creates the
> tap

Template interface is -i not -I.

-I is the guest interface name - that should take precedence if
specified, regardless of late binding or not.

> device with the default name (tap0). Later, when late binding discovers
> the actual template interface, we now rename the device to match it.
> 
> This ensures the namespace interface has the expected name, matching
> the discovered host interface.
> 
> Key changes:
> - Add nl_link_rename() function to rename network interfaces
> - When late binding triggers with the default TAP name, bring it
>   down, rename it to match the discovered interface, then bring it up
> 
> Signed-off-by: Jon Maloy <jmaloy@redhat.com>
> ---
>  netlink.c | 37 +++++++++++++++++++++++++++++++++++++
>  tap.c     | 27 +++++++++++++++++++++++++++
>  tap.h     |  1 +
>  3 files changed, 65 insertions(+)
> 
> diff --git a/netlink.c b/netlink.c
> index de04fb7..71089ab 100644
> --- a/netlink.c
> +++ b/netlink.c
> @@ -44,6 +44,8 @@
>  /* Default namespace interface name from conf.c */
>  extern const char *pasta_default_ifn;
>  
> +static int nl_link_rename(int s, unsigned int ifi, const char *name);
> +
>  /* Same as RTA_NEXT() but for nexthops: RTNH_NEXT() doesn't take 'attrlen' */
>  #define RTNH_NEXT_AND_DEC(rtnh, attrlen)				\
>  	((attrlen) -= RTNH_ALIGN((rtnh)->rtnh_len), RTNH_NEXT(rtnh))
> @@ -315,6 +317,14 @@ static void nl_linkaddr_host_msg_read(struct ctx *c, const struct nlmsghdr *nh)
>  			}
>  			late_binding = true;
>  
> +			/* Rename interface if it is still using default name */
> +			if (is_default && strcmp(ifname, pasta_default_ifn)) {
> +				nl_link_set_flags(nl_sock_ns, c->pasta_ifi,
> +						  0, IFF_UP);
> +				nl_link_rename(nl_sock_ns, c->pasta_ifi, ifname);
> +				debug("Renamed tap: %s -> %s",
> +				      pasta_default_ifn, ifname);
> +			}
>  			if (is_default)
>  				snprintf(c->pasta_ifn, sizeof(c->pasta_ifn),
>  					 "%s", ifname);
> @@ -1943,6 +1953,33 @@ int nl_link_set_flags(int s, unsigned int ifi,
>  	return nl_do(s, &req, RTM_NEWLINK, 0, sizeof(req));
>  }
>  
> +/**
> + * nl_link_rename() - Rename a network interface
> + * @s:		Netlink socket
> + * @ifi:	Interface index
> + * @name:	New interface name
> + *
> + * Return: 0 on success, negative error code on failure
> + */
> +static int nl_link_rename(int s, unsigned int ifi, const char *name)
> +{
> +	struct req_t {
> +		struct nlmsghdr nlh;
> +		struct ifinfomsg ifm;
> +		struct rtattr rta;
> +		char name[IFNAMSIZ];
> +	} req = {
> +		.ifm.ifi_family	  = AF_UNSPEC,
> +		.ifm.ifi_index	  = ifi,
> +		.rta.rta_type	  = IFLA_IFNAME,
> +		.rta.rta_len	  = RTA_LENGTH(IFNAMSIZ),
> +	};
> +
> +	snprintf(req.name, IFNAMSIZ, "%s", name);
> +
> +	return nl_do(s, &req, RTM_NEWLINK, 0, sizeof(req));
> +}
> +
>  /**
>   * nl_neigh_msg_read() - Interpret a neighbour state message from netlink
>   * @c:		Execution context
> diff --git a/tap.c b/tap.c
> index a2a4459..cd59160 100644
> --- a/tap.c
> +++ b/tap.c
> @@ -1502,6 +1502,33 @@ static void tap_sock_tun_init(struct ctx *c)
>  	tap_start_connection(c);
>  }
>  
> +/**
> + * tap_backend_init_late() - Create tap device for late binding
> + * @c:		Execution context
> + *
> + * Called when late binding discovers the template interface name.
> + * Creates the TAP device with the discovered name.
> + */
> +void tap_backend_init_late(struct ctx *c)

Uh... why would you need a second tap device?  Also this function
doesn't seem to be called.

> +{
> +	if (c->mode != MODE_PASTA || c->fd_tap != -1)
> +		return;
> +
> +	if (!*c->pasta_ifn) {
> +		warn("%s called with empty pasta_ifn", __func__);
> +		return;
> +	}
> +
> +	NS_CALL(tap_ns_tun, c);
> +	if (c->fd_tap == -1) {
> +		err("Failed to set up tap device in namespace");
> +		return;
> +	}
> +
> +	tap_start_connection(c);
> +	info("Created tap device %s for late binding", c->pasta_ifn);
> +}
> +
>  /**
>   * tap_sock_update_pool() - Set the buffer base and size for the pool of packets
>   * @base:	Buffer base
> diff --git a/tap.h b/tap.h
> index ee22a9d..b41eae9 100644
> --- a/tap.h
> +++ b/tap.h
> @@ -118,6 +118,7 @@ void tap_handler_passt(struct ctx *c, uint32_t events,
>  int tap_sock_unix_open(char *sock_path);
>  void tap_sock_reset(struct ctx *c);
>  void tap_backend_init(struct ctx *c);
> +void tap_backend_init_late(struct ctx *c);
>  void tap_flush_pools(void);
>  void tap_handler(struct ctx *c, const struct timespec *now);
>  void tap_add_packet(struct ctx *c, struct iov_tail *data,
> -- 
> 2.51.1
> 

-- 
David Gibson (he or they)	| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you, not the other way
				| around.
http://www.ozlabs.org/~dgibson

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