linux-kernel - Re: Network Device Naming mechanism and policy

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Date:	Wed, 5 Aug 2009 22:49:04 +0100
From:	Alan Jenkins <sourcejedi.lkml@...glemail.com>
To:	Jordan_Hargrave@...l.com
Cc:	linux-kernel@...r.kernel.org, Matt_Domsch@...l.com,
	Narendra_K@...l.com
Subject: Re: Network Device Naming mechanism and policy

On 8/5/09, Jordan_Hargrave@...l.com <Jordan_Hargrave@...l.com> wrote:
> This is from an old discussion several months ago:
> http://lkml.org/lkml/2009/3/24/357
> http://lkml.org/lkml/2009/3/24/380
>
> Basically the issue is that between a race in udev and PCI scan order the
> ethX IDs may not
> be consistent between reboots.  The idea is to use a mechanism similar to
> how disks now can
> be accessed by their LABEL/PATH/UUID instead of raw /dev/sdX ids.
>
> The following patch will create a device node for network devices based off
> their ifindex;
> udev can then use this device node for creating symlinks in /dev/net/xxxx
> similar to the
> way that disks now use by-label and by-path symlinks.
>
> Combining this with the biosdevname utility and patches to common network
> utilities,
> it could be possible to access ethernet devices by their PCI path or BIOS
> Label.
>
> eg. ifconfig Embedded_NIC_1
>
> --- include/linux/major.h~	2009-07-30 18:34:47.000000000 -0400
> +++ include/linux/major.h	2009-08-05 14:52:10.000000000 -0400
> @@ -169,6 +169,7 @@
>  #define IBM_FS3270_MAJOR	228
>
>  #define VIOTAPE_MAJOR		230
> +#define NETDEV_MAJOR		234
>
>  #define BLOCK_EXT_MAJOR		259
>  #define SCSI_OSD_MAJOR		260	/* open-osd's OSD scsi device */
> --- net/core/net-sysfs.cx	2009-08-05 15:00:13.000000000 -0400
> +++ net/core/net-sysfs.c	2009-08-05 15:01:20.000000000 -0400
> @@ -11,6 +11,7 @@
>
>  #include <linux/capability.h>
>  #include <linux/kernel.h>
> +#include <linux/major.h>
>  #include <linux/netdevice.h>
>  #include <linux/if_arp.h>
>  #include <net/sock.h>
> @@ -496,6 +497,7 @@ int netdev_register_kobject(struct net_d
>  	dev->class = &net_class;
>  	dev->platform_data = net;
>  	dev->groups = groups;
> +	dev->devt = MKDEV(NETDEV_MAJOR, net->ifindex);
>
>  	BUILD_BUG_ON(BUS_ID_SIZE < IFNAMSIZ);
>  	dev_set_name(dev, "%s", net->name);
>
>
> --jordan hargrave
> Dell Enterprise Linux Engineering

I guess opening such a device would yield ENODEV, which is
unfortunately misleading.  Pity to add code just to get ENOTSUPP,
but...

Idea: allow opening the device.  You needn't provide any operations.
The only effect would be to pin the ifindex.  That would allow tools
to avoid racing with hot-unplug.

Alan "Ideas are cheap" Jenkins

P.S. Is there any risk of ifindex overflowing when converted to a
device minor number?
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