netdev - RE: [PATCH] net: loopback: use NET_NAME_PREDICTABLE for name_assign

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Date:   Wed, 23 Nov 2022 20:16:32 +0000
From:   "Keller, Jacob E" <jacob.e.keller@...el.com>
To:     Rasmus Villemoes <linux@...musvillemoes.dk>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>
CC:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] net: loopback: use NET_NAME_PREDICTABLE for
 name_assign_type



> -----Original Message-----
> From: Rasmus Villemoes <linux@...musvillemoes.dk>
> Sent: Wednesday, November 23, 2022 6:18 AM
> To: David S. Miller <davem@...emloft.net>; Eric Dumazet
> <edumazet@...gle.com>; Jakub Kicinski <kuba@...nel.org>; Paolo Abeni
> <pabeni@...hat.com>
> Cc: Rasmus Villemoes <linux@...musvillemoes.dk>; netdev@...r.kernel.org;
> linux-kernel@...r.kernel.org
> Subject: [PATCH] net: loopback: use NET_NAME_PREDICTABLE for
> name_assign_type
> 
> When the name_assign_type attribute was introduced (commit
> 685343fc3ba6, "net: add name_assign_type netdev attribute"), the
> loopback device was explicitly mentioned as one which would make use
> of NET_NAME_PREDICTABLE:
> 
>     The name_assign_type attribute gives hints where the interface name of a
>     given net-device comes from. These values are currently defined:
> ...
>       NET_NAME_PREDICTABLE:
>         The ifname has been assigned by the kernel in a predictable way
>         that is guaranteed to avoid reuse and always be the same for a
>         given device. Examples include statically created devices like
>         the loopback device [...]
> 

Heh, so the doc says loopback is an example of this but we weren't using it for that :D

> Switch to that so that reading /sys/class/net/lo/name_assign_type
> produces something sensible instead of returning -EINVAL.
> 

This seems reasonable to me.

> Signed-off-by: Rasmus Villemoes <linux@...musvillemoes.dk>
> ---
> 
> This is mostly cosmetic, but ideally I'd like to get to a situation
> where I don't need to do
> 
>   assign_type=$(cat /sys/class/net/$dev/name_assign_type 2> /dev/null || echo
> 0)
> 
> or otherwise special-case [ $dev = "lo" ].
> 
> As always, there's a small chance that this could cause a regression,
> but it seems extremely unlikely that anybody relies on
> /sys/class/net/lo/name_assign_type being unreadable and thus
> effectively is known to be NET_NAME_UNKNOWN.
> 

I don't think I would consider this a regression. Previously name_assign_type was returning an error here, now it reports something useful. And we know the name is predictable because 
it is the loopback device.

Reviewed-by: Jacob Keller <jacob.e.keller@...el.com>

>  drivers/net/loopback.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/net/loopback.c b/drivers/net/loopback.c
> index 14e8d04cb434..2e9742952c4e 100644
> --- a/drivers/net/loopback.c
> +++ b/drivers/net/loopback.c
> @@ -211,7 +211,7 @@ static __net_init int loopback_net_init(struct net *net)
>  	int err;
> 
>  	err = -ENOMEM;
> -	dev = alloc_netdev(0, "lo", NET_NAME_UNKNOWN, loopback_setup);
> +	dev = alloc_netdev(0, "lo", NET_NAME_PREDICTABLE, loopback_setup);
>  	if (!dev)
>  		goto out;
> 
> --
> 2.37.2