linux-kernel - RE: [Patch v2] hv_netvsc: Mark VF as slave before exposing it to user-mode

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Message-ID: <PH7PR21MB32639C8A7EE6431FF990B5F1CEA1A@PH7PR21MB3263.namprd21.prod.outlook.com>
Date:   Mon, 30 Oct 2023 22:29:02 +0000
From:   Long Li <longli@...rosoft.com>
To:     Jakub Kicinski <kuba@...nel.org>,
        "longli@...uxonhyperv.com" <longli@...uxonhyperv.com>
CC:     KY Srinivasan <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Wei Liu <wei.liu@...nel.org>, Dexuan Cui <decui@...rosoft.com>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Paolo Abeni <pabeni@...hat.com>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [Patch v2] hv_netvsc: Mark VF as slave before exposing it to
 user-mode

> Subject: Re: [Patch v2] hv_netvsc: Mark VF as slave before exposing it to user-
> mode
> 
> On Fri, 27 Oct 2023 13:59:50 -0700 longli@...uxonhyperv.com wrote:
> > When a VF is being exposed form the kernel, it should be marked as "slave"
> > before exposing to the user-mode. The VF is not usable without netvsc
> > running as master. The user-mode should never see a VF without the "slave"
> flag.
> >
> > This commit moves the code of setting the slave flag to the time
> > before VF is exposed to user-mode.
> 
> Can you give a real example in the commit message of a flow in user space
> which would get confused by seeing the VF netdev without IFF_SLAVE?

A user-mode program may see the VF netdev show up without SLAVE flag before seeing the NETVSC netdev. It may try to configure the VF before it will be bonded to a NETVSC.

With the IFF_SLAVE correctly set at the time of VF showing up to the user-mode, it can rely on this flag to decide if this device should be ignored. (without implementing some timeout logic to detect a potential NETVSC device that may show up later)

> 
> You're only moving setting IFF_SLAVE but not linking the master, is there no
> code which would assume that if SLAVE is set there is a master?

The same (taking IFF_SLAVE without linking to master) is done in the original code before VF is joined, but it was for another purpose. I think there is a gap between when the VF is acted upon by other parts of the system and when it's bonded.