netdev - Re: [PATCH] net: linkwatch: ignore events for unregistered netdevs

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Date:   Mon, 25 Apr 2022 08:23:39 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     Jann Horn <jannh@...gle.com>
Cc:     Jakub Kicinski <kuba@...nel.org>, Lukas Wunner <lukas@...ner.de>,
        Paolo Abeni <pabeni@...hat.com>,
        Oliver Neukum <oneukum@...e.com>,
        "David S. Miller" <davem@...emloft.net>,
        Oleksij Rempel <o.rempel@...gutronix.de>,
        netdev <netdev@...r.kernel.org>,
        USB list <linux-usb@...r.kernel.org>,
        Andrew Lunn <andrew@...n.ch>,
        Jacky Chou <jackychou@...x.com.tw>, Willy Tarreau <w@....eu>,
        Lino Sanfilippo <LinoSanfilippo@....de>,
        Philipp Rosenberger <p.rosenberger@...bus.com>,
        Heiner Kallweit <hkallweit1@...il.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH] net: linkwatch: ignore events for unregistered netdevs

On Mon, Apr 25, 2022 at 8:19 AM Jann Horn <jannh@...gle.com> wrote:
>
> On Mon, Apr 25, 2022 at 5:13 PM Eric Dumazet <edumazet@...gle.com> wrote:
> > On Mon, Apr 25, 2022 at 8:01 AM Jakub Kicinski <kuba@...nel.org> wrote:
> > >
> > > On Mon, 25 Apr 2022 16:49:34 +0200 Jann Horn wrote:
> > > > > Doesn't mean we should make it legal. We can add a warning to catch
> > > > > abuses.
> > > >
> > > > That was the idea with
> > > > https://lore.kernel.org/netdev/20220128014303.2334568-1-jannh@google.com/,
> > > > but I didn't get any replies when I asked what the precise semantics
> > > > of dev_hold() are supposed to be
> > > > (https://lore.kernel.org/netdev/CAG48ez1-OyZETvrYAfaHicYW1LbrQUVp=C0EukSWqZrYMej73w@mail.gmail.com/),
> > > > so I don't know how to proceed...
> > >
> > > Yeah, I think after you pointed out that the netdev per cpu refcounting
> > > is fundamentally broken everybody decided to hit themselves with the
> > > obliviate spell :S
> >
> > dev_hold() has been an increment of a refcount, and dev_put() a decrement.
> >
> > Not sure why it is fundamentally broken.
>
> Well, it's not quite a refcount. It's a count that can be incremented
> and decremented but can't be read while the device is alive, and then
> at some point it turns into a count that can be read and decremented
> but can't be incremented (see
> https://lore.kernel.org/netdev/CAG48ez1-OyZETvrYAfaHicYW1LbrQUVp=C0EukSWqZrYMej73w@mail.gmail.com/).
> Normal refcounts allow anyone who is holding a reference to add
> another reference.

On a live netdev nothing wants to read the 'current refcount'.
We basically do not care.

>
> > There are specific steps at device dismantles making sure no more
> > users can dev_hold()
>
> So you're saying it's intentional that even if you're already holding
> a dev_hold() reference, you may not be allowed to call dev_hold()
> again?

I think you can/should not.
We might add a test in dev_hold() and catch offenders.

Then add a new api, (dev_hold() is void and can not propagate an
error), and eventually
fix offenders.