linux-kernel - Re: [PATCH net-next 07/12] net: dpaa2: publish MAC stringset to ethtool -S even if MAC is missing

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Message-ID: <Y4ZVaA1pNEvqcNud@lunn.ch>
Date:   Tue, 29 Nov 2022 19:54:32 +0100
From:   Andrew Lunn <andrew@...n.ch>
To:     Vladimir Oltean <vladimir.oltean@....com>
Cc:     netdev@...r.kernel.org, Ioana Ciornei <ioana.ciornei@....com>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Russell King <linux@...linux.org.uk>,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next 07/12] net: dpaa2: publish MAC stringset to
 ethtool -S even if MAC is missing

On Tue, Nov 29, 2022 at 04:12:16PM +0200, Vladimir Oltean wrote:
> DPNIs and DPSW objects can connect and disconnect at runtime from DPMAC
> objects on the same fsl-mc bus. The DPMAC object also holds "ethtool -S"
> unstructured counters. Those counters are only shown for the entity
> owning the netdev (DPNI, DPSW) if it's connected to a DPMAC.
> 
> The ethtool stringset code path is split into multiple callbacks, but
> currently, connecting and disconnecting the DPMAC takes the rtnl_lock().
> This blocks the entire ethtool code path from running, see
> ethnl_default_doit() -> rtnl_lock() -> ops->prepare_data() ->
> strset_prepare_data().
> 
> This is going to be a problem if we are going to no longer require
> rtnl_lock() when connecting/disconnecting the DPMAC, because the DPMAC
> could appear between ops->get_sset_count() and ops->get_strings().
> If it appears out of the blue, we will provide a stringset into an array
> that was dimensioned thinking the DPMAC wouldn't be there => array
> accessed out of bounds.
> 
> There isn't really a good way to work around that, and I don't want to
> put too much pressure on the ethtool framework by playing locking games.
> Just make the DPMAC counters be always available. They'll be zeroes if
> the DPNI or DPSW isn't connected to a DPMAC.
> 
> Signed-off-by: Vladimir Oltean <vladimir.oltean@....com>

Reviewed-by: Andrew Lunn <andrew@...n.ch>

    Andrew