netdev - Re: [PATCH v3 net-next 10/12] net: bonding: ensure .ndo_get

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Date:   Thu, 7 Jan 2021 13:33:13 +0200
From:   Vladimir Oltean <olteanv@...il.com>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        netdev <netdev@...r.kernel.org>, Andrew Lunn <andrew@...n.ch>,
        Florian Fainelli <f.fainelli@...il.com>,
        Cong Wang <xiyou.wangcong@...il.com>,
        Stephen Hemminger <stephen@...workplumber.org>,
        George McCollister <george.mccollister@...il.com>,
        Oleksij Rempel <o.rempel@...gutronix.de>,
        Jay Vosburgh <j.vosburgh@...il.com>,
        Veaceslav Falico <vfalico@...il.com>,
        Andy Gospodarek <andy@...yhouse.net>,
        Arnd Bergmann <arnd@...db.de>, Taehee Yoo <ap420073@...il.com>,
        Jiri Pirko <jiri@...lanox.com>, Florian Westphal <fw@...len.de>
Subject: Re: [PATCH v3 net-next 10/12] net: bonding: ensure .ndo_get_stats64
 can sleep

On Thu, Jan 07, 2021 at 12:18:28PM +0100, Eric Dumazet wrote:
> What a mess really.

Thanks, that's at least _some_ feedback :)

> You chose to keep the assumption that ndo_get_stats() would not fail,
> since we were providing the needed storage from callers.
>
> If ndo_get_stats() are now allowed to sleep, and presumably allocate
> memory, we need to make sure
> we report potential errors back to the user.
>
> I think your patch series is mostly good, but I would prefer not
> hiding errors and always report them to user space.
> And no, netdev_err() is not appropriate, we do not want tools to look
> at syslog to guess something went wrong.

Well, there are only 22 dev_get_stats callers in the kernel, so I assume
that after the conversion to return void, I can do another conversion to
return int, and then I can convert the ndo_get_stats64 method to return
int too. I will keep the plain ndo_get_stats still void (no reason not
to).

> Last point about drivers having to go to slow path, talking to
> firmware : Make sure that malicious/innocent users
> reading /proc/net/dev from many threads in parallel wont brick these devices.
>
> Maybe they implicitly _relied_ on the fact that firmware was gently
> read every second and results were cached from a work queue or
> something.

How? I don't understand how I can make sure of that.

There is an effort initiated by Jakub to standardize the ethtool
statistics. My objection was that you can't expect that to happen unless
dev_get_stats is sleepable just like ethtool -S is. So I think the same
reasoning should apply to ethtool -S too, really.