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Date:   Tue, 27 Sep 2022 18:09:21 +0300
From:   Vladimir Oltean <olteanv@...il.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Vladimir Oltean <vladimir.oltean@....com>, netdev@...r.kernel.org,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Paolo Abeni <pabeni@...hat.com>,
        Xiaoliang Yang <xiaoliang.yang_1@....com>,
        Rui Sousa <rui.sousa@....com>,
        Claudiu Manoil <claudiu.manoil@....com>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>,
        UNGLinuxDriver@...rochip.com,
        Horatiu Vultur <horatiu.vultur@...rochip.com>,
        Andrew Lunn <andrew@...n.ch>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        Michael Walle <michael@...le.cc>,
        Vinicius Costa Gomes <vinicius.gomes@...el.com>,
        Maxim Kochetkov <fido_max@...ox.ru>,
        Colin Foster <colin.foster@...advantage.com>,
        Richie Pearn <richard.pearn@....com>,
        Kurt Kanzenbach <kurt@...utronix.de>,
        Jesse Brandeburg <jesse.brandeburg@...el.com>,
        Tony Nguyen <anthony.l.nguyen@...el.com>,
        Giuseppe Cavallaro <peppe.cavallaro@...com>,
        Alexandre Torgue <alexandre.torgue@...s.st.com>,
        Jose Abreu <joabreu@...opsys.com>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Cong Wang <xiyou.wangcong@...il.com>,
        Jiri Pirko <jiri@...nulli.us>,
        Gerhard Engleder <gerhard@...leder-embedded.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 net-next 01/12] net/sched: taprio: allow user input of
 per-tc max SDU

On Mon, Sep 26, 2022 at 01:38:29PM -0700, Jakub Kicinski wrote:
> On Fri, 23 Sep 2022 19:32:59 +0300 Vladimir Oltean wrote:
> > +	if (!tb[TCA_TAPRIO_TC_ENTRY_INDEX]) {
> > +		NL_SET_ERR_MSG_MOD(extack, "TC entry index missing");
> 
> NL_SET_ERR_ATTR_MISS() ?
> 
> > +		return -EINVAL;
> > +	}
> > +
> > +	tc = nla_get_u32(tb[TCA_TAPRIO_TC_ENTRY_INDEX]);
> > +	if (tc >= TC_QOPT_MAX_QUEUE) {
> > +		NL_SET_ERR_MSG_MOD(extack, "TC entry index out of range");
> 
> NLA_POLICY_MAX()
> 
> Are you not using those on purpose? :(

I don't exactly see it as being super user friendly to leave it to the
policy validator (or to use NL_SET_ERR_ATTR_MISS()) because all that
will be reported back to user space will be the offset to the original
attribute in the nlmsghdr, which is pretty hard to retrieve and
re-interpret (at least in the iproute2 tc source code, I can't seem to
find a way to stringify it or something like that). For the NLA_POLICY_MAX(),
all I'll get now is an uninformative "Error: integer out of range."
What integer?  What range?

I don't understand what is the gain of removing extack message strings
and just pointing to the netlink attribute via NLMSGERR_ATTR_OFFS? Could
I at least use the NL_SET_ERR_ATTR_MISS() helper *and* set a custom message?
That's for the missing nlattr. Regarding the range checking in the
policy, I'd like a custom message there as well, but the NLA_POLICY_MAX()
doesn't provide one. However, I see that struct nla_policy has a const
char *reject_message for NLA_REJECT types. Would it be an abuse to move
this outside of the union and allow U32 policies and such to also
provide it?

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