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Date:   Tue, 27 Sep 2022 18:09:21 +0300
From:   Vladimir Oltean <olteanv@gmail.com>
To:     Jakub Kicinski <kuba@kernel.org>
Cc:     Vladimir Oltean <vladimir.oltean@nxp.com>, netdev@vger.kernel.org,
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        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 net-next 01/12] net/sched: taprio: allow user input of
 per-tc max SDU
Message-ID: <20220927150921.ffjdliwljccusxad@skbuf>
References: <20220923163310.3192733-1-vladimir.oltean@nxp.com>
 <20220923163310.3192733-2-vladimir.oltean@nxp.com>
 <20220926133829.6bb62b8a@kernel.org>
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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?