netdev - Re: [PATCH net-next 5/6] netlink: add mask validation

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Message-ID: <f302acfdb9558ebe0ddedc7726beba7d388fffda.camel@sipsolutions.net>
Date:   Tue, 06 Oct 2020 13:52:43 +0200
From:   Johannes Berg <johannes@...solutions.net>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Michal Kubecek <mkubecek@...e.cz>, davem@...emloft.net,
        netdev@...r.kernel.org, kernel-team@...com, jiri@...nulli.us,
        andrew@...n.ch, dsahern@...il.com, pablo@...filter.org
Subject: Re: [PATCH net-next 5/6] netlink: add mask validation

On Tue, 2020-10-06 at 08:37 +0200, Johannes Berg wrote:
> On Mon, 2020-10-05 at 15:21 -0700, Jakub Kicinski wrote:
> 
> > > > Nice, easy & useful, maybe I'll code it up tomorrow.  
> > > 
> > > OK I thought about it a bit more and looked at the code, and it's not
> > > actually possible to do easily right now, because we can't actually
> > > point to the bad attribute from the general lib/nlattr.c code ...
> > > 
> > > Why? Because we don't know right now, e.g. for nla_validate(), where in
> > > the message we started validation, i.e. the offset of the "head" inside
> > > the particular message.
> > > 
> > > For nlmsg_parse() and friends that's a bit easier, but it needs more
> > > rejiggering than I'm willing to do tonight ;)
> > 
> > I thought we'd record the const struct nla_policy *tp for the failing
> > attr in struct netlink_ext_ack and output based on that.
> 
> We could, but it's a bit useless if you know "which" attribute caused
> the issue, but you don't know where it was in the message? That way you
> wouldn't know the nesting level etc.
> 
> I mean, we actually have that problem today - the generic lib/nlattr.c
> policy violation doesn't tell you where exactly the problem occurred, so
> it'd be good to fix that regardless.

Just for the record: I'm obviously *completely* confused, of course we
do say which attribute was bad, I was just looking for the offset, but
we carry around a pointer and calculate the offset later,
with NL_SET_ERR_MSG_ATTR() or NL_SET_BAD_ATTR().

Which means this isn't so hard ...

johannes