netdev - Re: [PATCH v6 net-next 1/7] netlink: Add a macro to set policy message with format string

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Date:   Thu, 23 Mar 2023 13:34:22 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Shay Agroskin <shayagr@...zon.com>
Cc:     Paolo Abeni <pabeni@...hat.com>,
        David Miller <davem@...emloft.net>, <netdev@...r.kernel.org>,
        "Woodhouse, David" <dwmw@...zon.com>,
        "Machulsky, Zorik" <zorik@...zon.com>,
        "Matushevsky, Alexander" <matua@...zon.com>,
        Saeed Bshara <saeedb@...zon.com>,
        "Wilson, Matt" <msw@...zon.com>,
        "Liguori, Anthony" <aliguori@...zon.com>,
        "Bshara, Nafea" <nafea@...zon.com>,
        "Belgazal, Netanel" <netanel@...zon.com>,
        "Saidi, Ali" <alisaidi@...zon.com>,
        "Herrenschmidt, Benjamin" <benh@...zon.com>,
        "Kiyanovski, Arthur" <akiyano@...zon.com>,
        "Dagan, Noam" <ndagan@...zon.com>,
        "Arinzon, David" <darinzon@...zon.com>,
        "Itzko, Shahar" <itzko@...zon.com>,
        "Abboud, Osama" <osamaabb@...zon.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Vladimir Oltean <vladimir.oltean@....com>,
        Andrew Lunn <andrew@...n.ch>,
        Guangbin Huang <huangguangbin2@...wei.com>,
        Jie Wang <wangjie125@...wei.com>,
        Johannes Berg <johannes@...solutions.net>,
        Edward Cree <ecree.xilinx@...il.com>,
        "Florian Westphal" <fw@...len.de>
Subject: Re: [PATCH v6 net-next 1/7] netlink: Add a macro to set policy
 message with format string

On Thu, 23 Mar 2023 21:44:52 +0200 Shay Agroskin wrote:
> > That's why we have the local variable called __extack, that we 
> > *can*
> > use multiple times, because it's a separate variable, (extack) 
> > is
> > evaluated only once, to initialize it...
> >
> > We don't need to copy the string formatting, unless I'm missing
> > something. Paolo was just asking for:  
> 
> There is an issue with shadowing __extack by NL_SET_ERR_MSG_FMT 
> causing the changes to __extack not to be propagated back to the 
> caller.
> I'm not that big of an expert in C but changing __extack -> 
> _extack fixes the shadowing issue.
> 
> Might not be the most robust solution, though it might suffice for 
> this use case.

TBH the hierarchy should be the other way around, NL_SET_ERR_MSG_FMT()
should be converted to be:

#define NL_SET_ERR_MSG_FMT(extack, attr, msg, args...) \
	NL_SET_ERR_MSG_ATTR_POL_FMT(extack, NULL, NULL, msg, ##args)

and that'd fix the shadowing, right?