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 18:38:59 +0200
From:   Shay Agroskin <shayagr@...zon.com>
To:     Jakub Kicinski <kuba@...nel.org>
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


Jakub Kicinski <kuba@...nel.org> writes:

> On Wed, 22 Mar 2023 14:39:49 +0200 Shay Agroskin wrote:
>> > You should use '__extack' even above, to avoid multiple 
>> > evaluation of
>> > the 'extack' expression.  
>> 
>> I've got to admit that I don't understand the cost of such 
>> evaluations. I thought that it was added to help readers of the 
>> source code to understand what is the type of this attribute 
>> and 
>> have a better warning message when the wrong variable is passed 
>> (kind of typing in Python which isn't strictly needed).
>> What cost is there for casting a pointer ?
>
> It's not about the cost, the macros are unfolded by the 
> preprocessor,
> in the unlikely case someone passes extack++ as an argument 
> using it
> twice inside the body of the macro will increment the value 
> twice.
>
> #define MACRO(arg) function_bla(arg, arg) // use arg twice
>
> int a = 1;
> MACRO(a++);
> print(a); // should print 2, will print 3

Thanks for explaining, that's quite cool how something like this 
can cause a very hard to find bug.
Couldn't find a way to avoid both code duplication and evaluating 
extact only once \= Submitted a new patchset with the modified 
version of this macro.

Also added Reviewed-by tags where appropriate