linux-kernel - Re: [PATCH] UAPI: net/sched: Open-code __struct_group() in flex struct tc_u32

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Message-ID: <b9a20b9e-c871-451d-8b16-0704eec27329@intel.com>
Date: Tue, 17 Dec 2024 17:54:34 +0100
From: Alexander Lobakin <aleksander.lobakin@...el.com>
To: "Gustavo A. R. Silva" <gustavo@...eddedor.com>
CC: Kees Cook <kees@...nel.org>, Jakub Kicinski <kuba@...nel.org>,
	<cferris@...gle.com>, Jamal Hadi Salim <jhs@...atatu.com>, Cong Wang
	<xiyou.wangcong@...il.com>, Jiri Pirko <jiri@...nulli.us>,
	<netdev@...r.kernel.org>, "Gustavo A. R. Silva" <gustavoars@...nel.org>,
	<linux-kernel@...r.kernel.org>, <linux-hardening@...r.kernel.org>
Subject: Re: [PATCH] UAPI: net/sched: Open-code __struct_group() in flex
 struct tc_u32_sel

From: Gustavo A. R. Silva <gustavo@...eddedor.com>
Date: Tue, 17 Dec 2024 10:25:29 -0600

> 
> 
> On 17/12/24 10:04, Alexander Lobakin wrote:
>> From: Gustavo A. R. Silva <gustavo@...eddedor.com>
>> Date: Tue, 17 Dec 2024 09:58:28 -0600
>>
>>>
>>>
>>> On 17/12/24 08:55, Alexander Lobakin wrote:
>>>> From: Kees Cook <kees@...nel.org>
>>>> Date: Mon, 16 Dec 2024 18:59:55 -0800
>>>>
>>>>> This switches to using a manually constructed form of struct tagging
>>>>> to avoid issues with C++ being unable to parse tagged structs within
>>>>> anonymous unions, even under 'extern "C"':
>>>>>
>>>>>     ../linux/include/uapi/linux/pkt_cls.h:25124: error: ‘struct
>>>>> tc_u32_sel::<unnamed union>::tc_u32_sel_hdr,’ invalid; an anonymous
>>>>> union may only have public non-static data members [-fpermissive]
>>>>
>>>> I worked around that like this in the past: [0]
>>>> As I'm not sure it would be fine to fix every such occurrence manually
>>>> by open-coding.
>>>> What do you think?
>>>
>>> The thing is that, in this particular case, we need a struct tag to
>>> change
>>> the type of an object in another struct. See:
>>
>> But the fix I mentioned still allows you to specify a tag in C code...
>> cxgb4 is for sure not C++.
> 
> 
> Oh yes, I see what you mean. If it works, then you should probably
> submit that
> patch upstream. :)

I added it to my CI tree and will wait for a report (24-36 hrs) before
sending. In the meantime, feel free to test whether it solves your issue
and give a Tested-by (or an error report :)).

BTW, I mentioned in the commit message back in 2022 that some C++
standards support tagged structs with anonymous unions (I don't remember
that already). Would it make sense to use a separate #define not for the
whole __cplusplus, but only for certain standards?

> 
> Thanks
> -- 
> Gustavo

Thanks,
Olek