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

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Message-ID: <49add42f-42d9-4f34-b4ad-cff31e473f40@embeddedor.com>
Date: Tue, 17 Dec 2024 12:30:54 -0600
From: "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To: Alexander Lobakin <aleksander.lobakin@...el.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



On 17/12/24 10:54, Alexander Lobakin wrote:
> 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 :)).

Hopefully, Christopher can confirm whether this[0] resolves the issue he's seeing.

> 
> 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?

I'd say entirely preventing C++ from seeing the tag is cleaner and safer for
now.

Thanks
-Gustavo

[0] https://github.com/alobakin/linux/commit/2a065c7bae821f5fa85fff6f97fbbd460f4aa0f3