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

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Message-ID: <a077e758-44cd-476e-8377-5f6f80923494@embeddedor.com>
Date: Tue, 17 Dec 2024 13:19:44 -0600
From: "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To: Christopher Ferris <cferris@...gle.com>
Cc: Alexander Lobakin <aleksander.lobakin@...el.com>,
 Kees Cook <kees@...nel.org>, Jakub Kicinski <kuba@...nel.org>,
 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 13:10, Christopher Ferris wrote:
> I verified that this does fix the compilation problem on Android. Thanks
> for working on this.

Awesome! :)

Thanks for confirming.

-Gustavo

> 
> Christopher
> 
> On Tue, Dec 17, 2024 at 10:31 AM Gustavo A. R. Silva <gustavo@...eddedor.com>
> wrote:
> 
>>
>>
>> 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
>>
>