Re: [PATCH] UAPI: net/sched: Open-code __struct_group() in flex struct tc_u32_sel

[Gustavo A. R. Silva]

On 17/12/24 10:54, Alexander Lobakin wrote:
> From: Gustavo A. R. Silva <gustavo@xxxxxxxxxxxxxx>
> Date: Tue, 17 Dec 2024 10:25:29 -0600
>
> > On 17/12/24 10:04, Alexander Lobakin wrote:
> > > From: Gustavo A. R. Silva <gustavo@xxxxxxxxxxxxxx>
> > > Date: Tue, 17 Dec 2024 09:58:28 -0600
> > >
> > > > On 17/12/24 08:55, Alexander Lobakin wrote:
> > > > > From: Kees Cook <kees@xxxxxxxxxx>
> > > > > 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