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

[Gustavo A. R. Silva]

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:

diff --git a/drivers/net/ethernet/chelsio/cxgb4/cxgb4_tc_u32_parse.h b/drivers/net/ethernet/chelsio/cxgb4/cxgb4_tc_u32_parse.h
index 9050568a034c..64663112cad8 100644
--- a/drivers/net/ethernet/chelsio/cxgb4/cxgb4_tc_u32_parse.h
+++ b/drivers/net/ethernet/chelsio/cxgb4/cxgb4_tc_u32_parse.h
@@ -242,7 +242,7 @@ struct cxgb4_next_header {
 	 * field's value to jump to next header such as IHL field
 	 * in IPv4 header.
 	 */
-	struct tc_u32_sel sel;
+	struct tc_u32_sel_hdr sel;
 	struct tc_u32_key key;
 	/* location of jump to make */
 	const struct cxgb4_match_field *jump;;

You can also take a look at the original series:

https://lore.kernel.org/linux-hardening/cover.1723586870.git.gustavoars@xxxxxxxxxx/

Thanks
--
Gustavo