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

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Message-ID: <bbed49c7-56c0-4642-afec-e47b14425f76@embeddedor.com>
Date: Tue, 17 Dec 2024 09:58:28 -0600
From: "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To: Alexander Lobakin <aleksander.lobakin@...el.com>,
 Kees Cook <kees@...nel.org>
Cc: 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 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:

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@kernel.org/

Thanks
--
Gustavo