linux-kernel - Re: [PATCH v2][next] UAPI: net/sched: Avoid -Wflex-array-member-not-at-end warning

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Message-ID: <20240809094203.378b5224@hermes.local>
Date: Fri, 9 Aug 2024 09:42:03 -0700
From: Stephen Hemminger <stephen@...workplumber.org>
To: "Gustavo A. R. Silva" <gustavoars@...nel.org>
Cc: Potnuri Bharat Teja <bharat@...lsio.com>, "David S. Miller"
 <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski
 <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, Jamal Hadi Salim
 <jhs@...atatu.com>, Cong Wang <xiyou.wangcong@...il.com>, Jiri Pirko
 <jiri@...nulli.us>, netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-hardening@...r.kernel.org
Subject: Re: [PATCH v2][next] UAPI: net/sched: Avoid
 -Wflex-array-member-not-at-end warning

On Fri, 9 Aug 2024 09:27:51 -0600
"Gustavo A. R. Silva" <gustavoars@...nel.org> wrote:

> -Wflex-array-member-not-at-end has been introduced in GCC-14, and we
> are getting ready to enable it, globally.
> 
> So, in order to avoid ending up with a flexible-array member in the
> middle of multiple other structs, we use the `__struct_group()`
> helper to create a new tagged `struct tc_u32_sel_hdr`. This structure
> groups together all the members of the flexible `struct tc_u32_sel`
> except the flexible array.
> 
> As a result, the array is effectively separated from the rest of the
> members without modifying the memory layout of the flexible structure.
> We then change the type of the middle struct member currently causing
> trouble from `struct tc_u32_sel` to `struct tc_u32_sel_hdr`.
> 
> This approach avoids having to implement `struct tc_u32_sel_hdr`
> as a completely separate structure, thus preventing having to maintain
> two independent but basically identical structures, closing the door
> to potential bugs in the future.
> 
> So, with these changes, fix the following warning:
> drivers/net/ethernet/chelsio/cxgb4/cxgb4_tc_u32_parse.h:245:27: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
> 
> Signed-off-by: Gustavo A. R. Silva <gustavoars@...nel.org>

Tested this with iproute2-next and clang and no problem.

The patch might be better split into two parts?