netdev - Re: [PATCH 4/5][next] uapi: net: arp: Avoid -Wflex-array-member-not-at-end warnings

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Message-ID: <ac2ea738-09fb-4d03-b91c-d54bcfb893c6@lunn.ch>
Date: Wed, 16 Oct 2024 14:30:02 +0200
From: Andrew Lunn <andrew@...n.ch>
To: "Gustavo A. R. Silva" <gustavoars@...nel.org>
Cc: Andrew Lunn <andrew+netdev@...n.ch>, David Ahern <dsahern@...nel.org>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-hardening@...r.kernel.org, Kees Cook <kees@...nel.org>
Subject: Re: [PATCH 4/5][next] uapi: net: arp: Avoid
 -Wflex-array-member-not-at-end warnings

On Tue, Oct 15, 2024 at 06:32:43PM -0600, Gustavo A. R. Silva wrote:
> -Wflex-array-member-not-at-end was introduced in GCC-14, and we are
> getting ready to enable it, globally.
> 
> Address the following warnings by changing the type of the middle struct
> members in a couple of composite structs, which are currently causing
> trouble, from `struct sockaddr` to `struct sockaddr_legacy`. Note that
> the latter struct doesn't contain a flexible-array member.
> 
> include/uapi/linux/if_arp.h:118:25: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
> include/uapi/linux/if_arp.h:119:25: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
> include/uapi/linux/if_arp.h:121:25: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
> include/uapi/linux/if_arp.h:126:25: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
> include/uapi/linux/if_arp.h:127:25: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
> 
> Also, update some related code, accordingly.
> 
> No binary differences are present after these changes.

These are clearly UAPI files. It would be good to state in the commit
message why this is a safe change, at the source level.

Could user space code expect the type struct sockaddr and we are going
to get warnings when struct sockaddr_legacy is found? Have you tried
compiling an old arp binary, one that uses the IOCTL? The netfilter
code also references it:

http://charette.no-ip.com:81/programming/doxygen/netfilter/structarpreq.html

You also need to submit a patch to the man page:

https://man7.org/linux/man-pages/man7/arp.7.html

You might also want to build some Rust code:

https://docs.diesel.rs/master/libc/struct.arpreq.html

I've no idea what Rust does when a C structure has a type change.

	Andrew