Re: [PATCH 1/5][next] net: dev: Introduce struct sockaddr_legacy

[Kuniyuki Iwashima]

From: "Gustavo A. R. Silva" <gustavoars@xxxxxxxxxx>
Date: Tue, 15 Oct 2024 18:27:16 -0600
> We are currently working on enabling the -Wflex-array-member-not-at-end
> compiler option. This option has helped us detect several objects of
> the type `struct sockaddr` that appear in the middle of composite
> structures like `struct rtentry`, `struct compat_rtentry`, and others:
> 
> include/uapi/linux/wireless.h:751:33: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
> include/uapi/linux/wireless.h:776:25: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
> include/uapi/linux/wireless.h:833:25: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
> include/uapi/linux/wireless.h:857:25: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
> include/uapi/linux/wireless.h:864:25: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
> include/uapi/linux/route.h:33:25: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
> include/uapi/linux/route.h:34:25: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
> include/uapi/linux/route.h:35:25: 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: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]
> include/net/compat.h:34:25: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
> include/net/compat.h:35:25: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
> fs/nfsd/nfsd.h:74:33: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
> fs/nfsd/nfsd.h:75:33: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
> 
> In order to fix the warnings above, we introduce `struct sockaddr_legacy`.
> The intention is to use it to replace the type of several struct members
> in the middle of composite structures, currently of type `struct sockaddr`.
> 
> These middle struct members are currently causing thousands of warnings
> because `struct sockaddr` contains a flexible-array member, introduced
> by commit b5f0de6df6dce ("net: dev: Convert sa_data to flexible array in
> struct sockaddr").
> 
> The new `struct sockaddr_legacy` doesn't include a flexible-array
> member, making it suitable for use as the type of middle members
> in composite structs that don't really require the flexible-array
> member in `struct sockaddr`, thus avoiding -Wflex-array-member-not-at-end
> warnings.
> 
> Signed-off-by: Gustavo A. R. Silva <gustavoars@xxxxxxxxxx>
> ---
>  include/linux/socket.h | 19 +++++++++++++++++++
>  1 file changed, 19 insertions(+)
> 
> diff --git a/include/linux/socket.h b/include/linux/socket.h
> index d18cc47e89bd..f370ae0e6c82 100644
> --- a/include/linux/socket.h
> +++ b/include/linux/socket.h
> @@ -40,6 +40,25 @@ struct sockaddr {
>  	};
>  };
>  
> +/*
> + * This is the legacy form of `struct sockaddr`. The original `struct sockaddr`
> + * was modified in commit b5f0de6df6dce ("net: dev: Convert sa_data to flexible
> + * array in struct sockaddR") due to the fact that "One of the worst offenders

s/sockaddR/sockaddr/

The same typo? exists in the cover letter.

With it fixed,

Reviewed-by: Kuniyuki Iwashima <kuniyu@xxxxxxxxxx>


> + * of "fake flexible arrays" is struct sockaddr". This means that the original
> + * `char sa_data[14]` behaved as a flexible array at runtime, so a proper
> + * flexible-array member was introduced.
> + *
> + * This caused several flexible-array-in-the-middle issues:
> + * https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html#index-Wflex-array-member-not-at-end
> + *
> + * `struct sockaddr_legacy` replaces `struct sockaddr` in all instances where
> + * objects of this type do not appear at the end of composite structures.
> + */
> +struct sockaddr_legacy {
> +	sa_family_t	sa_family;	/* address family, AF_xxx	*/
> +	char 		sa_data[14];	/* 14 bytes of protocol address	*/
> +};
> +
>  struct linger {
>  	int		l_onoff;	/* Linger active		*/
>  	int		l_linger;	/* How long to linger for	*/
> -- 
> 2.34.1