linux-kernel - RE: [PATCH v2] net/socket: fix GCC8+ Wpacked-not-aligned warnings

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Date:   Tue, 30 Jul 2019 14:59:12 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Qian Cai' <cai@....pw>,
        "davem@...emloft.net" <davem@...emloft.net>
CC:     "vyasevich@...il.com" <vyasevich@...il.com>,
        "nhorman@...driver.com" <nhorman@...driver.com>,
        "marcelo.leitner@...il.com" <marcelo.leitner@...il.com>,
        "linux-sctp@...r.kernel.org" <linux-sctp@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH v2] net/socket: fix GCC8+ Wpacked-not-aligned warnings

From: Qian Cai
> Sent: 30 July 2019 15:38
...
> Use an implicit alignment for "struct __kernel_sockaddr_storage" so it
> can keep the same alignments as a member in both packed and un-packed
> structures without breaking UAPI.
> 
> Suggested-by: David Laight <David.Laight@...LAB.COM>
...

Although I suggested it needs a bit of tidy up.

Add a comment maybe:

/* The definition uses anonymous union and struct in order to
 * control the default alignment. */

>  struct __kernel_sockaddr_storage {
> -	__kernel_sa_family_t	ss_family;		/* address family */
> -	/* Following field(s) are implementation specific */
> -	char		__data[_K_SS_MAXSIZE - sizeof(unsigned short)];
> +	union {
> +		void *__align; /* implementation specific desired alignment */

Move the 'void *' below the struct so the first member is the 'public one.

> +		struct {
> +			__kernel_sa_family_t	ss_family; /* address family */
> +			/* Following field(s) are implementation specific */
> +			char __data[_K_SS_MAXSIZE - sizeof(unsigned short)];
>  				/* space to achieve desired size, */
>  				/* _SS_MAXSIZE value minus size of ss_family */
> -} __attribute__ ((aligned(_K_SS_ALIGNSIZE)));	/* force desired alignment */
> +			};
> +		};
> +};
> 
>  #endif /* _UAPI_LINUX_SOCKET_H */
> --
> 1.8.3.1

	David

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