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

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Date:   Tue, 30 Jul 2019 09:01:43 +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] net/socket: fix GCC8+ Wpacked-not-aligned warnings

From: Qian Cai
> Sent: 29 July 2019 21:24
..
> To fix this, "struct sockaddr_storage" needs to be aligned to 4-byte as
> it is only used in those packed sctp structure which is part of UAPI,
> and "struct __kernel_sockaddr_storage" is used in some other
> places of UAPI that need not to change alignments in order to not
> breaking userspace.
> 
> One option is use typedef between "sockaddr_storage" and
> "__kernel_sockaddr_storage" but it needs to change 35 or 370 lines of
> codes. The other option is to duplicate this simple 2-field structure to
> have a separate "struct sockaddr_storage" so it can use a different
> alignment than "__kernel_sockaddr_storage". Also the structure seems
> stable enough, so it will be low-maintenance to update two structures in
> the future in case of changes.

Does it all work if the 8 byte alignment is implicit, not explicit?
For instance if unnamed union and struct are used eg:

struct sockaddr_storage {
	union {
		void * __ptr;  /* Force alignment */
		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 */
		};
	};
};

I suspect unnamed unions and structs have to be allowed by the compiler.

	David

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