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

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Date:   Tue, 30 Jul 2019 09:18:24 -0400
From:   Qian Cai <cai@....pw>
To:     David Laight <David.Laight@...LAB.COM>,
        "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

On Tue, 2019-07-30 at 09:01 +0000, David Laight wrote:
> 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.

I believe this will suffer the same problem in that will break UAPI,

https://lore.kernel.org/lkml/20190726213045.GL6204@localhost.localdomain/