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

From: Qian Cai

quoted

Sent: 29 July 2019 21:24

..

quoted

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/ (local)