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

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

[Qian Cai]

In file included from ./include/linux/sctp.h:42,
                 from net/core/skbuff.c:47:
./include/uapi/linux/sctp.h:395:1: warning: alignment 4 of 'struct
sctp_paddr_change' is less than 8 [-Wpacked-not-aligned]
 } __attribute__((packed, aligned(4)));
 ^
./include/uapi/linux/sctp.h:728:1: warning: alignment 4 of 'struct
sctp_setpeerprim' is less than 8 [-Wpacked-not-aligned]
 } __attribute__((packed, aligned(4)));
 ^
./include/uapi/linux/sctp.h:727:26: warning: 'sspp_addr' offset 4 in
'struct sctp_setpeerprim' isn't aligned to 8 [-Wpacked-not-aligned]
  struct sockaddr_storage sspp_addr;
                          ^~~~~~~~~
./include/uapi/linux/sctp.h:741:1: warning: alignment 4 of 'struct
sctp_prim' is less than 8 [-Wpacked-not-aligned]
 } __attribute__((packed, aligned(4)));
 ^
./include/uapi/linux/sctp.h:740:26: warning: 'ssp_addr' offset 4 in
'struct sctp_prim' isn't aligned to 8 [-Wpacked-not-aligned]
  struct sockaddr_storage ssp_addr;
                          ^~~~~~~~
./include/uapi/linux/sctp.h:792:1: warning: alignment 4 of 'struct
sctp_paddrparams' is less than 8 [-Wpacked-not-aligned]
 } __attribute__((packed, aligned(4)));
 ^
./include/uapi/linux/sctp.h:784:26: warning: 'spp_address' offset 4 in
'struct sctp_paddrparams' isn't aligned to 8 [-Wpacked-not-aligned]
  struct sockaddr_storage spp_address;
                          ^~~~~~~~~~~
./include/uapi/linux/sctp.h:905:1: warning: alignment 4 of 'struct
sctp_paddrinfo' is less than 8 [-Wpacked-not-aligned]
 } __attribute__((packed, aligned(4)));
 ^
./include/uapi/linux/sctp.h:899:26: warning: 'spinfo_address' offset 4
in 'struct sctp_paddrinfo' isn't aligned to 8 [-Wpacked-not-aligned]
  struct sockaddr_storage spinfo_address;
                          ^~~~~~~~~~~~~~

This is because the commit 20c9c825b12f ("[SCTP] Fix SCTP socket options
to work with 32-bit apps on 64-bit kernels.") added "packed, aligned(4)"
GCC attributes to some structures but one of the members, i.e, "struct
sockaddr_storage" in those structures has the attribute,
"aligned(__alignof__ (struct sockaddr *)" which is 8-byte on 64-bit
systems, so the commit overwrites the designed alignments for
"sockaddr_storage".

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.

Signed-off-by: Qian Cai <cai@lca.pw>
---
 include/linux/socket.h | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/include/linux/socket.h b/include/linux/socket.h
index 97523818cb14..301119657125 100644
--- a/include/linux/socket.h
+++ b/include/linux/socket.h
@@ -38,7 +38,13 @@ struct linger {
 	int		l_linger;	/* How long to linger for	*/
 };
 
-#define sockaddr_storage __kernel_sockaddr_storage
+struct sockaddr_storage {
+	__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 */
+} __aligned(4);			/* force desired alignment */
 
 /*
  *	As we do 4.4BSD message passing we use a 4.4BSD message passing
-- 
1.8.3.1

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

[David Laight]

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

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)

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

[Qian Cai]

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/

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

[David Laight]

From: Qian Cai 
> Sent: 30 July 2019 14:18
> 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/

You are missing the bit where the UAPI structure is packed.
If the compiler won't let you 'pack' a structure that contains structures
(rather than just integers) then the compiler is broken!

The hope here was that it would be ok is the alignment was implicit not explicit.

	David

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)