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

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Date:   Tue, 30 Jul 2019 11:45:49 -0300
From:   Marcelo Ricardo Leitner <marcelo.leitner@...il.com>
To:     Qian Cai <cai@....pw>
Cc:     davem@...emloft.net, vyasevich@...il.com, nhorman@...driver.com,
        David.Laight@...lab.com, linux-sctp@...r.kernel.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] net/socket: fix GCC8+ Wpacked-not-aligned warnings

On Tue, Jul 30, 2019 at 10:38:08AM -0400, Qian Cai wrote:
> 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.
> 
> 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>
> Signed-off-by: Qian Cai <cai@....pw>
> ---
> 
> v2: Use an implicit alignment for "struct __kernel_sockaddr_storage".
> 
>  include/uapi/linux/socket.h | 15 +++++++++------
>  1 file changed, 9 insertions(+), 6 deletions(-)
> 

SCTP-wise, LGTM.