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SubjectRe: [PATCH] net/sctp: fix GCC8+ -Wpacked-not-aligned warnings
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> On Jul 26, 2019, at 5:30 PM, Marcelo Ricardo Leitner <marcelo.leitner@gmail.com> wrote:
>
> On Fri, Jul 26, 2019 at 04:57:39PM -0400, Qian Cai wrote:
>> There are a lot of those warnings with GCC8+ 64bit,
>>
>> 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;
>> ^~~~~~~~~~~~~~
>> Fix them by aligning the structures and fields to 8 bytes instead.
>>
>> Signed-off-by: Qian Cai <cai@lca.pw>
>> ---
>> include/uapi/linux/sctp.h | 18 +++++++++---------
>> 1 file changed, 9 insertions(+), 9 deletions(-)
>>
>> diff --git a/include/uapi/linux/sctp.h b/include/uapi/linux/sctp.h
>> index b8f2c4d56532..e7bd71cde882 100644
>> --- a/include/uapi/linux/sctp.h
>> +++ b/include/uapi/linux/sctp.h
>
> These changes gotta be more careful, if possible at all. As is, it's breaking UAPI.

Could you please elaborate how this breaks userspace? I read through all the information
I can find about UAPI and still have no clue. All I can see if that some field alignments changed
which are expected, and it still take on 3 cachelines which should not hurt the performance.

>
> -before
> +after patch
>
> struct sctp_paddrparams {
> sctp_assoc_t spp_assoc_id; /* 0 4 */
> - struct __kernel_sockaddr_storage spp_address __attribute__((__aligned__(1))); /* 4 128 */
> - /* --- cacheline 2 boundary (128 bytes) was 4 bytes ago --- */
> - __u32 spp_hbinterval; /* 132 4 */
> - __u16 spp_pathmaxrxt; /* 136 2 */
> - __u32 spp_pathmtu; /* 138 4 */
> - __u32 spp_sackdelay; /* 142 4 */
> - __u32 spp_flags; /* 146 4 */
> - __u32 spp_ipv6_flowlabel; /* 150 4 */
> - __u8 spp_dscp; /* 154 1 */
>
> - /* size: 156, cachelines: 3, members: 9 */
> + /* XXX 4 bytes hole, try to pack */
> +
> + struct __kernel_sockaddr_storage spp_address __attribute__((__aligned__(8))); /* 8 128 */
> + /* --- cacheline 2 boundary (128 bytes) was 8 bytes ago --- */
> + __u32 spp_hbinterval; /* 136 4 */
> + __u16 spp_pathmaxrxt; /* 140 2 */
> + __u32 spp_pathmtu; /* 142 4 */
> + __u32 spp_sackdelay; /* 146 4 */
> + __u32 spp_flags; /* 150 4 */
> + __u32 spp_ipv6_flowlabel; /* 154 4 */
> + __u8 spp_dscp; /* 158 1 */
> +
> + /* size: 160, cachelines: 3, members: 9 */
> + /* sum members: 155, holes: 1, sum holes: 4 */
> /* padding: 1 */
> - /* forced alignments: 1 */
> - /* last cacheline: 28 bytes */
> -} __attribute__((__aligned__(4)));
> + /* forced alignments: 1, forced holes: 1, sum forced holes: 4 */
> + /* last cacheline: 32 bytes */
> +} __attribute__((__aligned__(8)));
>
>
>> @@ -392,7 +392,7 @@ struct sctp_paddr_change {
>> int spc_state;
>> int spc_error;
>> sctp_assoc_t spc_assoc_id;
>> -} __attribute__((packed, aligned(4)));
>> +} __attribute__((packed, aligned(8)));
>>
>> /*
>> * spc_state: 32 bits (signed integer)
>> @@ -724,8 +724,8 @@ struct sctp_assocparams {
>> */
>> struct sctp_setpeerprim {
>> sctp_assoc_t sspp_assoc_id;
>> - struct sockaddr_storage sspp_addr;
>> -} __attribute__((packed, aligned(4)));
>> + struct sockaddr_storage sspp_addr __attribute__((aligned(8)));
>> +} __attribute__((packed, aligned(8)));
>>
>> /*
>> * 7.1.10 Set Primary Address (SCTP_PRIMARY_ADDR)
>> @@ -737,8 +737,8 @@ struct sctp_setpeerprim {
>> */
>> struct sctp_prim {
>> sctp_assoc_t ssp_assoc_id;
>> - struct sockaddr_storage ssp_addr;
>> -} __attribute__((packed, aligned(4)));
>> + struct sockaddr_storage ssp_addr __attribute__((aligned(8)));
>> +} __attribute__((packed, aligned(8)));
>>
>> /* For backward compatibility use, define the old name too */
>> #define sctp_setprim sctp_prim
>> @@ -781,7 +781,7 @@ enum sctp_spp_flags {
>>
>> struct sctp_paddrparams {
>> sctp_assoc_t spp_assoc_id;
>> - struct sockaddr_storage spp_address;
>> + struct sockaddr_storage spp_address __attribute__((aligned(8)));
>> __u32 spp_hbinterval;
>> __u16 spp_pathmaxrxt;
>> __u32 spp_pathmtu;
>> @@ -789,7 +789,7 @@ struct sctp_paddrparams {
>> __u32 spp_flags;
>> __u32 spp_ipv6_flowlabel;
>> __u8 spp_dscp;
>> -} __attribute__((packed, aligned(4)));
>> +} __attribute__((packed, aligned(8)));
>>
>> /*
>> * 7.1.18. Add a chunk that must be authenticated (SCTP_AUTH_CHUNK)
>> @@ -896,13 +896,13 @@ struct sctp_stream_value {
>> */
>> struct sctp_paddrinfo {
>> sctp_assoc_t spinfo_assoc_id;
>> - struct sockaddr_storage spinfo_address;
>> + struct sockaddr_storage spinfo_address __attribute__((aligned(8)));
>> __s32 spinfo_state;
>> __u32 spinfo_cwnd;
>> __u32 spinfo_srtt;
>> __u32 spinfo_rto;
>> __u32 spinfo_mtu;
>> -} __attribute__((packed, aligned(4)));
>> +} __attribute__((packed, aligned(8)));
>>
>> /* Peer addresses's state. */
>> /* UNKNOWN: Peer address passed by the upper layer in sendmsg or connect[x]
>> --
>> 1.8.3.1

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