Subject: Re: [PATCHv3 ipsec-next] xfrm: Do not parse 32bits compiled xfrm netlink msg on 64bits host

From: Fan Du <[email-masked]>
Date: Wed, 28 Jan 2015 12:34:29 +0800

于 2015年01月27日 17:46, David Laight 写道:
> From: Fan Du
>> structure like xfrm_usersa_info or xfrm_userpolicy_info
>> has different sizeof when compiled as 32bits and 64bits
>> due to not appending pack attribute in their definition.
>
> Don't 'pack' the structure, just ensure that all the fields
> are fixed sized and on their natural boundary.

Is that simple??

The layout is exactly the same between 32bits and 64bits, only difference
is the last cache line padding, because even if last cache line is not fully
occupied, but at least use even bytes, not odd bytes. I think this relates
to cache HW behaviour, not how the structure is naturally aligned or not.
Actually, the structure members is aligned indeed.

#### 64bits
struct xfrm_usersa_info {
     struct xfrm_selector       sel;                  /*     0    56 */
     struct xfrm_id             id;                   /*    56    24 */

     /* XXX last struct has 3 bytes of padding */

     /* --- cacheline 1 boundary (64 bytes) was 16 bytes ago --- */
     xfrm_address_t             saddr;                /*    80    16 */
     struct xfrm_lifetime_cfg   lft;                  /*    96    64 */
     /* --- cacheline 2 boundary (128 bytes) was 32 bytes ago --- */
     struct xfrm_lifetime_cur   curlft;               /*   160    32 */
     /* --- cacheline 3 boundary (192 bytes) --- */
     struct xfrm_stats          stats;                /*   192    12 */
     __u32                      seq;                  /*   204     4 */
     __u32                      reqid;                /*   208     4 */
     __u16                      family;               /*   212     2 */
     __u8                       mode;                 /*   214     1 */
     __u8                       replay_window;        /*   215     1 */
     __u8                       flags;                /*   216     1 */

     /* size: 224, cachelines: 4, members: 12 */
     /* padding: 7 */
     /* paddings: 1, sum paddings: 3 */
     /* last cacheline: 32 bytes */
};

#### 32bits
struct xfrm_usersa_info {
     struct xfrm_selector       sel;                  /*     0    56 */
     struct xfrm_id             id;                   /*    56    24 */

     /* XXX last struct has 3 bytes of padding */

     /* --- cacheline 1 boundary (64 bytes) was 16 bytes ago --- */
     xfrm_address_t             saddr;                /*    80    16 */
     struct xfrm_lifetime_cfg   lft;                  /*    96    64 */
     /* --- cacheline 2 boundary (128 bytes) was 32 bytes ago --- */
     struct xfrm_lifetime_cur   curlft;               /*   160    32 */
     /* --- cacheline 3 boundary (192 bytes) --- */
     struct xfrm_stats          stats;                /*   192    12 */
     __u32                      seq;                  /*   204     4 */
     __u32                      reqid;                /*   208     4 */
     __u16                      family;               /*   212     2 */
     __u8                       mode;                 /*   214     1 */
     __u8                       replay_window;        /*   215     1 */
     __u8                       flags;                /*   216     1 */

     /* size: 220, cachelines: 4, members: 12 */
     /* padding: 3 */
     /* paddings: 1, sum paddings: 3 */
     /* last cacheline: 28 bytes */
};

> Possibly add a compile-time check that the structure is
> of the expected size.
>
> 	David
>

-- 
No zuo no die but I have to try.