struct nbd_reply not packed => trouble?

[卜弋天]

在 2013-1-11，0:42，"Yann Droneaud" <ydroneaud at opteya.com> 写道：

>> Hello kernel hackers,
>> 
>> 
>> I noticed that in [linux]/include/uapi/linux/nbd.h the structure
>> 
>>   struct nbd_request {
>>           __be32 magic;
>>           __be32 type;    /* == READ || == WRITE */
>>           char handle[8];
>>           __be64 from;
>>           __be32 len;
>>   } __attribute__((packed));
>> 
>> is packed
> 
> Given a target where ABI requires types to be aligned on their size (1 =>
> 1, 2 => 2, 4 => 4, 8 => 8, etc.)
> 
> 
> 4 + 4 + 8 + 8 + 4 = 28 bytes
> 
> To satisfy __be64 alignement (the strictest), it must be aligned on a 8
> bytes boundary, but 28 is not multiple of 8. 32 is. Padding is required.
> 
> [The padding is required so that in a array of struct nbd_request[],
> all members of all structures are aligned to their requirement alignement]
> 
> 
>> but its reply counter part
>> 
>>   struct nbd_reply {
>>           __be32 magic;
>>           __be32 error;    /* 0 = ok, else error */
>>      char handle[8];  /* handle you got from request */
>>   };
> 
> 4 + 4 + 8 = 16 bytes.
> To satisfy __be32 alignement (the strictest), it must be aligned on a 4
> bytes boundary and 16 is a multiple of 4: no need for padding.
> 
>> is not.  Since Linux seems to read sizeof(nbd_reply) bytes from the
>> network (see [1]), the number of bytes read varies with the number of
>> bytes of the structure.
> 
> 
> It won't.
> 
>> 
>> So my understanding is that if the size of struct nbd_reply varies from
>> platform/compiler to another that means trouble.  I wonder:
>> 
>>  - Is there anything about struct nbd_reply that would keep its size
>>    equal everywhere or a reason why variance in size is no problem here?
>> 
>>  - Any ideas for a platform where you would expect struct nbd_reply to
>>    be other than 4 + 4 + 8 = 16 bytes in size?
> 
> Look for an ABI that require 'char' with either property:
> - size > 1 byte
> - alignment > 8 (16 bytes !)
> 
> 

      Do you mean this structure size will vary on different platform? If yes, why Linux does not packed here? Thanks


> Regards.
> 
> -- 
> Yann Droneaud
> OPTEYA
> 
> 
> 
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