Re: struct nbd_reply not packed = trouble?

[Yann Droneaud]

Hi,

Le mardi 15 janvier 2013 à 07:59 +0800, 卜弋天 a écrit :
 
 在 2013-1-11，0:42，Yann Droneaud ydrone...@opteya.com 写道：

   - 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
 

There's probably no such architecture for a general purpose processor
with the ABI properties I've mentionned. eg. (sizeof(char)  1 ||
alignof(char)  8).

And if one day Linux is going to be ported on such target, the packing
of the nbd_reply struct will be the smallest problem to fix.

Regards

-- 
Yann Droneaud
OPTEYA



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Re: struct nbd_reply not packed = trouble?

[卜弋天]

在 2013-1-11，0:42，Yann Droneaud ydrone...@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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Re: struct nbd_reply not packed = trouble?

[Sebastian Pipping]

I see, thank you!

Best,



Sebastian


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struct nbd_reply not packed = trouble?

[Sebastian Pipping]

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 but its reply counter part

   struct nbd_reply {
   __be32 magic;
   __be32 error;/* 0 = ok, else error */
  char handle[8];  /* handle you got from request */
   };

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.

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?

Best,



Sebastian


[1] http://lxr.linux.no/#linux+v3.7.1/drivers/block/nbd.c#L336

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Re: struct nbd_reply not packed = trouble?

[Yann Droneaud]

 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 !)


Regards.

-- 
Yann Droneaud
OPTEYA



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