struct nbd_reply not packed => trouble?

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?

Quoted 13 lines

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));

Quoted 1 line

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]

Quoted 7 lines

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.

Quoted 3 lines

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.

Quoted 7 lines

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?

Quoted 2 lines

- 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.

Signature · 9 lines

Yann Droneaud
OPTEYA



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? 2013-1-11?0:42?"Yann Droneaud" <[email protected]> ???

Quoted 3 lines

Quoted 2 lines

- Any ideas for a platform where you would expect struct nbd_reply to be other than 4 + 4 + 8 = 16 bytes in size?

Quoted 5 lines

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