Re: [Qemu-devel] [PATCH 4/5] block: cow - used QEMU_PACKED for on-disk structures

[Richard Henderson <rth@twiddle.net>]

On 09/19/2013 11:43 AM, Jeff Cody wrote:
> cow_header_v2 is read and written directly from the image file
> with bdrv_pread()/bdrv_pwrite(), and as such should be packed to
> avoid unintentional padding.
> 
> Also change struct cow_header_v2 to a typedef, and some minor
> code style changes to keep checkpatch.pl happy.
> 
> Signed-off-by: Jeff Cody <jcody@redhat.com>
> ---
>  block/cow.c | 21 +++++++++++----------
>  1 file changed, 11 insertions(+), 10 deletions(-)
> 
> diff --git a/block/cow.c b/block/cow.c
> index 909c3e7..9c15afb 100644
> --- a/block/cow.c
> +++ b/block/cow.c
> @@ -32,14 +32,14 @@
>  #define COW_MAGIC 0x4f4f4f4d  /* MOOO */
>  #define COW_VERSION 2
>  
> -struct cow_header_v2 {
> +typedef struct QEMU_PACKED cow_header_v2 {
>      uint32_t magic;
>      uint32_t version;
>      char backing_file[1024];
>      int32_t mtime;
>      uint64_t size;
>      uint32_t sectorsize;
> -};
> +} COWHeaderV2;

This changes the layout of this struct.  In particular, there's padding
(depending on the host) between mtime and size.

I don't know what the right solution is: COWHeaderV3 with the bug fix, leaving
V2 alone; adding an int32_t dummy there where the padding was; nothing,
considering the padding to be gone a good thing.


r~