Re: [Qemu-devel] [PATCH v2 1/5] qcow1: Make padding in the header explicit

[Eric Blake <eblake@redhat.com>]

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On 05/15/2014 08:21 AM, Kevin Wolf wrote:
> We were relying on all compilers inserting the same padding in the
> header struct that is used for the on-disk format. Let's not do that.
> Mark the struct as packed and insert an explicit padding field for
> compatibility.
> 
> Cc: qemu-stable@nongnu.org
> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> Reviewed-by: Benoit Canet <benoit@irqsave.net>
> ---
>  block/qcow.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/block/qcow.c b/block/qcow.c
> index 937dd6d..3684794 100644
> --- a/block/qcow.c
> +++ b/block/qcow.c
> @@ -48,9 +48,10 @@ typedef struct QCowHeader {
>      uint64_t size; /* in bytes */
>      uint8_t cluster_bits;
>      uint8_t l2_bits;
> +    uint16_t padding;
>      uint32_t crypt_method;
>      uint64_t l1_table_offset;
> -} QCowHeader;
> +} QEMU_PACKED QCowHeader;

Is it worth a compile-time assertion that the correct size is achieved?

[I don't know if glib provides such a macro, but gnulib has a verify()
macro that could be used as:

verify(sizeof(QCowHeader) == NNN)

which expands to _Static_assert(sizeof(QCowHeader) == NNN) in new enough
C compilers, and to something like

extern int (*dummy1(void)) [sizeof (struct dummy2 {
  int dummy3: (sizeof(QCowHeader) == NNN) ? 1 : -1; })]

on older compilers for reliable compile-time detection]

But not a show-stopper to this patch as-is.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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