Re: [PATCH v2] vpc: Read images exported from Azure correctly

[Eric Blake]

On Mon, Nov 18, 2024 at 03:36:46PM +0100, Vitaly Kuznetsov wrote:
> It was found that 'qemu-nbd' is not able to work with some disk images
> exported from Azure. Looking at the 512b footer (which contains VPC
> metadata):
> 
> 00000000  63 6f 6e 65 63 74 69 78  00 00 00 02 00 01 00 00  |conectix........|
> 00000010  ff ff ff ff ff ff ff ff  2e c7 9b 96 77 61 00 00  |............wa..|
> 00000020  00 07 00 00 57 69 32 6b  00 00 00 01 40 00 00 00  |....Wi2k....@...|
> 00000030  00 00 00 01 40 00 00 00  28 a2 10 3f 00 00 00 02  |....@...(..?....|
> 00000040  ff ff e7 47 8c 54 df 94  bd 35 71 4c 94 5f e5 44  |...G.T...5qL._.D|
> 00000050  44 53 92 1a 00 00 00 00  00 00 00 00 00 00 00 00  |DS..............|
> 00000060  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
> 
> we can see that Azure uses a different 'Creator application' --
> 'wa\0\0' (offset 0x1c, likely reads as 'Windows Azure') and QEMU uses this
> field to determine how it can get image size. Apparently, Azure uses 'new'
> method, just like Hyper-V.
> 
> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
> ---
> Alternatively, we can probably make 'current_size' the default and only use
> CHS for 'vpc '/'qemu'.
> ---
>  block/vpc.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/block/vpc.c b/block/vpc.c
> index d95a204612b7..b67798697c15 100644
> --- a/block/vpc.c
> +++ b/block/vpc.c
> @@ -321,6 +321,7 @@ static int vpc_open(BlockDriverState *bs, QDict *options, 
> int flags,
>       *      'qemu'  :  CHS              QEMU (uses disk geometry)
>       *      'qem2'  :  current_size     QEMU (uses current_size)
>       *      'win '  :  current_size     Hyper-V
> +     *      'wa\0\0':  current_size     Azure
>       *      'd2v '  :  current_size     Disk2vhd
>       *      'tap\0' :  current_size     XenServer
>       *      'CTXS'  :  current_size     XenConverter
> @@ -330,6 +331,7 @@ static int vpc_open(BlockDriverState *bs, QDict *options, 
> int flags,
>       *  that have CHS geometry of the maximum size.
>       */
>      use_chs = (!!strncmp(footer->creator_app, "win ", 4) &&
> +               !!memcmp(footer->creator_app, "wa\0", 4) &&

While this is literally correct (a string literal with 3 characters
spelled out includes the implicit NUL byte; sizeof("wa\0") == 4), it
is a bit odd to see a memcmp() of 4 bytes against a literal containing
only 3 characters, especially when the comments above spelled it out
with four characters.  For the sake of avoiding further confusion, it
might be nice to use memcmp() against explicit 4-byte patterns for all
of the strings (not just the Azure witness).

>                 !!strncmp(footer->creator_app, "qem2", 4) &&
>                 !!strncmp(footer->creator_app, "d2v ", 4) &&
>                 !!strncmp(footer->creator_app, "CTXS", 4) &&

I also don't know if it would be any easier to read by creating a
`static const char table[][4] = { "qemu", "qem2", "wa", ...}` (where
you don't have to write any explicit \0, because the compiler is
guaranteed to NUL-pad short strings into the char[4] table entry) and
then write a loop over each entry in the table, rather than having to
spell out a separate strncmp/memcmp line for each string in the table.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.
Virtualization:  qemu.org | libguestfs.org