Eric Blake <ebl...@redhat.com> writes:
> 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 <vkuzn...@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).
Yea, it's just that we already have
!!memcmp(footer->creator_app, "tap", 4))
down below so I decided to stick to the style :-)
>
>> !!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.
I like the idea but I'm still trying to understand whether we need to
keep adding new entries there or just flip the default and say that only
'vpc ' and 'qemu' are legacy and deserve CHS.
--
Vitaly
