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Re: [libvirt] [PATCH] Add basic support for VDI images



On 02/04/2013 07:03 PM, Daniel P. Berrange wrote:
> On Mon, Feb 04, 2013 at 06:53:34PM +0100, Martin Kletzander wrote:
>> On 02/04/2013 04:53 PM, Daniel P. Berrange wrote:
>>> On Mon, Feb 04, 2013 at 04:46:53PM +0100, Martin Kletzander wrote:
>> [...]
>>>> +
>>>> +    /* not fully supported, but qemu knows it, so we should be able to
>>>> +     * handle this at least basically */
>>>> +    [VIR_STORAGE_FILE_VDI] = { NULL, ".vdi", LV_LITTLE_ENDIAN,
>>>> +                               -2, 0, 0, 0, 0, -1, NULL},
>>>
>>> We can do better than that - look at block/vdi.c in QEMU GIT tree
>>> to find out the offsets / values for version number, magic signature
>>> at least, even if you ignore backing files for now.
>>>
>>
>> Thanks for pointing that out, I've got a v2 and will send it
>> after some testing.  However, it looks like VDI doesn't have a
>> magic, because there is a 40 Byte string at the start that cacn
>> contain anything the creating binary puts there.  For example
>> qemu-img uses "<<< QEMU VM Virtual Disk Image >>>\n".  According
>> to [1], I'm adding "<<<" as the magic, because we don't fail if
>> it doesn't match and we can still match it by the extension then.
> 
> No, you need to look at the 'signature'  field of the header
> instead, which has a fixed magic value
> 
>   #define VDI_SIGNATURE 0xbeda107f
> 

But the struct mapped to the header starts like this:

typedef struct {
    char text[0x40];
    uint32_t signature;
...

Of course that would just mean adding one more parameter to the
FileTypeInfo struct, but I thought it's not magic then.  However, I
checked that and you're right, I was wrong, because according to
magic(5) it can be anywhere.  The magic file '/usr/share/misc/magic.mgc'
also matches the signature, so I'm using that.

Martin


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