Re: [Qemu-devel] [PATCH] Make default invocation of block drivers safer

[Kevin Wolf <kwolf@redhat.com>]

Am 15.07.2010 18:20, schrieb Anthony Liguori:
> I have another idea that I hope will solve the problem in a more 
> complete way.  The fundamental issue is that it's impossible to probe 
> raw images reliably.  We can probe qcow2, vmdk, etc but not raw.
> 
> So, let's do the following: have raw_probe() always fail.  Probing 
> shouldn't be a heuristic, it should be an absolute.  We can't prove it's 
> a raw image, so we should always fail.
> 
> To accomodate current use-cases with raw, let's introduce a new format 
> called "probed_raw".  probed_raw's semantics will be the following:
> 
> The signature of a probed_raw will be ~{'QFI\xfb', 'VMDK', 'COWD', 
> 'OOOM', ...}.  If the signature is 'QRAW', then instead of reading the 
> first sector at offset 0, we read the first sector at offset LENGTH.  If 
> the signature is 'QRAW', LENGTH is computed by calculating FILE_SIZE - 512.
> 
> For probed_raw, write requests to sector 0 are checked.  If the first 
> four bytes is an invalid probed_raw signature or QRAW, we write a QRAW 
> signature to file offset 0 and copy the first sector to the end of the 
> file redirecting reads and writes to the end of file.
> 
> An approach like this has the following properties:
> 
> 1) We can make the bdrv_probe check 100% reliable and return a boolean.
> 2) In the cases where we known format=raw, none of this code is ever 
> invoked.
> 3) probed_raw images usually look exactly like raw images in most cases
> 4) In the degenerate cases, probe_raw images are still mountable in the 
> normal way.
> 5) Even after the QRAW signature is applied, if the guest writes a valid 
> signature, we can truncate the file and make it appear as a normal raw 
> image.
> 
> Christoph/Markus/Stefan, does this seem like a more reasonable approach?

I hope I may answer even though you didn't ask me. ;-)

This qraw format you introduce is a weird thing, mainly because
depending on a signature the first sectors moves to somewhere completely
else.

If we want to introduce something, that's _almost_ raw and that we can
use by default, what about doing something similar to what statically
sized VHD images look like: take a raw image and attach a footer? With
this approach you should still be able to mount it in the host etc. We
could automatically append this footer when opening a probed qraw image r/w.

Kevin