Re: [PATCH] qemu-img: add seek and -n option to dd command

[Max Reitz]

On 04.02.21 21:09, Peter Lieven wrote:
> Am 02.02.21 um 16:51 schrieb Eric Blake:
> > On 1/28/21 8:07 AM, Peter Lieven wrote:
> > > Signed-off-by: Peter Lieven <pl@kamp.de>
> > Your commit message says 'what', but not 'why'.  Generally, the one-line
> > 'what' works well as the subject line, but you want the commit body to
> > give an argument why your patch should be applied, rather than blank.
> >
> > Here's the last time we tried to improve qemu-img dd:
> > https://lists.gnu.org/archive/html/qemu-devel/2018-08/msg02618.html
>
>
> I was not aware of that story. My use case is that I want to be
>
> able to "patch" an image that Qemu is able to handle by overwriting
>
> certain sectors. And I especially do not want to "mount" that image
>
> via qemu-nbd because I might not trust it. I totally want to avoid that the host
>
> system tries to analyse that image in terms of scanning the bootsector, 
> partprobe,
>
> lvm etc. pp.

qemu will have FUSE exporting as of 6.0 (didn’t quite make it into 5.2), 
so you can do something like this:

$ qemu-storage-daemon \
    --blockdev node-name=export,driver=qcow2,\
file.driver=file,file.filename=image.qcow2 \
    --export fuse,id=fuse,node-name=export,mountpoint=image.qcow2

This exports the image on image.qcow2 (i.e., on itself) and so by 
accessing the image file you then get raw access to its contents (so you 
can use system tools like dd).

Doesn’t require root rights, and shouldn’t make the kernel scan 
anything, because it’s exported as just a regular file.

Max