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

[Max Reitz]

On 05.02.21 10:16, Max Reitz wrote:
> On 05.02.21 09:47, Peter Lieven wrote:
> > Am 05.02.21 um 09:18 schrieb 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.
> >
> >
> > Okay, but that is still more housekeeping than just invoking a single 
> > command.
>
> Yes, but I personally see this as much better than copying all of dd’s 
> functionality into qemu-img.
>
> My personal complaint is only that it’s a pain in the ass to invoke QSD 
> this way.  It would be nice to have a script that does the same via
>
> $ qemu-blk-fuse-export image.qcow2
>
> Would probably be trivial to write, but well, first we gotta do it, and 
> have justification to keep it as part of qemu...
>
> And if that’s still too much housekeeping, we could even write a qemu-dd 
> script that scans all file arguments for non-raw images, launches a QSD 
> instance to present them as raw, and then invokes dd.

Since today’s Day of Learning at Red Hat, I decided to have some fun 
writing a qemu-dd.py script.

Max

qemu-dd.py
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