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

From: Peter Lieven
Subject: Re: [PATCH] qemu-img: add seek and -n option to dd command
Date: Fri, 5 Feb 2021 09:47:38 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0

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.

Would it be an option to extend qemu-io to write data at a certain offset which 
it reads from STDIN?


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