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Date: Mon, 22 Feb 2010 16:39:33 +0200
From: Naphtali Sprei <nsprei@redhat.com>
MIME-Version: 1.0
Subject: Re: [Qemu-devel] [BUG] Regression: readonly raw images no longer work
References: <4B771C34.4040308@mail.berlios.de>
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To: Stefan Weil <weil@mail.berlios.de>
Cc: Paolo Bonzini <pbonzini@redhat.com>, QEMU Developers <qemu-devel@nongnu.org>, Markus Armbruster <armbru@redhat.com>

Stefan Weil wrote:
> This command used to work, but fails now:
> 
> $ i386-softmmu/qemu -snapshot /dev/sda
> qemu: could not open disk image /dev/sda: Permission denied
> 
> $ ls -l /dev/sda
> brw-rw-r-- 1 root disk 8, 0 13. Feb 08:55 /dev/sda
> 
> The original file of a snapshot needs only read access,
> but QEMU tries read/write access and fails.
> 
> Variants of above command using -hda or -drive
> also fail with the same error message.
> 
> I did not test whether the regression affects other
> kinds of images, too. Maybe only raw images trigger
> no longer work.
> 
> Regards
> Stefan Weil
> 
> 
> 

Sorry for the late reply,
this is my fault. It will fail for any image format.

It's already fixed with commit 4dca4b639cb20fee38f6eec0a391aecc0ad8848d :
"block: more read-only changes, related to backing files"

Now, since the /dev/sda is the backing-file, it's opened as read-only, no permission problem.

Notice that if you try to commit your changes to the read-only disk (in monitor command), 
it will be silently ignored, no error or warning displayed.
Should I add a warning/error printing ? 

 Naphtali