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From: Paolo Bonzini <pbonzini@redhat.com>
Date: Sat, 13 Feb 2010 23:38:12 +0100
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Subject: [Qemu-devel] Re: [BUG] Regression: readonly raw images no longer
	work
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To: qemu-devel@nongnu.org

On 02/13/2010 10:40 PM, 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.

Caused by

commit 03cbdac7efc20994d0a87015e24e835d0139df7b
Author: Naphtali Sprei <nsprei@redhat.com>  2010-01-17 15:48:15
Committer: Anthony Liguori <aliguori@us.ibm.com>  2010-01-20 15:25:22
Follows: v0.12.0-rc0

     Disable fall-back to read-only when cannot open drive's file for
     read-write

     Signed-off-by: Naphtali Sprei <nsprei@redhat.com>
     Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

... because before that it was working like this:

open("/dev/sda", O_RDWR|O_SYNC|O_CLOEXEC) = -1 EACCES
open("/dev/sda", O_RDONLY|O_SYNC|O_CLOEXEC) = 10

BTW, because of other bugs in the middle of the history you need a 
command line "-hda /dev/null -drive file=/dev/sda,snapshot=on" to bisect it.

Paolo