Bug 856505

Description of problem:
Boot up a guest with a image with parameter snapshot=on, readonly=on. Write in the image inside guest, then commit it to the backing files in monitor. The commit will success without any error message. But in fact the backing image is readonly and the commit is not saved in the image files.




Version-Release number of selected component (if applicable):
host kernel:  2.6.32-303.el6.x86_64
# rpm -qa |grep qemu
qemu-kvm-tools-0.12.1.2-2.309.el6.x86_64
gpxe-roms-qemu-0.9.7-6.9.el6.noarch
qemu-guest-agent-0.12.1.2-2.309.el6.x86_64
qemu-kvm-0.12.1.2-2.309.el6.x86_64
qemu-kvm-debuginfo-0.12.1.2-2.309.el6.x86_64
qemu-img-0.12.1.2-2.309.el6.x86_64


How reproducible:
always

Steps to Reproduce:
1. boot up guest with a data image and set the image parameter like this: ...,readonly=on,snapshot=on

2. dd a file in the guest to the data image set to readonly

3. do commit in monitor

  
Actual results:
commit will not return error message

Expected results:
commit return an error message mentioned that backing image is readonly

Additional info:
1. host cpu
processor	: 3
vendor_id	: GenuineIntel
cpu family	: 6
model		: 23
model name	: Intel(R) Core(TM)2 Quad CPU    Q9400  @ 2.66GHz
stepping	: 10
cpu MHz		: 1998.000
cache size	: 3072 KB
physical id	: 0
siblings	: 4
core id		: 2
cpu cores	: 4
apicid		: 2
initial apicid	: 2
fpu		: yes
fpu_exception	: yes
cpuid level	: 13
wp		: yes
flags		: fpu vme de pse tsc msr pae mce cx8 apic mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe syscall nx lm constant_tsc arch_perfmon pebs bts rep_good aperfmperf pni dtes64 monitor ds_cpl vmx smx est tm2 ssse3 cx16 xtpr pdcm sse4_1 xsave lahf_lm dts tpr_shadow vnmi flexpriority
bogomips	: 5333.08
clflush size	: 64
cache_alignment	: 64
address sizes	: 36 bits physical, 48 bits virtual
power management: