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Re: [libvirt] [PATCH 2/2] Do not generate security_model when fs driver is anything but 'path'



On Mon, Dec 26, 2011 at 02:49:55PM +0530, Deepak C Shetty wrote:
> On 12/22/2011 11:00 PM, Daniel P. Berrange wrote:
> >On Thu, Dec 22, 2011 at 10:24:57AM +0530, Deepak C Shetty wrote:
> >>On 12/21/2011 08:11 PM, Daniel P. Berrange wrote:
> >>>On Wed, Dec 21, 2011 at 11:17:17AM +0530, Deepak C Shetty wrote:
> >>>>QEMU does not support security_model for anything but 'path' fs driver type.
> >>>>Currently in libvirt, when security_model ( accessmode attribute) is not
> >>>>specified it auto-generates it irrespective of the fs driver type. Also
> >>>>when virt-manager (vmm) adds a new fs device with default security_model
> >>>>the input xml passed to libvirt does not contain accessmode attribute, but
> >>>>libvirt generates it as part of the virDomainDefine flow, which should
> >>>>only be done if fs driver is of type 'path', else not. This patch fixes
> >>>>these issues.
> >>>>
> >>>>Signed-off-by: Deepak C Shetty<deepakcs linux vnet ibm com>
> >>>>---
> >>>>
> >>>>  src/conf/domain_conf.c  |   13 +++++++++----
> >>>>  src/qemu/qemu_command.c |   15 +++++++++------
> >>>>  2 files changed, 18 insertions(+), 10 deletions(-)
> >>>>
> >>>>diff --git a/src/conf/domain_conf.c b/src/conf/domain_conf.c
> >>>>index 8b89a0b..2c91f82 100644
> >>>>--- a/src/conf/domain_conf.c
> >>>>+++ b/src/conf/domain_conf.c
> >>>>@@ -10019,10 +10019,15 @@ virDomainFSDefFormat(virBufferPtr buf,
> >>>>          return -1;
> >>>>      }
> >>>>
> >>>>-
> >>>>-    virBufferAsprintf(buf,
> >>>>-                      "<filesystem type='%s' accessmode='%s'>\n",
> >>>>-                      type, accessmode);
> >>>>+    if (def->fsdriver == VIR_DOMAIN_FS_DRIVER_TYPE_PATH ||
> >>>>+        def->fsdriver == VIR_DOMAIN_FS_DRIVER_TYPE_DEFAULT) {
> >>>>+        virBufferAsprintf(buf,
> >>>>+                          "<filesystem type='%s' accessmode='%s'>\n",
> >>>>+                          type, accessmode);
> >>>>+    } else {
> >>>>+        virBufferAsprintf(buf,
> >>>>+                          "<filesystem type='%s'>\n", type);
> >>>>+    }
> >>>No, this isn't right. We should *always* include the accessmode
> >>>in the XML. Only at time of use should we decide whether the
> >>>requested accessmode can be supported or not.
> >>fsdriver type 'handle' does not support 'accessmode', if we include it
> >>in the xml, wouldn't it be misleading at the xml level ?. Also when
> >>viewed from virsh/VMM, we do
> >>not want to show accessmode attribute, if user selected fs driver type
> >>as 'handle'. If we end up including accessmode in the xml always, it would
> >>be misleading there too, correct ?
> >You are missing my point. For every filesystem we pass through to
> >the guest, there is an access mode. Some drivers only support
> >one access mode, other drivers support several access modes. The
> >XML should show the access mode at all times, even if there is only
> >a choice of one mode for certain drivers. The fact that there is
> >only a choice of one mode, is an implementation detail of QEMU.
> >Individual driver impl details should not leak up into the XML
> >parsing code, which is why we should be dealing with this by
> >reporting an error in the QEMU driver.
> >
> >Daniel
> Hi Dan,
>     Its possible for fs driver to not support accessmode at all. fs
> driver type proxy
> and synthfs are examples, qemu already supports these today. Proxy
> driver is for
> passthru + TOCTOU vulnerability fix and synthfs help export
> host/hypervisor info
> to a trusted guest/domain. In both of these cases, accessmode does not make
> any sense. So i feel it should be ok for accessmode not to be supported for
> a particular fs driver type.
> 
>    For  the 'handle' fs driver type, qemu currently throws error if
> security_model
> is present when fs driver is handle, hence the need to check for
> anything but
> local/default fs driver in libvirt and not to generate
> security_model, else it would just
> result in qemu error and user won't be able to create a domain.
> 
>    Having said the above, I also think its logical to have
> accessmode for 'handle' case
> since the behaviour maps to 'passthru' mode, Aneesh can correct me
> here if i am
> wrong, but since qemu won't allow us to specify security_model when
> its handle type,
> i was thinking to support a 4th accessmode in libvirt called
> 'default' and set that in xml
> when fs driver is handle, will that work ? If acessmode is set to
> 'default" it also takes care
> of not generating the security_model when fs driver is handle in
> libvirt, and qemu
> cmdline will be a valid one. Ofcouse will add code libvirt to
> generate unsuppported error
> for anything but accessmode= 'default'  when fs driver is handle.
> 
> Pls let me know your comments, based on which will post v2.

The core requirement here is that the XML parsing/formatting is
not tied to specifics of the QEMU implementation. If certain
combinations of XML attributes do not make sense for QEMU, then
the QEMU driver code in libvirt should report errors. The XML
code should not care what combinations QEMU supports.


Daniel
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