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Re: [libvirt] [PATCH] (v2) avoid chowning domain devices if higer-level mgmt does it for us



On Wed, Nov 25, 2009 at 10:31:03AM +0100, Daniel Veillard wrote:
> On Wed, Nov 25, 2009 at 09:27:13AM +0200, Dan Kenigsberg wrote:
> > this is particularily important if said device is a file sitting on a
> > root_squashing nfs export.
> > 
> > my previous attempt for a patch missed 3 chowns that should be avoided.
> > ---
> >  src/qemu/qemu.conf     |    4 ++++
> >  src/qemu/qemu_conf.c   |    3 +++
> >  src/qemu/qemu_conf.h   |    1 +
> >  src/qemu/qemu_driver.c |    8 ++++----
> >  4 files changed, 12 insertions(+), 4 deletions(-)
> > 
> > diff --git a/src/qemu/qemu.conf b/src/qemu/qemu.conf
> > index bca858a..892a50b 100644
> > --- a/src/qemu/qemu.conf
> > +++ b/src/qemu/qemu.conf
> > @@ -96,6 +96,10 @@
> >  # The group ID for QEMU processes run by the system instance
> >  #group = "root"
> >  
> > +# should libvirt assume that devices are accessible to the above user:group.
> > +# by default, libvirt tries to chown devices before starting up a domain and
> > +# restore ownership to root when domain comes down.
> > +#assume_devices_accessible = 0
> >  
> >  # What cgroup controllers to make use of with QEMU guests
> >  #
> > diff --git a/src/qemu/qemu_conf.c b/src/qemu/qemu_conf.c
> > index b1b9e5f..520a395 100644
> > --- a/src/qemu/qemu_conf.c
> > +++ b/src/qemu/qemu_conf.c
> > @@ -232,6 +232,9 @@ int qemudLoadDriverConfig(struct qemud_driver *driver,
> >          return -1;
> >      }
> >  
> > +    p = virConfGetValue (conf, "assume_devices_accessible");
> > +    CHECK_TYPE ("assume_devices_accessible", VIR_CONF_LONG);
> > +    if (p) driver->avoid_dev_chown = p->l;
> 
>   an explicit initialization of the field would be better if p is NULL.
> 
> >      if (virGetGroupID(NULL, group, &driver->group) < 0) {
> >          VIR_FREE(group);
> > diff --git a/src/qemu/qemu_conf.h b/src/qemu/qemu_conf.h
> > index 675c636..3a9da73 100644
> > --- a/src/qemu/qemu_conf.h
> > +++ b/src/qemu/qemu_conf.h
> > @@ -87,6 +87,7 @@ struct qemud_driver {
> >  
> >      uid_t user;
> >      gid_t group;
> > +    int avoid_dev_chown;
> >  
> >      unsigned int qemuVersion;
> >      int nextvmid;
> > diff --git a/src/qemu/qemu_driver.c b/src/qemu/qemu_driver.c
> > index 2f273eb..5f02aa2 100644
> > --- a/src/qemu/qemu_driver.c
> > +++ b/src/qemu/qemu_driver.c
> > @@ -1968,7 +1968,7 @@ static int qemuDomainSetDeviceOwnership(virConnectPtr conn,
> >      uid_t uid;
> >      gid_t gid;
> >  
> > -    if (!driver->privileged)
> > +    if (!driver->privileged || driver->avoid_dev_chown)
> >          return 0;
> >  
> >      /* short circuit case of root:root */
> > @@ -2002,7 +2002,7 @@ static int qemuDomainSetAllDeviceOwnership(virConnectPtr conn,
> >      uid_t uid;
> >      gid_t gid;
> >  
> > -    if (!driver->privileged)
> > +    if (!driver->privileged || driver->avoid_dev_chown)
> >          return 0;
> >  
> >      /* short circuit case of root:root */
> > @@ -3438,7 +3438,7 @@ static int qemudDomainSave(virDomainPtr dom,
> >      }
> >      fd = -1;
> >  
> > -    if (driver->privileged &&
> > +    if (driver->privileged && !driver->avoid_dev_chown &&
> >          chown(path, driver->user, driver->group) < 0) {
> >          virReportSystemError(NULL, errno,
> >                               _("unable to set ownership of '%s' to user %d:%d"),
> > @@ -3473,7 +3473,7 @@ static int qemudDomainSave(virDomainPtr dom,
> >      if (rc < 0)
> >          goto endjob;
> >  
> > -    if (driver->privileged &&
> > +    if (driver->privileged && !driver->avoid_dev_chown &&
> >          chown(path, 0, 0) < 0) {
> >          virReportSystemError(NULL, errno,
> >                               _("unable to set ownership of '%s' to user %d:%d"),
> 
> The core question is having this as another manual user tweak, it always
> makes me a bit uncomfortable if proper working of software requires manually
> editing a config file. If we really need this kind of options for proper
> operations in specific conditions can we make sure it can be set via
> APIs too ? I don't think we should expose something as generic as the
> internal Conf APIs, but these runtime option in src/qemu/qemu.conf
> start to accumulate and I start to wonder how to properly manage all
> this.

Aren't we here to babysit software? :-)

For this particular behavior, I personally feel that the default is
wrong. I don't like it that libvirt takes the liberty to (try to) change
ownership, I like it less when it does not truely restore ownership, and
even less, that it fails to use files over nfs.

I understand that this behavior was meant to assist admins who upgrade
from running qemu as root to running it as a meager user. But personally
I believe that we could have politely asked them: if you use this new
feature, please make sure this new qemu user has access to your
files/devices. Now that there are people expecting current behavior, I
don't really see a nice way out.

Dan


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