[libvirt] [PATCH 1 of 4] [LXC] Detect support for NETNS in lxc driver initialization

[Dan Smith]

DL> You call
DL> clone(CLONE_NEWPID|CLONE_NEWNS|CLONE_NEWUTS|CLONE_NEWUSER|CLONE_NEWIPC|SIGCHLD|CLONE_NEWNET)

DL> When this call fails, you 'assume' netns is not compiled in.

No, actually, I do this:

     int flags = CLONE_NEWPID|CLONE_NEWNS|CLONE_NEWUTS|CLONE_NEWUSER|
                 CLONE_NEWIPC|SIGCHLD|extra_flags;

Where extra_flags=0 for the LXC detection and extra_flags=CLONE_NEWNET
for the NETNS detection.  See the lxcCheckContainerSupport() calls in
lxcProbe() and lxcCheckNetNsSupport().

DL> In your code, you launch the ip command and if it fails with a
DL> particular exit code, you 'assume' netns is not supported. Another
DL> assumption

The ip command returns a different error code for an invalid
subcommand than for a failure of a known subcommand.  That seems like
a pretty reasonable sentinel (and certainly better than scraping the
help output), IMHO.

DL> ... IMHO you should rely on the package dependencies/command
DL> version. Or if you absolutely want to detect that at startup,
DL> perhaps doing "ip link help | grep netns" is more secure :)

DV has already said he'd like to see it done dynamically at the driver
probe stage.  I'll let him comment on his preferred way of doing that.

-- 
Dan Smith
IBM Linux Technology Center
Open Hypervisor Team
email: danms at us.ibm.com
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