[libvirt] LXC broken on Linux >= 3.15

[Richard Weinberger]

Hi!

Kernel commit 23adbe12 ("fs,userns: Change inode_capable to capable_wrt_inode_uidgid")
uncovered a libvirt-lxc issue.
Starting with that commit the kernel correctly checks also the gid of an inode.

Sadly this change breaks libvirt-lxc in a way such that openpty() will always fail
with -EPERM within a container. Therefore ssh and other programs are no longer usable.

Libvirt's virLXCControllerSetupDevPTS() has a hardcoded mount
string for mounting devpts, namely "newinstance,ptmxmode=0666,mode=0620,gid=5",
devpts correctly translates the uid and gid while mounting but libvirt
mounts devpts _before_ setting up the uid/gid mappings.
Therefore the internal gid for the new devpts instance is still 5 instead the mapped gid
and the new check in the kernel will always fail.

We have two options to fix that:
a) virLXCControllerSetupDevPTS() translates the gid (5) by hand and passes the correct
value to devpts. (IMHO hacky)

b) We setup devpts and therefore also the consoles after installing the mappings.
This needs maybe a bit of work.
First I thought a trivial patch like the appended one will do it, but then libvirt
fails to start a guest with no further explanation. Maybe I've later the time to
investigate further.

What do you think?

Thanks,
//richard

diff --git a/src/lxc/lxc_controller.c b/src/lxc/lxc_controller.c
index 2d220eb..3435f42 100644
--- a/src/lxc/lxc_controller.c
+++ b/src/lxc/lxc_controller.c
@@ -2157,9 +2157,6 @@ virLXCControllerRun(virLXCControllerPtr ctrl)
     if (virLXCControllerSetupResourceLimits(ctrl) < 0)
         goto cleanup;

-    if (virLXCControllerSetupDevPTS(ctrl) < 0)
-        goto cleanup;
-
     if (virLXCControllerPopulateDevices(ctrl) < 0)
         goto cleanup;

@@ -2172,9 +2169,6 @@ virLXCControllerRun(virLXCControllerPtr ctrl)
     if (virLXCControllerSetupFuse(ctrl) < 0)
         goto cleanup;

-    if (virLXCControllerSetupConsoles(ctrl, containerTTYPaths) < 0)
-        goto cleanup;
-
     if (lxcSetPersonality(ctrl->def) < 0)
         goto cleanup;

@@ -2198,6 +2192,12 @@ virLXCControllerRun(virLXCControllerPtr ctrl)
     if (virLXCControllerSetupUserns(ctrl) < 0)
         goto cleanup;

+    if (virLXCControllerSetupDevPTS(ctrl) < 0)
+        goto cleanup;
+
+    if (virLXCControllerSetupConsoles(ctrl, containerTTYPaths) < 0)
+        goto cleanup;
+
     if (virLXCControllerMoveInterfaces(ctrl) < 0)
         goto cleanup;


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