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[libvirt] [PATCH v2 16/17] vircgroupv2: use dummy process to workaround kernel bug with systemd



If some program is attached to cgroup and that cgroup is removed before
detaching the program it might not be freed and will remain in the
system until it's rebooted.

This would not be that big deal to workaround if we wouldn't use
machined to track our guests.  Systemd tries to be the nice guy and if
there is no process attached to TransientUnit it will destroy the unit
and remove the cgroup from system which will hit the kernel bug.

In order to prevent this behavior of systemd we can move another process
into the guest cgroup and the cgroup will remain active even if we kill
qemu process while destroying guest.  We can then detach our BPF program
from that cgroup and call machined terminate API to cleanup the cgroup.

Signed-off-by: Pavel Hrdina <phrdina redhat com>
---
 src/libvirt_private.syms      |  1 +
 src/lxc/lxc_cgroup.c          |  1 +
 src/qemu/qemu_cgroup.c        |  6 ++-
 src/util/vircgroup.c          | 16 ++++++++
 src/util/vircgroup.h          |  1 +
 src/util/vircgrouppriv.h      |  2 +
 src/util/vircgroupv2.c        |  2 +
 src/util/vircgroupv2devices.c | 70 ++++++++++++++++++++++++++++++++++-
 src/util/virsystemd.c         |  2 +-
 src/util/virsystemd.h         |  2 +
 10 files changed, 99 insertions(+), 4 deletions(-)

diff --git a/src/libvirt_private.syms b/src/libvirt_private.syms
index f1da5ede71..c22c81cebe 100644
--- a/src/libvirt_private.syms
+++ b/src/libvirt_private.syms
@@ -3031,6 +3031,7 @@ virSystemdCanHybridSleep;
 virSystemdCanSuspend;
 virSystemdCreateMachine;
 virSystemdGetMachineNameByPID;
+virSystemdHasMachined;
 virSystemdHasMachinedResetCachedValue;
 virSystemdMakeScopeName;
 virSystemdMakeSliceName;
diff --git a/src/lxc/lxc_cgroup.c b/src/lxc/lxc_cgroup.c
index d93a19d684..a8cef74bb9 100644
--- a/src/lxc/lxc_cgroup.c
+++ b/src/lxc/lxc_cgroup.c
@@ -455,6 +455,7 @@ virCgroupPtr virLXCCgroupCreate(virDomainDefPtr def,
                             nnicindexes, nicindexes,
                             def->resource->partition,
                             -1,
+                            LXC_STATE_DIR,
                             &cgroup) < 0)
         goto cleanup;
 
diff --git a/src/qemu/qemu_cgroup.c b/src/qemu/qemu_cgroup.c
index 4931fb6575..9374a799a1 100644
--- a/src/qemu/qemu_cgroup.c
+++ b/src/qemu/qemu_cgroup.c
@@ -946,6 +946,7 @@ qemuInitCgroup(virDomainObjPtr vm,
                             nnicindexes, nicindexes,
                             vm->def->resource->partition,
                             cfg->cgroupControllers,
+                            cfg->stateDir,
                             &priv->cgroup) < 0) {
         if (virCgroupNewIgnoreError())
             goto done;
@@ -1256,16 +1257,19 @@ int
 qemuRemoveCgroup(virDomainObjPtr vm)
 {
     qemuDomainObjPrivatePtr priv = vm->privateData;
+    int rc = 0;
 
     if (priv->cgroup == NULL)
         return 0; /* Not supported, so claim success */
 
+    rc = virCgroupRemove(priv->cgroup);
+
     if (virCgroupTerminateMachine(priv->machineName) < 0) {
         if (!virCgroupNewIgnoreError())
             VIR_DEBUG("Failed to terminate cgroup for %s", vm->def->name);
     }
 
-    return virCgroupRemove(priv->cgroup);
+    return rc;
 }
 
 
diff --git a/src/util/vircgroup.c b/src/util/vircgroup.c
index 9d284e9bf4..506cc49b9a 100644
--- a/src/util/vircgroup.c
+++ b/src/util/vircgroup.c
@@ -1105,6 +1105,7 @@ virCgroupNewMachineSystemd(const char *name,
                            int *nicindexes,
                            const char *partition,
                            int controllers,
+                           const char *stateDir,
                            virCgroupPtr *group)
 {
     int rv;
@@ -1151,6 +1152,16 @@ virCgroupNewMachineSystemd(const char *name,
         return -1;
     }
 
+    if (VIR_STRDUP((*group)->stateDir, stateDir) < 0) {
+        virCgroupFree(group);
+        return -1;
+    }
+
+    if (VIR_STRDUP((*group)->vmName, name) < 0) {
+        virCgroupFree(group);
+        return -1;
+    }
+
     if (virCgroupAddProcess(*group, pidleader) < 0) {
         virErrorPtr saved = virSaveLastError();
         virCgroupRemove(*group);
@@ -1233,6 +1244,7 @@ virCgroupNewMachine(const char *name,
                     int *nicindexes,
                     const char *partition,
                     int controllers,
+                    const char *stateDir,
                     virCgroupPtr *group)
 {
     int rv;
@@ -1249,6 +1261,7 @@ virCgroupNewMachine(const char *name,
                                          nicindexes,
                                          partition,
                                          controllers,
+                                         stateDir,
                                          group)) == 0)
         return 0;
 
@@ -1301,6 +1314,8 @@ virCgroupFree(virCgroupPtr *group)
     VIR_FREE((*group)->unified.placement);
 
     VIR_FREE((*group)->path);
+    VIR_FREE((*group)->stateDir);
+    VIR_FREE((*group)->vmName);
     VIR_FREE(*group);
 }
 
@@ -2897,6 +2912,7 @@ virCgroupNewMachine(const char *name ATTRIBUTE_UNUSED,
                     int *nicindexes ATTRIBUTE_UNUSED,
                     const char *partition ATTRIBUTE_UNUSED,
                     int controllers ATTRIBUTE_UNUSED,
+                    const char *stateDir ATTRIBUTE_UNUSED,
                     virCgroupPtr *group ATTRIBUTE_UNUSED)
 {
     virReportSystemError(ENXIO, "%s",
diff --git a/src/util/vircgroup.h b/src/util/vircgroup.h
index 372009de4a..0cd90d3651 100644
--- a/src/util/vircgroup.h
+++ b/src/util/vircgroup.h
@@ -98,6 +98,7 @@ int virCgroupNewMachine(const char *name,
                         int *nicindexes,
                         const char *partition,
                         int controllers,
+                        const char *stateDir,
                         virCgroupPtr *group)
     ATTRIBUTE_NONNULL(1) ATTRIBUTE_NONNULL(2)
     ATTRIBUTE_NONNULL(3);
diff --git a/src/util/vircgrouppriv.h b/src/util/vircgrouppriv.h
index 085fea375c..e0ccce88a0 100644
--- a/src/util/vircgrouppriv.h
+++ b/src/util/vircgrouppriv.h
@@ -62,6 +62,8 @@ typedef virCgroupV2Controller *virCgroupV2ControllerPtr;
 
 struct _virCgroup {
     char *path;
+    char *stateDir;
+    char *vmName;
 
     virCgroupBackendPtr backends[VIR_CGROUP_BACKEND_TYPE_LAST];
 
diff --git a/src/util/vircgroupv2.c b/src/util/vircgroupv2.c
index 44c4c7fc90..03efabe198 100644
--- a/src/util/vircgroupv2.c
+++ b/src/util/vircgroupv2.c
@@ -37,6 +37,8 @@
 #include "virerror.h"
 #include "virfile.h"
 #include "virlog.h"
+#include "virpidfile.h"
+#include "virprocess.h"
 #include "virstring.h"
 #include "virsystemd.h"
 
diff --git a/src/util/vircgroupv2devices.c b/src/util/vircgroupv2devices.c
index a481ba89b7..9d8fc5557e 100644
--- a/src/util/vircgroupv2devices.c
+++ b/src/util/vircgroupv2devices.c
@@ -31,10 +31,15 @@
 #include "vircgrouppriv.h"
 
 #include "virbpf.h"
+#include "vircommand.h"
 #include "vircgroup.h"
 #include "vircgroupv2devices.h"
 #include "virfile.h"
 #include "virlog.h"
+#include "virpidfile.h"
+#include "virprocess.h"
+#include "virstring.h"
+#include "virsystemd.h"
 
 VIR_LOG_INIT("util.cgroup");
 
@@ -365,17 +370,74 @@ virCgroupV2DevicesReallocMap(int mapfd,
 }
 
 
+static pid_t
+virCgroupV2DevicesStartDummyProc(virCgroupPtr group)
+{
+    pid_t ret = -1;
+    pid_t pid = -1;
+    virCommandPtr cmd = NULL;
+    VIR_AUTOFREE(char *) pidfile = NULL;
+    VIR_AUTOFREE(char *) fileName = NULL;
+
+    if (virAsprintf(&fileName, "cgroup-%s", group->vmName) < 0)
+        return -1;
+
+    pidfile = virPidFileBuildPath(group->stateDir, fileName);
+    if (!pidfile)
+        return -1;
+
+    cmd = virCommandNewArgList("/usr/bin/tail", "-f", "/dev/null",
+                               "-s", "3600", NULL);
+    if (!cmd)
+        return -1;
+
+    virCommandDaemonize(cmd);
+    virCommandSetPidFile(cmd, pidfile);
+
+    if (virCommandRun(cmd, NULL) < 0)
+        goto cleanup;
+
+    if (virPidFileReadPath(pidfile, &pid) < 0) {
+        virReportError(VIR_ERR_INTERNAL_ERROR, "%s",
+                       _("failed to start dummy cgroup helper"));
+        goto cleanup;
+    }
+
+    if (virCgroupAddMachineProcess(group, pid) < 0)
+        goto cleanup;
+
+    ret = pid;
+ cleanup:
+    if (ret < 0) {
+        virCommandAbort(cmd);
+        if (pid > 0)
+            virProcessKillPainfully(pid, true);
+    }
+    if (pidfile)
+        unlink(pidfile);
+    virCommandFree(cmd);
+    return ret;
+}
+
+
 int
 virCgroupV2DevicesCreateProg(virCgroupPtr group)
 {
-    int mapfd;
+    int mapfd = -1;
+    pid_t pid = -1;
 
     if (group->unified.devices.progfd > 0 && group->unified.devices.mapfd > 0)
         return 0;
 
+    if (virSystemdHasMachined() == 0) {
+        pid = virCgroupV2DevicesStartDummyProc(group);
+        if (pid < 0)
+            return -1;
+    }
+
     mapfd = virCgroupV2DevicesCreateMap(VIR_CGROUP_V2_INITIAL_BPF_MAP_SIZE);
     if (mapfd < 0)
-        return -1;
+        goto error;
 
     if (virCgroupV2DevicesAttachProg(group, mapfd,
                                      VIR_CGROUP_V2_INITIAL_BPF_MAP_SIZE) < 0) {
@@ -385,6 +447,7 @@ virCgroupV2DevicesCreateProg(virCgroupPtr group)
     return 0;
 
  error:
+    virProcessKillPainfully(pid, true);
     VIR_FORCE_CLOSE(mapfd);
     return -1;
 }
@@ -453,6 +516,9 @@ virCgroupV2DevicesRemoveProg(virCgroupPtr group)
     if (group->unified.devices.progfd >= 0)
         VIR_FORCE_CLOSE(group->unified.devices.progfd);
 
+    if (virSystemdHasMachined() == 0)
+        virCgroupKillPainfully(group);
+
     ret = 0;
 
  cleanup:
diff --git a/src/util/virsystemd.c b/src/util/virsystemd.c
index f492ac1859..ca35b12a5c 100644
--- a/src/util/virsystemd.c
+++ b/src/util/virsystemd.c
@@ -138,7 +138,7 @@ void virSystemdHasMachinedResetCachedValue(void)
  * -1 = error
  *  0 = machine1 is available
  */
-static int
+int
 virSystemdHasMachined(void)
 {
     int ret;
diff --git a/src/util/virsystemd.h b/src/util/virsystemd.h
index 7d9c0ebd62..553d4196f1 100644
--- a/src/util/virsystemd.h
+++ b/src/util/virsystemd.h
@@ -51,4 +51,6 @@ int virSystemdCanHybridSleep(bool *result);
 
 char *virSystemdGetMachineNameByPID(pid_t pid);
 
+int virSystemdHasMachined(void);
+
 #endif /* LIBVIRT_VIRSYSTEMD_H */
-- 
2.20.1


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