[libvirt] [PATCH 02/53] vircgroup: introduce virCgroupV2Available

[Pavel Hrdina]

We cannot detect only mount points to figure out whether cgroup v2
is available because systemd uses cgroup v2 for process tracking and
all controllers are mounted as cgroup v1 controllers.

To make sure that this is no the situation we need to check
'cgroup.controllers' file if it's not empty to make sure that cgroup
v2 is not mounted only for process tracking.

Signed-off-by: Pavel Hrdina <phrdina at redhat.com>
---
 src/util/vircgroupv2.c | 52 ++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 52 insertions(+)

diff --git a/src/util/vircgroupv2.c b/src/util/vircgroupv2.c
index 23bf81dae2..78497bd31d 100644
--- a/src/util/vircgroupv2.c
+++ b/src/util/vircgroupv2.c
@@ -19,6 +19,10 @@
  */
 #include <config.h>
 
+#ifdef __linux__
+# include <mntent.h>
+#endif /* __linux__ */
+
 #include "internal.h"
 
 #define __VIR_CGROUP_ALLOW_INCLUDE_PRIV_H__
@@ -28,7 +32,9 @@
 #include "vircgroup.h"
 #include "vircgroupbackend.h"
 #include "vircgroupv2.h"
+#include "virfile.h"
 #include "virlog.h"
+#include "virstring.h"
 
 VIR_LOG_INIT("util.cgroup");
 
@@ -41,8 +47,54 @@ VIR_ENUM_IMPL(virCgroupV2Controller, VIR_CGROUP_CONTROLLER_LAST,
 
 #ifdef __linux__
 
+/* We're looking for one 'cgroup2' fs mount which has some
+ * controllers enabled. */
+static bool
+virCgroupV2Available(void)
+{
+    bool ret = false;
+    FILE *mounts = NULL;
+    struct mntent entry;
+    char buf[CGROUP_MAX_VAL];
+
+    if (!(mounts = fopen("/proc/mounts", "r")))
+        return false;
+
+    while (getmntent_r(mounts, &entry, buf, sizeof(buf)) != NULL) {
+        if (STREQ(entry.mnt_type, "cgroup2")) {
+            ret = true;
+            break;
+        }
+    }
+
+    /* Systemd uses cgroup v2 for process tracking but no controller is
+     * available. We should consider this configuration as cgroup v2 is
+     * not available. */
+    if (ret) {
+        int rc;
+        VIR_AUTOFREE(char *) contFile = NULL;
+        VIR_AUTOFREE(char *) contStr = NULL;
+
+        if (virAsprintf(&contFile, "%s/cgroup.controllers", entry.mnt_dir) < 0)
+            return false;
+
+        rc = virFileReadAll(contFile, 1024 * 1024, &contStr);
+        if (rc < 0)
+            return false;
+
+        if (STREQ(contStr, ""))
+            return false;
+    }
+
+    VIR_FORCE_FCLOSE(mounts);
+    return ret;
+}
+
+
 virCgroupBackend virCgroupV2Backend = {
     .type = VIR_CGROUP_BACKEND_TYPE_V2,
+
+    .available = virCgroupV2Available,
 };
 
 
-- 
2.17.1