Support running podman under a root v2 cgroup

Signed-off-by: Jason T. Greene <jason.greene@redhat.com>

libpod/container_internal_linux.go

@@ -3108,7 +3108,7 @@ func (c *Container) getOCICgroupPath() (string, error) {
 	case c.config.NoCgroups:
 		return "", nil
 	case c.config.CgroupsMode == cgroupSplit:
-		selfCgroup, err := utils.GetOwnCgroup()
+		selfCgroup, err := utils.GetOwnCgroupDisallowRoot()
 		if err != nil {
 			return "", err
 		}

pkg/specgen/generate/validate.go

@@ -1,6 +1,7 @@
 package generate
 
 import (
+	"io/ioutil"
 	"os"
 	"path/filepath"
 
@@ -166,6 +167,14 @@ func verifyContainerResourcesCgroupV2(s *specgen.SpecGenerator) ([]string, error
 		if err != nil {
 			return warnings, err
 		}
+
+		if own == "/" {
+			// If running under the root cgroup try to create or reuse a "probe" cgroup to read memory values
+			own = "podman_probe"
+			_ = os.MkdirAll(filepath.Join("/sys/fs/cgroup", own), 0o755)
+			_ = ioutil.WriteFile("/sys/fs/cgroup/cgroup.subtree_control", []byte("+memory"), 0o644)
+		}
+
 		memoryMax := filepath.Join("/sys/fs/cgroup", own, "memory.max")
 		memorySwapMax := filepath.Join("/sys/fs/cgroup", own, "memory.swap.max")
 		_, errMemoryMax := os.Stat(memoryMax)

utils/testdata/cgroup.other

@@ -0,0 +1 @@
+0::/other

utils/testdata/cgroup.root

@@ -0,0 +1 @@
+0::/

utils/utils_supported.go

@@ -64,7 +64,7 @@ func RunUnderSystemdScope(pid int, slice string, unitName string) error {
 	return nil
 }
 
-func getCgroupProcess(procFile string) (string, error) {
+func getCgroupProcess(procFile string, allowRoot bool) (string, error) {
 	f, err := os.Open(procFile)
 	if err != nil {
 		return "", err
@@ -72,7 +72,7 @@ func getCgroupProcess(procFile string) (string, error) {
 	defer f.Close()
 
 	scanner := bufio.NewScanner(f)
-	cgroup := "/"
+	cgroup := ""
 	for scanner.Scan() {
 		line := scanner.Text()
 		parts := strings.SplitN(line, ":", 3)
@@ -87,7 +87,7 @@ func getCgroupProcess(procFile string) (string, error) {
 			cgroup = parts[2]
 		}
 	}
-	if cgroup == "/" {
+	if len(cgroup) == 0 || (!allowRoot && cgroup == "/") {
 		return "", errors.Errorf("could not find cgroup mount in %q", procFile)
 	}
 	return cgroup, nil
@@ -95,12 +95,16 @@ func getCgroupProcess(procFile string) (string, error) {
 
 // GetOwnCgroup returns the cgroup for the current process.
 func GetOwnCgroup() (string, error) {
-	return getCgroupProcess("/proc/self/cgroup")
+	return getCgroupProcess("/proc/self/cgroup", true)
+}
+
+func GetOwnCgroupDisallowRoot() (string, error) {
+	return getCgroupProcess("/proc/self/cgroup", false)
 }
 
 // GetCgroupProcess returns the cgroup for the specified process process.
 func GetCgroupProcess(pid int) (string, error) {
-	return getCgroupProcess(fmt.Sprintf("/proc/%d/cgroup", pid))
+	return getCgroupProcess(fmt.Sprintf("/proc/%d/cgroup", pid), true)
 }
 
 // MoveUnderCgroupSubtree moves the PID under a cgroup subtree.

utils/utils_test.go

@@ -0,0 +1,26 @@
+//go:build linux || darwin
+// +build linux darwin
+
+package utils
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestCgroupProcess(t *testing.T) {
+	val, err := getCgroupProcess("testdata/cgroup.root", true)
+	assert.Nil(t, err)
+	assert.Equal(t, "/", val)
+
+	_, err = getCgroupProcess("testdata/cgroup.root", false)
+	assert.NotNil(t, err)
+
+	val, err = getCgroupProcess("testdata/cgroup.other", true)
+	assert.Nil(t, err)
+	assert.Equal(t, "/other", val)
+
+	_, err = getCgroupProcess("testdata/cgroup.empty", true)
+	assert.NotNil(t, err)
+}

utils/utils_windows.go

@@ -17,6 +17,10 @@ func GetOwnCgroup() (string, error) {
 	return "", errors.New("not implemented for windows")
 }
 
+func GetOwnCgroupDisallowRoot() (string, error) {
+	return "", errors.New("not implemented for windows")
+}
+
 func GetCgroupProcess(pid int) (string, error) {
 	return "", errors.New("not implemented for windows")
 }