userns: Fix running tests inside a userns

containerd creates a userns and inside there, it runs the critest tool.
However, in that setup, the length of containerd's userns is not the
whole UID space.

Let's verify that the length of the userns inside the pod, when we
created it with NamespaceMode_NODE (IOW, when not using a new userns for
the pod) is the same as outside the pod.

This works fine when contained itself runs inside a userns.

Signed-off-by: Rodrigo Campos <rodrigoca@microsoft.com>

pkg/validate/security_context_linux.go

@@ -24,6 +24,7 @@ import (
 	"os"
 	"os/exec"
 	"path/filepath"
+	"slices"
 	"strings"
 	"sync"
 	"syscall"
@@ -936,13 +937,29 @@ var _ = framework.KubeDescribe("Security Context", func() {
 				podID, podConfig = createNamespacePodSandbox(rc, namespaceOption, podName, podLogPath)
 				containerName := runUserNamespaceContainer(rc, ic, podID, podConfig)
 
-				// 4294967295 means that the entire range is available
+				// If this test is run inside a userns, we need to check the
+				// container userns is the same as the one we see outside.
 				expectedOutput := hostUsernsContent()
 				if expectedOutput == "" {
 					Fail("failed to get host userns content")
 				}
-				// 4294967295 means that the entire range is available
-				matchContainerOutputRe(podConfig, containerName, `\s+0\s+0\s+4294967295\n`)
+				// The userns mapping can have several lines, we match each of them.
+				for _, line := range strings.Split(expectedOutput, "\n") {
+					if line == "" {
+						continue
+					}
+					mapping := parseUsernsMappingLine(line)
+					if len(mapping) != 3 {
+						msg := fmt.Sprintf("slice: %#v, len: %v", mapping, len(mapping))
+						Fail("Unexpected host mapping line: " + msg)
+					}
+
+					// The uid_map file should match host (separated with
+					// spaces).
+					containerId, hostId, length := mapping[0], mapping[1], mapping[2]
+					regex := fmt.Sprintf(`\s+%v\s+%v\s+%v`, containerId, hostId, length)
+					matchContainerOutputRe(podConfig, containerName, regex)
+				}
 			})
 
 			It("runtime should fail if more than one mapping provided", func() {
@@ -1564,3 +1581,24 @@ func rootfsPath(info map[string]string) string {
 	// always exist.
 	return filepath.Join(cfg.StateDir, "../")
 }
+
+func hostUsernsContent() string {
+	uidMapPath := "/proc/self/uid_map"
+	uidMapContent, err := os.ReadFile(uidMapPath)
+	if err != nil {
+		return ""
+	}
+	return string(uidMapContent)
+}
+
+func parseUsernsMappingLine(line string) []string {
+	// The line format is:
+	// <container-id> <host-id> <length>
+	// But there could be a lot of spaces between the fields.
+	line = strings.TrimSpace(line)
+	m := strings.Split(line, " ")
+	m = slices.DeleteFunc(m, func(s string) bool {
+		return s == ""
+	})
+	return m
+}