Let flags be overriden from OCI annotations

This allows runsc flags to be set per sandbox instance. For
example, K8s pod annotations can be used to enable
--debug for a single pod, making troubleshoot much easier.
Similarly, features like --vfs2 can be enabled for
experimentation without affecting other pods in the node.

Closes #3494

PiperOrigin-RevId: 329542815

3 files changed, 125 insertions, 0 deletions

diff --git a/runsc/config/config.go b/runsc/config/config.go
index bca27ebf1..df134bb2f 100644
--- a/runsc/config/config.go
+++ b/runsc/config/config.go
@@ -157,6 +157,8 @@ type Config struct {
 	// Enables FUSE usage.
 	FUSE bool `flag:"fuse"`
 
+	AllowFlagOverride bool `flag:"allow-flag-override"`
+
 	// TestOnlyAllowRunAsCurrentUserWithoutChroot should only be used in
 	// tests. It allows runsc to start the sandbox process as the current
 	// user, and without chrooting the sandbox process. This can be
diff --git a/runsc/config/config_test.go b/runsc/config/config_test.go
index af7867a2a..fb162b7eb 100644
--- a/runsc/config/config_test.go
+++ b/runsc/config/config_test.go
@@ -183,3 +183,90 @@ func TestValidationFail(t *testing.T) {
 		})
 	}
 }
+
+func TestOverride(t *testing.T) {
+	c, err := NewFromFlags()
+	if err != nil {
+		t.Fatal(err)
+	}
+	c.AllowFlagOverride = true
+
+	t.Run("string", func(t *testing.T) {
+		c.RootDir = "foobar"
+		if err := c.Override("root", "bar"); err != nil {
+			t.Fatalf("Override(root, bar) failed: %v", err)
+		}
+		defer setDefault("root")
+		if c.RootDir != "bar" {
+			t.Errorf("Override(root, bar) didn't work: %+v", c)
+		}
+	})
+
+	t.Run("bool", func(t *testing.T) {
+		c.Debug = true
+		if err := c.Override("debug", "false"); err != nil {
+			t.Fatalf("Override(debug, false) failed: %v", err)
+		}
+		defer setDefault("debug")
+		if c.Debug {
+			t.Errorf("Override(debug, false) didn't work: %+v", c)
+		}
+	})
+
+	t.Run("enum", func(t *testing.T) {
+		c.FileAccess = FileAccessShared
+		if err := c.Override("file-access", "exclusive"); err != nil {
+			t.Fatalf("Override(file-access, exclusive) failed: %v", err)
+		}
+		defer setDefault("file-access")
+		if c.FileAccess != FileAccessExclusive {
+			t.Errorf("Override(file-access, exclusive) didn't work: %+v", c)
+		}
+	})
+}
+
+func TestOverrideDisabled(t *testing.T) {
+	c, err := NewFromFlags()
+	if err != nil {
+		t.Fatal(err)
+	}
+	const errMsg = "flag override disabled"
+	if err := c.Override("root", "path"); err == nil || !strings.Contains(err.Error(), errMsg) {
+		t.Errorf("Override() wrong error: %v", err)
+	}
+}
+
+func TestOverrideError(t *testing.T) {
+	c, err := NewFromFlags()
+	if err != nil {
+		t.Fatal(err)
+	}
+	c.AllowFlagOverride = true
+	for _, tc := range []struct {
+		name  string
+		value string
+		error string
+	}{
+		{
+			name:  "invalid",
+			value: "valid",
+			error: `flag "invalid" not found`,
+		},
+		{
+			name:  "debug",
+			value: "invalid",
+			error: "error setting flag debug",
+		},
+		{
+			name:  "file-access",
+			value: "invalid",
+			error: "invalid file access type",
+		},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			if err := c.Override(tc.name, tc.value); err == nil || !strings.Contains(err.Error(), tc.error) {
+				t.Errorf("Override(%q, %q) wrong error: %v", tc.name, tc.value, err)
+			}
+		})
+	}
+}
diff --git a/runsc/config/flags.go b/runsc/config/flags.go
index 488a4b9fb..eff46e938 100644
--- a/runsc/config/flags.go
+++ b/runsc/config/flags.go
@@ -48,6 +48,7 @@ func RegisterFlags() {
 		flag.Bool("log-packets", false, "enable network packet logging.")
 		flag.String("debug-log-format", "text", "log format: text (default), json, or json-k8s.")
 		flag.Bool("alsologtostderr", false, "send log messages to stderr.")
+		flag.Bool("allow-flag-override", false, "allow OCI annotations (dev.gvisor.flag.<name>) to override flags for debugging.")
 
 		// Debugging flags: strace related
 		flag.Bool("strace", false, "enable strace.")
@@ -149,6 +150,41 @@ func (c *Config) ToFlags() []string {
 	return rv
 }
 
+// Override writes a new value to a flag.
+func (c *Config) Override(name string, value string) error {
+	if !c.AllowFlagOverride {
+		return fmt.Errorf("flag override disabled, use --allow-flag-override to enable it")
+	}
+
+	obj := reflect.ValueOf(c).Elem()
+	st := obj.Type()
+	for i := 0; i < st.NumField(); i++ {
+		f := st.Field(i)
+		fieldName, ok := f.Tag.Lookup("flag")
+		if !ok || fieldName != name {
+			// Not a flag field, or flag name doesn't match.
+			continue
+		}
+		fl := flag.CommandLine.Lookup(name)
+		if fl == nil {
+			// Flag must exist if there is a field match above.
+			panic(fmt.Sprintf("Flag %q not found", name))
+		}
+
+		// Use flag to convert the string value to the underlying flag type, using
+		// the same rules as the command-line for consistency.
+		if err := fl.Value.Set(value); err != nil {
+			return fmt.Errorf("error setting flag %s=%q: %w", name, value, err)
+		}
+		x := reflect.ValueOf(flag.Get(fl.Value))
+		obj.Field(i).Set(x)
+
+		// Validates the config again to ensure it's left in a consistent state.
+		return c.validate()
+	}
+	return fmt.Errorf("flag %q not found. Cannot set it to %q", name, value)
+}
+
 func getVal(field reflect.Value) string {
 	if str, ok := field.Addr().Interface().(fmt.Stringer); ok {
 		return str.String()