validation: set the blkio rules on a block device that exists

validation/linux_cgroups_blkio/linux_cgroups_blkio.go

@@ -13,11 +13,10 @@ func testBlkioCgroups(rate uint64, isEmpty bool) error {
 	var weight uint16 = 500
 	var leafWeight uint16 = 300
 
-	// It's assumed that a device /dev/sda (8:0) exists on the test system.
-	// The minor number must be always 0, as it's only allowed to set blkio
-	// weights to a whole block device /dev/sda, not to partitions like sda1.
-	var major int64 = 8
-	var minor int64
+	major, minor, err := util.GetBlockDevice()
+	if err != nil {
+		return err
+	}
 
 	t := tap.New()
 	t.Header(0)

validation/linux_cgroups_relative_blkio/linux_cgroups_relative_blkio.go

@@ -9,9 +9,13 @@ import (
 func main() {
 	var weight uint16 = 500
 	var leafWeight uint16 = 300
-	var major, minor int64 = 8, 0
 	var rate uint64 = 102400
 
+	major, minor, err := util.GetBlockDevice()
+	if err != nil {
+		util.Fatal(err)
+	}
+
 	t := tap.New()
 	t.Header(0)
 	defer t.AutoPlan()

validation/util/linux_resources_blkio.go

@@ -1,13 +1,47 @@
 package util
 
 import (
+	"errors"
 	"fmt"
+	"os"
+	"path/filepath"
+	"strings"
 
 	"github.com/mndrix/tap-go"
 	rspec "github.com/opencontainers/runtime-spec/specs-go"
 	"github.com/opencontainers/runtime-tools/cgroups"
 )
 
+// GetBlockDevice returns the device number of a whole block device on
+// the host, as blkio device rules have to name one that exists. Devices
+// backed by hardware are preferred over loop and RAM disks, which may
+// have nothing behind them.
+func GetBlockDevice() (int64, int64, error) {
+	devs, err := filepath.Glob("/sys/block/*/dev")
+	if err != nil {
+		return 0, 0, err
+	}
+
+	for _, hardware := range []bool{true, false} {
+		for _, dev := range devs {
+			if _, err := os.Stat(filepath.Join(filepath.Dir(dev), "device")); hardware && err != nil {
+				continue
+			}
+			contents, err := os.ReadFile(dev)
+			if err != nil {
+				continue
+			}
+			var major, minor int64
+			if _, err := fmt.Sscanf(strings.TrimSpace(string(contents)), "%d:%d", &major, &minor); err != nil {
+				continue
+			}
+			return major, minor, nil
+		}
+	}
+
+	return 0, 0, errors.New("no block device found in /sys/block")
+}
+
 // ValidateLinuxResourcesBlockIO validates linux.resources.blockIO.
 func ValidateLinuxResourcesBlockIO(config *rspec.Spec, t *tap.T, state *rspec.State) error {
 	cg, err := cgroups.FindCgroup()