beacon/config: fix LoadForks with non-string values (#32609)

Fixes a crash when loading the beacon chain config if new fields like
`BLOB_SCHEDULE: []` are present.
Previously, the config loader assumed all values were strings, causing
errors such as:

```
Fatal: Could not load beacon chain config '/network-configs/config.yaml': failed to parse beacon chain config file: yaml: unmarshal errors:
line 242: cannot unmarshal !!seq into string
```

This PR updates the parsing logic to handle non-string values correctly
and adds explicit validation for fork fields.

beacon/params/config.go

@@ -20,6 +20,7 @@ import (
 	"crypto/sha256"
 	"fmt"
 	"math"
+	"math/big"
 	"os"
 	"slices"
 	"sort"
@@ -90,12 +91,8 @@ func (c *ChainConfig) AddFork(name string, epoch uint64, version []byte) *ChainC
 
 // LoadForks parses the beacon chain configuration file (config.yaml) and extracts
 // the list of forks.
-func (c *ChainConfig) LoadForks(path string) error {
-	file, err := os.ReadFile(path)
-	if err != nil {
-		return fmt.Errorf("failed to read beacon chain config file: %v", err)
-	}
-	config := make(map[string]string)
+func (c *ChainConfig) LoadForks(file []byte) error {
+	config := make(map[string]any)
 	if err := yaml.Unmarshal(file, &config); err != nil {
 		return fmt.Errorf("failed to parse beacon chain config file: %v", err)
 	}
@@ -108,18 +105,36 @@ func (c *ChainConfig) LoadForks(path string) error {
 	for key, value := range config {
 		if strings.HasSuffix(key, "_FORK_VERSION") {
 			name := key[:len(key)-len("_FORK_VERSION")]
-			if v, err := hexutil.Decode(value); err == nil {
+			switch version := value.(type) {
+			case int:
+				versions[name] = new(big.Int).SetUint64(uint64(version)).FillBytes(make([]byte, 4))
+			case uint64:
+				versions[name] = new(big.Int).SetUint64(version).FillBytes(make([]byte, 4))
+			case string:
+				v, err := hexutil.Decode(version)
+				if err != nil {
+					return fmt.Errorf("failed to decode hex fork id %q in beacon chain config file: %v", version, err)
+				}
 				versions[name] = v
-			} else {
-				return fmt.Errorf("failed to decode hex fork id %q in beacon chain config file: %v", value, err)
+			default:
+				return fmt.Errorf("invalid fork version %q in beacon chain config file", version)
 			}
 		}
 		if strings.HasSuffix(key, "_FORK_EPOCH") {
 			name := key[:len(key)-len("_FORK_EPOCH")]
-			if v, err := strconv.ParseUint(value, 10, 64); err == nil {
+			switch epoch := value.(type) {
+			case int:
+				epochs[name] = uint64(epoch)
+			case uint64:
+				epochs[name] = epoch
+			case string:
+				v, err := strconv.ParseUint(epoch, 10, 64)
+				if err != nil {
+					return fmt.Errorf("failed to parse epoch number %q in beacon chain config file: %v", epoch, err)
+				}
 				epochs[name] = v
-			} else {
-				return fmt.Errorf("failed to parse epoch number %q in beacon chain config file: %v", value, err)
+			default:
+				return fmt.Errorf("invalid fork epoch %q in beacon chain config file", epoch)
 			}
 		}
 	}

beacon/params/config_test.go

@@ -0,0 +1,34 @@
+package params
+
+import (
+	"bytes"
+	"testing"
+)
+
+func TestChainConfig_LoadForks(t *testing.T) {
+	const config = `
+GENESIS_FORK_VERSION: 0x00000000
+
+ALTAIR_FORK_VERSION: 0x00000001
+ALTAIR_FORK_EPOCH: 1
+
+EIP7928_FORK_VERSION: 0xb0000038
+EIP7928_FORK_EPOCH: 18446744073709551615
+
+BLOB_SCHEDULE: []
+`
+	c := &ChainConfig{}
+	err := c.LoadForks([]byte(config))
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	for _, fork := range c.Forks {
+		if fork.Name == "GENESIS" && (fork.Epoch != 0) {
+			t.Errorf("unexpected genesis fork epoch %d", fork.Epoch)
+		}
+		if fork.Name == "ALTAIR" && (fork.Epoch != 1 || !bytes.Equal(fork.Version, []byte{0, 0, 0, 1})) {
+			t.Errorf("unexpected altair fork epoch %d version %x", fork.Epoch, fork.Version)
+		}
+	}
+}

cmd/utils/flags.go

@@ -1931,11 +1931,15 @@ func MakeBeaconLightConfig(ctx *cli.Context) bparams.ClientConfig {
 		} else {
 			Fatalf("Could not parse --%s: %v", BeaconGenesisRootFlag.Name, err)
 		}
-		configFile := ctx.String(BeaconConfigFlag.Name)
-		if err := config.ChainConfig.LoadForks(configFile); err != nil {
-			Fatalf("Could not load beacon chain config '%s': %v", configFile, err)
+		configPath := ctx.String(BeaconConfigFlag.Name)
+		file, err := os.ReadFile(configPath)
+		if err != nil {
+			Fatalf("failed to read beacon chain config file '%s': %v", configPath, err)
 		}
-		log.Info("Using custom beacon chain config", "file", configFile)
+		if err := config.ChainConfig.LoadForks(file); err != nil {
+			Fatalf("Could not load beacon chain config '%s': %v", configPath, err)
+		}
+		log.Info("Using custom beacon chain config", "file", configPath)
 	} else {
 		if ctx.IsSet(BeaconGenesisRootFlag.Name) {
 			Fatalf("Genesis root is specified but custom beacon chain config is missing")