chore: module-sig-verify cleanup

Make module-sig-verify code more idiomatic.

Co-authored-by: Andrey Smirnov <andrey.smirnov@talos-systems.com>
Signed-off-by: Noel Georgi <git@frezbo.dev>
(cherry picked from commit 07bb61e)

hack/module-sig-verify/main.go

@@ -11,12 +11,11 @@ import (
 	"crypto/rsa"
 	"crypto/sha512"
 	"crypto/x509"
-	"encoding/hex"
+	"encoding/binary"
 	"flag"
 	"fmt"
 	"io"
 	"os"
-	"strconv"
 
 	"go.mozilla.org/pkcs7"
 )
@@ -36,8 +35,10 @@ import (
 const (
 	// SignedModuleMagic is the magic string appended to the end of a signed module.
 	SignedModuleMagic = "~Module signature appended~\n"
+	// SignedModuleMagicLength is the length of the magic string.
+	SignedModuleMagicLength = int64(len(SignedModuleMagic))
 	// ModuleSignatureInfoLength is the length of the signature info.
-	ModuleSignatureInfoLength = 12
+	ModuleSignatureInfoLength int64 = 12
 )
 
 var (
@@ -72,56 +73,58 @@ func main() {
 		fmt.Println(err)
 		os.Exit(1)
 	}
+	defer moduleData.Close() //nolint:errcheck
 
 	if err := verifyModule(crt, moduleData); err != nil {
 		fmt.Println(err)
-		os.Exit(1)
+		os.Exit(1) //nolint:gocritic
 	}
 }
 
-func parseModuleInput(module string) (*bytes.Reader, error) {
+type noOPCloser struct {
+	io.ReadSeeker
+}
+
+func (noOPCloser) Close() error { return nil }
+
+func parseModuleInput(module string) (io.ReadSeekCloser, error) {
 	if module == "-" {
 		moduleData, err := io.ReadAll(os.Stdin)
 		if err != nil {
 			return nil, fmt.Errorf("failed to read module from stdin: %w", err)
 		}
 
-		return bytes.NewReader(moduleData), nil
+		return noOPCloser{bytes.NewReader(moduleData)}, nil
 	}
 
-	moduleData, err := os.ReadFile(module)
+	moduleData, err := os.Open(module)
 	if err != nil {
 		return nil, fmt.Errorf("failed to open file %s: %w", module, err)
 	}
 
-	return bytes.NewReader(moduleData), nil
+	return moduleData, nil
 }
 
-func verifyModule(crt *x509.Certificate, moduleData *bytes.Reader) error {
-	fileLen := moduleData.Size()
-	signedModuleMagicStart := fileLen - int64(len(SignedModuleMagic))
-
-	_, err := moduleData.Seek(signedModuleMagicStart, 0)
+func verifyModule(crt *x509.Certificate, moduleData io.ReadSeeker) error {
+	_, err := moduleData.Seek(-SignedModuleMagicLength, io.SeekEnd)
 	if err != nil {
-		return fmt.Errorf("failed to seek to %d in file %s: %w", signedModuleMagicStart, module, err)
+		return fmt.Errorf("failed to seek to %d in file %s: %w", -SignedModuleMagicLength, module, err)
 	}
 
 	magicBytes := make([]byte, len(SignedModuleMagic))
 
 	_, err = moduleData.Read(magicBytes)
 	if err != nil {
-		return fmt.Errorf("failed to read %d bytes from file %s: %w", len(SignedModuleMagic), module, err)
+		return fmt.Errorf("failed to read %d bytes from file %s: %w", SignedModuleMagicLength, module, err)
 	}
 
 	if string(magicBytes) != SignedModuleMagic {
 		return fmt.Errorf("file %s is not a signed module", module)
 	}
 
-	signatureInfoStart := signedModuleMagicStart - ModuleSignatureInfoLength
-
-	_, err = moduleData.Seek(signatureInfoStart, 0)
+	_, err = moduleData.Seek(-SignedModuleMagicLength-ModuleSignatureInfoLength, io.SeekCurrent)
 	if err != nil {
-		return fmt.Errorf("failed to seek to %d in file %s: %w", signatureInfoStart, module, err)
+		return fmt.Errorf("failed to seek to %d in file %s: %w", -SignedModuleMagicLength-ModuleSignatureInfoLength, module, err)
 	}
 
 	signatureBytes := make([]byte, ModuleSignatureInfoLength)
@@ -131,17 +134,13 @@ func verifyModule(crt *x509.Certificate, moduleData *bytes.Reader) error {
 		return fmt.Errorf("failed to read %d bytes from file %s: %w", ModuleSignatureInfoLength, module, err)
 	}
 
-	// The signature length is encoded in the last 2 bytes of the signature info.
-	signatureLength, err := strconv.ParseInt(hex.EncodeToString(signatureBytes[(len(signatureBytes)-2):]), 16, 64)
-	if err != nil {
-		return fmt.Errorf("failed to parse signature length %w", err)
-	}
+	// The signature length is encoded in the last 4 bytes of the signature info.
+	// https://github.com/torvalds/linux/blob/master/scripts/sign-file.c#L62-L70
+	signatureLength := int64(binary.BigEndian.Uint32(signatureBytes[(len(signatureBytes) - 4):]))
 
-	signatureStart := signatureInfoStart - signatureLength
-
-	_, err = moduleData.Seek(signatureStart, 0)
+	_, err = moduleData.Seek(-ModuleSignatureInfoLength-signatureLength, io.SeekCurrent)
 	if err != nil {
-		return fmt.Errorf("failed to seek to %d in file %s: %w", signatureStart, module, err)
+		return fmt.Errorf("failed to seek to %d in file %s: %w", -ModuleSignatureInfoLength-signatureLength, module, err)
 	}
 
 	signature := make([]byte, signatureLength)
@@ -151,26 +150,16 @@ func verifyModule(crt *x509.Certificate, moduleData *bytes.Reader) error {
 		return fmt.Errorf("failed to read %d bytes from file %s: %w", signatureLength, module, err)
 	}
 
-	moduleWithSignatureLength := fileLen - int64(len(SignedModuleMagic))
-
-	_, err = moduleData.Seek(0, 0)
+	unsignedModuleLength, err := moduleData.Seek(-signatureLength, io.SeekCurrent)
 	if err != nil {
 		return fmt.Errorf("failed to seek to %d in file %s: %w", 0, module, err)
 	}
 
-	moduleWithSignature := make([]byte, moduleWithSignatureLength)
-
-	_, err = moduleData.Read(moduleWithSignature)
-	if err != nil {
-		return fmt.Errorf("failed to read %d bytes from file %s: %w", moduleWithSignatureLength, module, err)
-	}
-
 	_, err = moduleData.Seek(0, 0)
 	if err != nil {
 		return fmt.Errorf("failed to seek to %d in file %s: %w", 0, module, err)
 	}
 
-	unsignedModuleLength := fileLen - signatureLength - ModuleSignatureInfoLength - int64(len(SignedModuleMagic))
 	unsignedModuleData := make([]byte, unsignedModuleLength)
 
 	_, err = moduleData.Read(unsignedModuleData)