metadata: reorganize and add consistency checks

This commit adds in IsValid() checks for the metadata structures that
let us enforce stronger invariants than those imposed by the protobuf
package. The main uses of this will be to check that metadata is valid
before writing it to the filesystem, and to check that the filesystem
contains valid metadata before returning it to the user. These functions
also will log the exact reason if the validity checks fail.

To have these checks in the metadata package, all of the various
constants have been moved to a single metadata/constants.go file. The
uses of these constants were changed accordingly.

Finally, this commit standardizes our use of errors so that they always
begin with an appropriate prefix.

Change-Id: I99008e2ee803ebe5f6236eb8d83fc83efcd22718

1 files changed, 22 insertions, 24 deletions

diff --git a/metadata/policy.go b/metadata/policy.go
index 8c67f52..102c587 100644
--- a/metadata/policy.go
+++ b/metadata/policy.go
@@ -23,6 +23,8 @@ package metadata
 import (
 	"encoding/hex"
 	"errors"
+	"fmt"
+	"log"
 	"os"
 	"unsafe"
 
@@ -31,16 +33,14 @@ import (
 	"fscrypt/util"
 )
 
-// DescriptorLen is the length of all Protector and Policy descriptors.
-const DescriptorLen = 2 * unix.FS_KEY_DESCRIPTOR_SIZE
-
 // Encryption specific errors
 var (
-	ErrEncryptionNotSupported = errors.New("filesystem encryption is not supported")
-	ErrEncryptionDisabled     = errors.New("filesystem encryption has been disabled in the kernel config")
-	ErrNotEncrypted           = errors.New("file or directory not encrypted")
-	ErrEncrypted              = errors.New("file or directory already encrypted")
-	ErrBadEncryptionOptions   = errors.New("invalid encryption options provided")
+	prefix                    = "filesystem encryption: "
+	ErrEncryptionNotSupported = errors.New(prefix + "not supported")
+	ErrEncryptionDisabled     = errors.New(prefix + "disabled in the kernel config")
+	ErrNotEncrypted           = errors.New(prefix + "file or directory not encrypted")
+	ErrEncrypted              = errors.New(prefix + "file or directory already encrypted")
+	ErrBadEncryptionOptions   = errors.New(prefix + "invalid options provided")
 )
 
 // policyIoctl is a wrapper for the ioctl syscall. If opens the file at the path
@@ -76,13 +76,6 @@ func policyIoctl(path string, request uintptr, policy *unix.FscryptPolicy) error
 	}
 }
 
-// DefaultOptions use the only supported encryption modes and maximum padding.
-var DefaultOptions = &EncryptionOptions{
-	Padding:       32,
-	ContentsMode:  EncryptionMode_XTS,
-	FilenamesMode: EncryptionMode_CTS,
-}
-
 // Maps EncryptionOptions.Padding <-> FscryptPolicy.Flags
 var (
 	paddingArray = []int64{4, 8, 16, 32}
@@ -101,17 +94,20 @@ func GetPolicy(path string) (*PolicyData, error) {
 
 	// Convert the padding flag into an amount of padding
 	paddingFlag := int64(policy.Flags & unix.FS_POLICY_FLAGS_PAD_MASK)
+
+	// This lookup should always succeed
 	padding, ok := util.Lookup(paddingFlag, flagsArray, paddingArray)
 	if !ok {
-		return nil, util.SystemErrorF("invalid padding flag: %x", paddingFlag)
+		log.Printf("padding flag of %x not found", paddingFlag)
+		util.NeverError(util.SystemError("invalid padding flag"))
 	}
 
 	return &PolicyData{
 		KeyDescriptor: hex.EncodeToString(policy.Master_key_descriptor[:]),
 		Options: &EncryptionOptions{
-			Padding:       padding,
-			ContentsMode:  EncryptionMode(policy.Contents_encryption_mode),
-			FilenamesMode: EncryptionMode(policy.Filenames_encryption_mode),
+			Padding:   padding,
+			Contents:  EncryptionOptions_Mode(policy.Contents_encryption_mode),
+			Filenames: EncryptionOptions_Mode(policy.Filenames_encryption_mode),
 		},
 	}, nil
 }
@@ -123,23 +119,25 @@ func SetPolicy(path string, data *PolicyData) error {
 	// Convert the padding value to a flag
 	paddingFlag, ok := util.Lookup(data.Options.Padding, paddingArray, flagsArray)
 	if !ok {
-		return util.InvalidInputF("padding of %d", data.Options.Padding)
+		return util.InvalidInput(fmt.Sprintf("padding of %d", data.Options.Padding))
 	}
 
 	// Convert the policyDescriptor to a byte array
 	if len(data.KeyDescriptor) != DescriptorLen {
-		return util.InvalidLengthError("policy descriptor", DescriptorLen, len(data.KeyDescriptor))
+		return util.InvalidLengthError(
+			"policy descriptor", DescriptorLen, len(data.KeyDescriptor))
 	}
 
 	descriptorBytes, err := hex.DecodeString(data.KeyDescriptor)
 	if err != nil {
-		return util.InvalidInputF("policy descriptor of %s: %v", data.KeyDescriptor, err)
+		return util.InvalidInput(
+			fmt.Sprintf("policy descriptor of %s: %v", data.KeyDescriptor, err))
 	}
 
 	policy := unix.FscryptPolicy{
 		Version:                   0, // Version must always be zero
-		Contents_encryption_mode:  uint8(data.Options.ContentsMode),
-		Filenames_encryption_mode: uint8(data.Options.FilenamesMode),
+		Contents_encryption_mode:  uint8(data.Options.Contents),
+		Filenames_encryption_mode: uint8(data.Options.Filenames),
 		Flags: uint8(paddingFlag),
 	}
 	copy(policy.Master_key_descriptor[:], descriptorBytes)