fusefrontend_reverse: fix 176-byte names

A file with a name of exactly 176 bytes length caused this error:

  ls: cannot access ./tmp/dsg/sXSGJLTuZuW1FarwIkJs0w/b6mGjdxIRpaeanTo0rbh0A/QjMRrQZC_4WLhmHI1UOBcA/gocryptfs.longname.QV-UipdDXeUVdl05WruoEzBNPrQCfpu6OzJL0_QnDKY: No such file or directory
  ls: cannot access ./tmp/dsg/sXSGJLTuZuW1FarwIkJs0w/b6mGjdxIRpaeanTo0rbh0A/QjMRrQZC_4WLhmHI1UOBcA/gocryptfs.longname.QV-UipdDXeUVdl05WruoEzBNPrQCfpu6OzJL0_QnDKY.name: No such file or directory
  -????????? ? ?     ?             ?            ? gocryptfs.longname.QV-UipdDXeUVdl05WruoEzBNPrQCfpu6OzJL0_QnDKY
  -????????? ? ?     ?             ?            ? gocryptfs.longname.QV-UipdDXeUVdl05WruoEzBNPrQCfpu6OzJL0_QnDKY.name

Root cause was a wrong shortNameMax constant that failed to
account for the obligatory padding byte.

Fix the constant and also expand the TestLongnameStat test case
to test ALL file name lengths from 1-255 bytes.

Fixes https://github.com/rfjakob/gocryptfs/issues/143 .

1 files changed, 9 insertions, 2 deletions

diff --git a/internal/fusefrontend_reverse/reverse_longnames.go b/internal/fusefrontend_reverse/reverse_longnames.go
index ec164f1..3bfe3af 100644
--- a/internal/fusefrontend_reverse/reverse_longnames.go
+++ b/internal/fusefrontend_reverse/reverse_longnames.go
@@ -17,7 +17,14 @@ import (
 )
 
 const (
-	shortNameMax = 176
+	// File names are padded to 16-byte multiples, encrypted and
+	// base64-encoded. We can encode at most 176 bytes to stay below the 255
+	// bytes limit:
+	// * base64(176 bytes) = 235 bytes
+	// * base64(192 bytes) = 256 bytes (over 255!)
+	// But the PKCS#7 padding is at least one byte. This means we can only use
+	// 175 bytes for the file name.
+	shortNameMax = 175
 )
 
 var longnameParentCache map[string]string
@@ -57,6 +64,7 @@ func (rfs *ReverseFS) findLongnameParent(dir string, dirIV []byte, longname stri
 	}
 	dirEntries, err := dirfd.Readdirnames(-1)
 	if err != nil {
+		tlog.Warn.Printf("findLongnameParent: Readdirnames failed: %v\n", err)
 		return "", err
 	}
 	longnameCacheLock.Lock()
@@ -78,7 +86,6 @@ func (rfs *ReverseFS) findLongnameParent(dir string, dirIV []byte, longname stri
 	if hit == "" {
 		return "", syscall.ENOENT
 	}
-
 	return hit, nil
 }