CipherSizeToPlainSize: Handle illegal states

A file never gets a cipherSize <= HEADER_LEN in normal operation.
However, this can happen if header write it interrupted or the
underlying filesystem does not support fallocate.

Noticed while trying to store a CIPHERDIR in another gocryptfs mount
(gocryptfs does not support fallocate)

1 files changed, 10 insertions, 0 deletions

diff --git a/cryptfs/address_translation.go b/cryptfs/address_translation.go
index dfc6ef9..147040c 100644
--- a/cryptfs/address_translation.go
+++ b/cryptfs/address_translation.go
@@ -30,6 +30,16 @@ func (be *CryptFS) CipherSizeToPlainSize(cipherSize uint64) uint64 {
 		return 0
 	}
 
+	if cipherSize == HEADER_LEN {
+		Warn.Printf("cipherSize %d == header size: interrupted write?\n", cipherSize)
+		return 0
+	}
+
+	if cipherSize < HEADER_LEN {
+		Warn.Printf("cipherSize %d < header size: corrupt file\n", cipherSize)
+		return 0
+	}
+
 	// Block number at last byte
 	blockNo := be.CipherOffToBlockNo(cipherSize - 1)
 	blockCount := blockNo + 1