storage: Fix capacity value for LUKS encrypted volumes

https://bugzilla.redhat.com/show_bug.cgi?id=1371892

The 'capacity' value (e.g. guest logical size) for a LUKS volume is
smaller than the 'physical' value of the file in the file system, so
we need to account for that.

When peeking at the encryption information about the volume add a fetch
of the payload_offset which is described as the offset to the start of
the volume data (in 512 byte sectors) in QEMU's QCryptoBlockLUKSHeader.

Then adjust the ->capacity appropriately when we determine that the
volume target encryption has a payload_offset value.

src/storage/storage_util.c

@@ -3436,6 +3436,9 @@ storageBackendProbeTarget(virStorageSourcePtr target,
         target->capacity = meta->capacity;
 
     if (encryption && meta->encryption) {
+        if (meta->encryption->payload_offset != -1)
+            target->capacity -= meta->encryption->payload_offset * 512;
+
         *encryption = meta->encryption;
         meta->encryption = NULL;
 

src/util/virstorageencryption.h

@@ -70,6 +70,7 @@ typedef struct _virStorageEncryption virStorageEncryption;
 typedef virStorageEncryption *virStorageEncryptionPtr;
 struct _virStorageEncryption {
     int format; /* virStorageEncryptionFormatType */
+    int payload_offset;
 
     size_t nsecrets;
     virStorageEncryptionSecretPtr *secrets;

src/util/virstoragefile.c

@@ -133,6 +133,8 @@ struct FileEncryptionInfo {
 
     int modeOffset; /* Byte offset of the format native encryption mode */
     char modeValue; /* Value expected at offset */
+
+    int payloadOffset; /* start offset of the volume data (in 512 byte sectors) */
 };
 
 /* Either 'magic' or 'extension' *must* be provided */
@@ -212,9 +214,18 @@ qedGetBackingStore(char **, int *, const char *, size_t);
 
 #define LUKS_HDR_MAGIC_LEN 6
 #define LUKS_HDR_VERSION_LEN 2
+#define LUKS_HDR_CIPHER_NAME_LEN 32
+#define LUKS_HDR_CIPHER_MODE_LEN 32
+#define LUKS_HDR_HASH_SPEC_LEN 32
+#define LUKS_HDR_PAYLOAD_LEN 4
 
 /* Format described by qemu commit id '3e308f20e' */
 #define LUKS_HDR_VERSION_OFFSET LUKS_HDR_MAGIC_LEN
+#define LUKS_HDR_PAYLOAD_OFFSET (LUKS_HDR_MAGIC_LEN+\
+                                 LUKS_HDR_VERSION_LEN+\
+                                 LUKS_HDR_CIPHER_NAME_LEN+\
+                                 LUKS_HDR_CIPHER_MODE_LEN+\
+                                 LUKS_HDR_HASH_SPEC_LEN)
 
 static struct FileEncryptionInfo const luksEncryptionInfo[] = {
     {
@@ -231,6 +242,8 @@ static struct FileEncryptionInfo const luksEncryptionInfo[] = {
 
         .modeOffset = -1,
         .modeValue = -1,
+
+        .payloadOffset = LUKS_HDR_PAYLOAD_OFFSET,
     },
     { 0 }
 };
@@ -249,6 +262,8 @@ static struct FileEncryptionInfo const qcow1EncryptionInfo[] = {
 
         .modeOffset = QCOW1_HDR_CRYPT,
         .modeValue = 1,
+
+        .payloadOffset = -1,
     },
     { 0 }
 };
@@ -267,6 +282,8 @@ static struct FileEncryptionInfo const qcow2EncryptionInfo[] = {
 
         .modeOffset = QCOW2_HDR_CRYPT,
         .modeValue = 1,
+
+        .payloadOffset = -1,
     },
     { 0 }
 };
@@ -921,6 +938,23 @@ virStorageFileHasEncryptionFormat(const struct FileEncryptionInfo *info,
 }
 
 
+static int
+virStorageFileGetEncryptionPayloadOffset(const struct FileEncryptionInfo *info,
+                                         char *buf)
+{
+    int payload_offset = -1;
+
+    if (info->payloadOffset != -1) {
+        if (info->endian == LV_LITTLE_ENDIAN)
+            payload_offset = virReadBufInt32LE(buf + info->payloadOffset);
+        else
+            payload_offset = virReadBufInt32BE(buf + info->payloadOffset);
+    }
+
+    return payload_offset;
+}
+
+
 /* Given a header in BUF with length LEN, as parsed from the storage file
  * assuming it has the given FORMAT, populate information into META
  * with information about the file and its backing store. Return format
@@ -967,6 +1001,8 @@ virStorageFileGetMetadataInternal(virStorageSourcePtr meta,
                         goto cleanup;
                     }
                 }
+                meta->encryption->payload_offset =
+                    virStorageFileGetEncryptionPayloadOffset(&fileTypeInfo[meta->format].cryptInfo[i], buf);
             }
         }
     }
@@ -3423,6 +3459,9 @@ virStorageSourceUpdateCapacity(virStorageSourcePtr src,
     else
         goto cleanup;
 
+    if (src->encryption && src->encryption->payload_offset != -1)
+        src->capacity -= src->encryption->payload_offset * 512;
+
     ret = 0;
 
  cleanup: