In qcow2 header data file is represented by incompitible feature bit
and its path is saved to header extension table.
Thus, we implement here the logic similar to backing file probing.

Signed-off-by: Nikolai Barybin <nikolai.barybin@virtuozzo.com>
---
 src/storage_file/storage_file_probe.c | 46 ++++++++++++++++++++++++---
 1 file changed, 42 insertions(+), 4 deletions(-)

diff --git a/src/storage_file/storage_file_probe.c b/src/storage_file/storage_file_probe.c
index ad14350edc..06868f5355 100644
--- a/src/storage_file/storage_file_probe.c
+++ b/src/storage_file/storage_file_probe.c
@@ -106,6 +106,7 @@ qcow2GetClusterSize(const char *buf,
                     size_t buf_size);
 static int qcowXGetBackingStore(char **, int *,
                                 const char *, size_t);
+static int qcow2GetDataFile(char **, virBitmap *, char *, size_t);
 static int qcow2GetFeatures(virBitmap **features, int format,
                             char *buf, ssize_t len);
 static int vmdk4GetBackingStore(char **, int *,
@@ -127,6 +128,7 @@ qedGetBackingStore(char **, int *, const char *, size_t);
 
 #define QCOW2_HDR_EXTENSION_END 0
 #define QCOW2_HDR_EXTENSION_BACKING_FORMAT 0xE2792ACA
+#define QCOW2_HDR_EXTENSION_DATA_FILE_NAME 0x44415441
 
 #define QCOW2v3_HDR_FEATURES_INCOMPATIBLE (QCOW2_HDR_TOTAL_SIZE)
 #define QCOW2v3_HDR_FEATURES_COMPATIBLE (QCOW2v3_HDR_FEATURES_INCOMPATIBLE+8)
@@ -314,7 +316,7 @@ static struct FileTypeInfo const fileTypeInfo[] = {
         qcow2EncryptionInfo,
         qcow2GetClusterSize,
         qcowXGetBackingStore,
-        NULL,
+        qcow2GetDataFile,
         qcow2GetFeatures
     },
     [VIR_STORAGE_FILE_QED] = {
@@ -361,7 +363,7 @@ enum qcow2IncompatibleFeature {
 static const virStorageFileFeature qcow2IncompatibleFeatureArray[] = {
     VIR_STORAGE_FILE_FEATURE_LAST, /* QCOW2_INCOMPATIBLE_FEATURE_DIRTY */
     VIR_STORAGE_FILE_FEATURE_LAST, /* QCOW2_INCOMPATIBLE_FEATURE_CORRUPT */
-    VIR_STORAGE_FILE_FEATURE_LAST, /* QCOW2_INCOMPATIBLE_FEATURE_DATA_FILE */
+    VIR_STORAGE_FILE_FEATURE_DATA_FILE, /* QCOW2_INCOMPATIBLE_FEATURE_DATA_FILE */
     VIR_STORAGE_FILE_FEATURE_LAST, /* QCOW2_INCOMPATIBLE_FEATURE_COMPRESSION */
     VIR_STORAGE_FILE_FEATURE_EXTENDED_L2, /* QCOW2_INCOMPATIBLE_FEATURE_EXTL2 */
 };
@@ -393,7 +395,8 @@ cowGetBackingStore(char **res,
 static int
 qcow2GetExtensions(const char *buf,
                    size_t buf_size,
-                   int *backingFormat)
+                   int *backingFormat,
+                   char **dataFilePath)
 {
     size_t offset;
     size_t extension_start;
@@ -488,6 +491,15 @@ qcow2GetExtensions(const char *buf,
             break;
         }
 
+        case QCOW2_HDR_EXTENSION_DATA_FILE_NAME: {
+            if (!dataFilePath)
+                break;
+
+            *dataFilePath = g_new0(char, len + 1);
+            memcpy(*dataFilePath, buf + offset, len);
+            break;
+        }
+
         case QCOW2_HDR_EXTENSION_END:
             return 0;
         }
@@ -554,13 +566,33 @@ qcowXGetBackingStore(char **res,
     memcpy(*res, buf + offset, size);
     (*res)[size] = '\0';
 
-    if (qcow2GetExtensions(buf, buf_size, format) < 0)
+    if (qcow2GetExtensions(buf, buf_size, format, NULL) < 0)
         return 0;
 
     return 0;
 }
 
 
+static int
+qcow2GetDataFile(char **res,
+                 virBitmap *features,
+                 char *buf,
+                 size_t buf_size)
+{
+    *res = NULL;
+
+    if (buf_size < QCOW2v3_HDR_FEATURES_INCOMPATIBLE + 8)
+        return 0;
+
+    if (features && virBitmapIsBitSet(features, VIR_STORAGE_FILE_FEATURE_DATA_FILE)) {
+        if (qcow2GetExtensions(buf, buf_size, NULL, res) < 0)
+            return 0;
+    }
+
+    return 0;
+}
+
+
 static int
 vmdk4GetBackingStore(char **res,
                      int *format,
@@ -968,6 +1000,12 @@ virStorageFileProbeGetMetadata(virStorageSource *meta,
         fileTypeInfo[meta->format].getFeatures(&meta->features, meta->format, buf, len) < 0)
         return -1;
 
+    VIR_FREE(meta->dataFileRaw);
+    if (fileTypeInfo[meta->format].getDataFile != NULL) {
+        fileTypeInfo[meta->format].getDataFile(&meta->dataFileRaw, meta->features,
+                                               buf, len);
+    }
+
     VIR_FREE(meta->compat);
     if (meta->format == VIR_STORAGE_FILE_QCOW2 && meta->features)
         meta->compat = g_strdup("1.1");
-- 
2.43.5

On Wed, Nov 20, 2024 at 18:48:41 +0300, Nikolai Barybin via Devel wrote:
> In qcow2 header data file is represented by incompitible feature bit
> and its path is saved to header extension table.
> Thus, we implement here the logic similar to backing file probing.
> 
> Signed-off-by: Nikolai Barybin <nikolai.barybin@virtuozzo.com>
> ---
>  src/storage_file/storage_file_probe.c | 46 ++++++++++++++++++++++++---
>  1 file changed, 42 insertions(+), 4 deletions(-)

[...]

> +static int
> +qcow2GetDataFile(char **res,
> +                 virBitmap *features,
> +                 char *buf,
> +                 size_t buf_size)
> +{
> +    *res = NULL;
> +
> +    if (buf_size < QCOW2v3_HDR_FEATURES_INCOMPATIBLE + 8)
> +        return 0;
> +
> +    if (features && virBitmapIsBitSet(features, VIR_STORAGE_FILE_FEATURE_DATA_FILE)) {
> +        if (qcow2GetExtensions(buf, buf_size, NULL, res) < 0)
> +            return 0;

This should be 'return -1'

> +    }
> +
> +    return 0;
> +}
> +
> +
>  static int
>  vmdk4GetBackingStore(char **res,
>                       int *format,

Reviewed-by: Peter Krempa <pkrempa@redhat.com>