[PATCH] Fix old QCow tapdisk image handling

[Miroslav Rezanina <mrezanin@redhat.com>]

When I tried to use QCow image, I found that only each second boot is successful. As I discovered, this is caused by wrong handling old qcow tapdisk images. Extended header flag is not stored correctly so the blktap tries to change endian fo L1 table on each startup.

Here is a fix for this problem:

--
diff -r 9a1d7caa2024 tools/blktap/drivers/block-qcow.c
--- a/tools/blktap/drivers/block-qcow.c	Mon Apr 26 12:13:23 2010 +0100
+++ b/tools/blktap/drivers/block-qcow.c	Fri Apr 30 11:00:41 2010 +0200
@@ -862,11 +862,12 @@
 		be32_to_cpus(&exthdr->xmagic);
 		if(exthdr->xmagic != XEN_MAGIC) 
 			goto end_xenhdr;
-    
+	
+		be32_to_cpus(&exthdr->flags);
 		/* Try to detect old tapdisk images. They have to be fixed because 
 		 * they don't use big endian but native endianess for the L1 table */
 		if ((exthdr->flags & EXTHDR_L1_BIG_ENDIAN) == 0) {
-
+			QCowHeader_ext *tmphdr = (QCowHeader_ext *)(buf2 + sizeof(QCowHeader));
 			/* 
 			   The image is broken. Fix it. The L1 table has already been 
 			   byte-swapped, so we can write it to the image file as it is
@@ -874,6 +875,11 @@
 			   for operation.
 			 */
 
+			/* Change ENDIAN flag and copy it to store buffer */
+			exthdr->flags |= EXTHDR_L1_BIG_ENDIAN;
+			tmphdr->flags = cpu_to_be32(exthdr->flags);
+
+
 			DPRINTF("qcow: Converting image to big endian L1 table\n");
 
 			memcpy(buf2 + s->l1_table_offset, s->l1_table, l1_table_size);
@@ -888,13 +894,6 @@
 				cpu_to_be64s(&s->l1_table[i]);
 			}
 
-			/* Write the big endian flag to the extended header */
-			exthdr->flags |= EXTHDR_L1_BIG_ENDIAN;
-
-			if (write(fd, buf, 512) != 512) {
-				DPRINTF("qcow: Failed to write extended header\n");
-				goto fail;
-			}
 		}
 
 		/*Finally check the L1 table cksum*/
@@ -905,7 +904,6 @@
 			goto end_xenhdr;
 			
 		be32_to_cpus(&exthdr->min_cluster_alloc);
-		be32_to_cpus(&exthdr->flags);
 		s->sparse = (exthdr->flags & SPARSE_FILE);
 		s->min_cluster_alloc = exthdr->min_cluster_alloc; 
 	}
@@ -1263,6 +1261,7 @@
 	} else
 		flags = SPARSE_FILE;
 
+	flags |= EXTHDR_L1_BIG_ENDIAN;
 	exthdr.flags = cpu_to_be32(flags);
 	
 	/* write all the data */
-- 
Miroslav Rezanina
Software Engineer - XEN Virtualization Team