[Qemu-devel] [PATCH 03/14] block-qcow2: keep backing file format in a qcow2 extension

[Uri Lublin <uril@redhat.com>]

Use a qcow2 extension to keep the backing file format.

By keeping the backing file format, we can:
1. Provide a way to know the backing file format without probing
   it (setting the format at creation time).
2. Enable using qcow2 format over host block devices.
   (only if the user specifically asks for it, by providing the format
   at creation time).

Also fixes a security flaw found by Daniel P. Berrange on [1]
which summarizes: "Autoprobing: just say no."

[1] http://lists.gnu.org/archive/html/qemu-devel/2008-12/msg01083.html

Signed-off-by: Uri Lublin <uril@redhat.com>
---
 block-qcow2.c |   55 +++++++++++++++++++++++++++++++++++++++++++++++++++++--
 1 files changed, 53 insertions(+), 2 deletions(-)

diff --git a/block-qcow2.c b/block-qcow2.c
index 23e66b0..8a910fa 100644
--- a/block-qcow2.c
+++ b/block-qcow2.c
@@ -84,6 +84,7 @@ typedef struct {
     uint32_t len;
 } QCowExtension;
 #define  QCOW_EXT_MAGIC_END 0
+#define  QCOW_EXT_MAGIC_BACKING_FORMAT 0xE2792ACA
 
 
 typedef struct __attribute__((packed)) QCowSnapshotHeader {
@@ -234,6 +235,24 @@ static int qcow_read_extensions(BlockDriverState *bs, uint64_t start_offset,
         switch (ext.magic) {
         case QCOW_EXT_MAGIC_END:
             return 0;
+
+        case QCOW_EXT_MAGIC_BACKING_FORMAT:
+            if (ext.len >= sizeof(bs->backing_format)) {
+                fprintf(stderr, "ERROR: ext_backing_format: len=%u too large"
+                        " (>=%lu)\n",
+                        ext.len, sizeof(bs->backing_format));
+                return 2;
+            }
+            if (bdrv_pread(s->hd, offset , bs->backing_format,
+                           ext.len) != ext.len)
+                return 3;
+            bs->backing_format[ext.len] = '\0';
+#ifdef DEBUG_EXT
+            printf("Qcow2: Got format extension %s\n", bs->backing_format);
+#endif
+            offset += ((ext.len + 7) & ~7);
+            break;
+
         default:
             /* unknown magic -- just skip it */
             offset += ((ext.len + 7) & ~7);
@@ -1525,13 +1544,18 @@ static void create_refcount_update(QCowCreateState *s,
     }
 }
 
-static int qcow_create(const char *filename, int64_t total_size,
-                      const char *backing_file, int flags)
+static int qcow_create2(const char *filename, int64_t total_size,
+                        const char *backing_file, const char *backing_format,
+                        int flags)
 {
+
     int fd, header_size, backing_filename_len, l1_size, i, shift, l2_bits;
+    int backing_format_len = 0;
     QCowHeader header;
     uint64_t tmp, offset;
     QCowCreateState s1, *s = &s1;
+    QCowExtension ext_bf = {0, 0};
+
 
     memset(s, 0, sizeof(*s));
 
@@ -1545,6 +1569,12 @@ static int qcow_create(const char *filename, int64_t total_size,
     header_size = sizeof(header);
     backing_filename_len = 0;
     if (backing_file) {
+        if (backing_format) {
+            ext_bf.magic = QCOW_EXT_MAGIC_BACKING_FORMAT;
+            backing_format_len = strlen(backing_format);
+            ext_bf.len = (backing_format_len + 7) & ~7;
+            header_size += ((sizeof(ext_bf) + ext_bf.len + 7) & ~7);
+        }
         header.backing_file_offset = cpu_to_be64(header_size);
         backing_filename_len = strlen(backing_file);
         header.backing_file_size = cpu_to_be32(backing_filename_len);
@@ -1589,6 +1619,19 @@ static int qcow_create(const char *filename, int64_t total_size,
     /* write all the data */
     write(fd, &header, sizeof(header));
     if (backing_file) {
+        if (backing_format_len) {
+            char zero[16];
+            int d = ext_bf.len - backing_format_len;
+
+            memset(zero, 0, sizeof(zero));
+            cpu_to_be32s(&ext_bf.magic);
+            cpu_to_be32s(&ext_bf.len);
+            write(fd, &ext_bf, sizeof(ext_bf));
+            write(fd, backing_format, backing_format_len);
+            if (d>0) {
+                write(fd, zero, d);
+            }
+        }
         write(fd, backing_file, backing_filename_len);
     }
     lseek(fd, s->l1_table_offset, SEEK_SET);
@@ -1608,6 +1651,12 @@ static int qcow_create(const char *filename, int64_t total_size,
     return 0;
 }
 
+static int qcow_create(const char *filename, int64_t total_size,
+                       const char *backing_file, int flags)
+{
+    return qcow_create2(filename, total_size, backing_file, NULL, flags);
+}
+
 static int qcow_make_empty(BlockDriverState *bs)
 {
 #if 0
@@ -2684,4 +2733,6 @@ BlockDriver bdrv_qcow2 = {
     .bdrv_snapshot_delete = qcow_snapshot_delete,
     .bdrv_snapshot_list	= qcow_snapshot_list,
     .bdrv_get_info	= qcow_get_info,
+
+    .bdrv_create2 = qcow_create2,
 };
-- 
1.6.0.6