qcow2: Fix header update with overridden backing file

In recent qemu versions, it is possible to override the backing file
name and format that is stored in the image file with values given at
runtime. In such cases, the temporary override could end up in the
image header if the qcow2 header was updated, while obviously correct
behaviour would be to leave the on-disk backing file path/format
unchanged.

Fix this and add a test case for it.

Reported-by: Michael Tokarev <mjt@tls.msk.ru>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 1428411796-2852-1-git-send-email-kwolf@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

block/qcow2.c

@@ -140,6 +140,7 @@ static int qcow2_read_extensions(BlockDriverState *bs, uint64_t start_offset,
                 return 3;
             }
             bs->backing_format[ext.len] = '\0';
+            s->image_backing_format = g_strdup(bs->backing_format);
 #ifdef DEBUG_EXT
             printf("Qcow2: Got format extension %s\n", bs->backing_format);
 #endif
@@ -884,6 +885,7 @@ static int qcow2_open(BlockDriverState *bs, QDict *options, int flags,
             goto fail;
         }
         bs->backing_file[len] = '\0';
+        s->image_backing_file = g_strdup(bs->backing_file);
     }
 
     /* Internal snapshots */
@@ -1457,6 +1459,9 @@ static void qcow2_close(BlockDriverState *bs)
     g_free(s->unknown_header_fields);
     cleanup_unknown_header_ext(bs);
 
+    g_free(s->image_backing_file);
+    g_free(s->image_backing_format);
+
     g_free(s->cluster_cache);
     qemu_vfree(s->cluster_data);
     qcow2_refcount_close(bs);
@@ -1622,9 +1627,10 @@ int qcow2_update_header(BlockDriverState *bs)
     }
 
     /* Backing file format header extension */
-    if (*bs->backing_format) {
+    if (s->image_backing_format) {
         ret = header_ext_add(buf, QCOW2_EXT_MAGIC_BACKING_FORMAT,
-                             bs->backing_format, strlen(bs->backing_format),
+                             s->image_backing_format,
+                             strlen(s->image_backing_format),
                              buflen);
         if (ret < 0) {
             goto fail;
@@ -1682,8 +1688,8 @@ int qcow2_update_header(BlockDriverState *bs)
     buflen -= ret;
 
     /* Backing file name */
-    if (*bs->backing_file) {
-        size_t backing_file_len = strlen(bs->backing_file);
+    if (s->image_backing_file) {
+        size_t backing_file_len = strlen(s->image_backing_file);
 
         if (buflen < backing_file_len) {
             ret = -ENOSPC;
@@ -1691,7 +1697,7 @@ int qcow2_update_header(BlockDriverState *bs)
         }
 
         /* Using strncpy is ok here, since buf is not NUL-terminated. */
-        strncpy(buf, bs->backing_file, buflen);
+        strncpy(buf, s->image_backing_file, buflen);
 
         header->backing_file_offset = cpu_to_be64(buf - ((char*) header));
         header->backing_file_size   = cpu_to_be32(backing_file_len);
@@ -1712,9 +1718,17 @@ fail:
 static int qcow2_change_backing_file(BlockDriverState *bs,
     const char *backing_file, const char *backing_fmt)
 {
+    BDRVQcowState *s = bs->opaque;
+
     pstrcpy(bs->backing_file, sizeof(bs->backing_file), backing_file ?: "");
     pstrcpy(bs->backing_format, sizeof(bs->backing_format), backing_fmt ?: "");
 
+    g_free(s->image_backing_file);
+    g_free(s->image_backing_format);
+
+    s->image_backing_file = backing_file ? g_strdup(bs->backing_file) : NULL;
+    s->image_backing_format = backing_fmt ? g_strdup(bs->backing_format) : NULL;
+
     return qcow2_update_header(bs);
 }
 
@@ -2751,8 +2765,9 @@ static int qcow2_amend_options(BlockDriverState *bs, QemuOpts *opts,
     }
 
     if (backing_file || backing_format) {
-        ret = qcow2_change_backing_file(bs, backing_file ?: bs->backing_file,
-                                        backing_format ?: bs->backing_format);
+        ret = qcow2_change_backing_file(bs,
+                    backing_file ?: s->image_backing_file,
+                    backing_format ?: s->image_backing_format);
         if (ret < 0) {
             return ret;
         }

block/qcow2.h

@@ -283,6 +283,12 @@ typedef struct BDRVQcowState {
     QLIST_HEAD(, Qcow2UnknownHeaderExtension) unknown_header_ext;
     QTAILQ_HEAD (, Qcow2DiscardRegion) discards;
     bool cache_discards;
+
+    /* Backing file path and format as stored in the image (this is not the
+     * effective path/format, which may be the result of a runtime option
+     * override) */
+    char *image_backing_file;
+    char *image_backing_format;
 } BDRVQcowState;
 
 struct QCowAIOCB;

tests/qemu-iotests/130

@@ -0,0 +1,95 @@
+#!/bin/bash
+#
+# Test that temporary backing file overrides (on the command line or in
+# blockdev-add) don't replace the original path stored in the image during
+# header updates.
+#
+# Copyright (C) 2015 Red Hat, Inc.
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 2 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+#
+
+# creator
+owner=kwolf@redhat.com
+
+seq="$(basename $0)"
+echo "QA output created by $seq"
+
+here="$PWD"
+tmp=/tmp/$$
+status=1	# failure is the default!
+
+_cleanup()
+{
+    _cleanup_test_img
+}
+trap "_cleanup; exit \$status" 0 1 2 3 15
+
+# get standard environment, filters and checks
+. ./common.rc
+. ./common.filter
+. ./common.qemu
+
+_supported_fmt qcow2
+_supported_proto generic
+_supported_os Linux
+
+qemu_comm_method="monitor"
+
+
+TEST_IMG="$TEST_IMG.orig" _make_test_img 64M
+TEST_IMG="$TEST_IMG.base" _make_test_img 64M
+_make_test_img 64M
+_img_info | _filter_img_info
+
+echo
+echo "=== HMP commit ==="
+echo
+# bdrv_make_empty() involves a header update for qcow2
+
+# Test that a backing file isn't written
+_launch_qemu -drive file="$TEST_IMG",backing.file.filename="$TEST_IMG.base"
+_send_qemu_cmd $QEMU_HANDLE "commit ide0-hd0" "(qemu)"
+_send_qemu_cmd $QEMU_HANDLE '' '(qemu)'
+_cleanup_qemu
+_img_info | _filter_img_info
+
+# Make sure that if there was a backing file that was just overridden on the
+# command line, that backing file is retained, with the right format
+_make_test_img -F raw -b "$TEST_IMG.orig" 64M
+_launch_qemu -drive file="$TEST_IMG",backing.file.filename="$TEST_IMG.base",backing.driver=$IMGFMT
+_send_qemu_cmd $QEMU_HANDLE "commit ide0-hd0" "(qemu)"
+_send_qemu_cmd $QEMU_HANDLE '' '(qemu)'
+_cleanup_qemu
+_img_info | _filter_img_info
+
+echo
+echo "=== Marking image dirty (lazy refcounts) ==="
+echo
+
+# Test that a backing file isn't written
+_make_test_img 64M
+$QEMU_IO -c "open -o backing.file.filename=$TEST_IMG.base,lazy-refcounts=on $TEST_IMG" -c "write 0 4k" | _filter_qemu_io
+_img_info | _filter_img_info
+
+# Make sure that if there was a backing file that was just overridden on the
+# command line, that backing file is retained, with the right format
+_make_test_img -F raw -b "$TEST_IMG.orig" 64M
+$QEMU_IO -c "open -o backing.file.filename=$TEST_IMG.base,backing.driver=$IMGFMT,lazy-refcounts=on $TEST_IMG" -c "write 0 4k" | _filter_qemu_io
+_img_info | _filter_img_info
+
+# success, all done
+echo '*** done'
+rm -f $seq.full
+status=0

tests/qemu-iotests/130.out

@@ -0,0 +1,43 @@
+QA output created by 130
+Formatting 'TEST_DIR/t.IMGFMT.orig', fmt=IMGFMT size=67108864
+Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=67108864
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864
+image: TEST_DIR/t.IMGFMT
+file format: IMGFMT
+virtual size: 64M (67108864 bytes)
+
+=== HMP commit ===
+
+QEMU X.Y.Z monitor - type 'help' for more information
+(qemu) ccocomcommcommicommitcommit commit icommit idcommit idecommit ide0commit ide0-commit ide0-hcommit ide0-hdcommit ide0-hd0
+(qemu) 
+image: TEST_DIR/t.IMGFMT
+file format: IMGFMT
+virtual size: 64M (67108864 bytes)
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864 backing_file='TEST_DIR/t.IMGFMT.orig' backing_fmt='raw'
+QEMU X.Y.Z monitor - type 'help' for more information
+(qemu) ccocomcommcommicommitcommit commit icommit idcommit idecommit ide0commit ide0-commit ide0-hcommit ide0-hdcommit ide0-hd0
+(qemu) 
+image: TEST_DIR/t.IMGFMT
+file format: IMGFMT
+virtual size: 64M (67108864 bytes)
+backing file: TEST_DIR/t.IMGFMT.orig
+backing file format: raw
+
+=== Marking image dirty (lazy refcounts) ===
+
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864
+wrote 4096/4096 bytes at offset 0
+4 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+image: TEST_DIR/t.IMGFMT
+file format: IMGFMT
+virtual size: 64M (67108864 bytes)
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864 backing_file='TEST_DIR/t.IMGFMT.orig' backing_fmt='raw'
+wrote 4096/4096 bytes at offset 0
+4 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+image: TEST_DIR/t.IMGFMT
+file format: IMGFMT
+virtual size: 64M (67108864 bytes)
+backing file: TEST_DIR/t.IMGFMT.orig
+backing file format: raw
+*** done

tests/qemu-iotests/group

@@ -124,3 +124,4 @@
 121 rw auto
 123 rw auto quick
 128 rw auto quick
+130 rw auto quick