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[PATCH 1/3] block/qcow2: Keep auto_backing_file if possible

From: Hanna Reitz
Subject: [PATCH 1/3] block/qcow2: Keep auto_backing_file if possible
Date: Wed, 3 Aug 2022 16:44:44 +0200

qcow2_do_open() is used by qcow2_co_invalidate_cache(), i.e. may be run
on an image that has been opened before.  When reading the backing file
string from the image header, compare it against the existing
bs->backing_file, and update bs->auto_backing_file only if they differ.

auto_backing_file should ideally contain the filename the backing BDS
will actually have after opening, i.e. a post-bdrv_refresh_filename()
version of what is in the image header.  So for example, if the image
header reports the following backing file string:

    json:{"driver": "qcow2", "file": {
        "driver": "file", "filename": "/tmp/backing.qcow2"

Then auto_backing_file should contain simply "/tmp/backing.qcow2".

Because bdrv_refresh_filename() only works on existing BDSs, though, the
way how we get this auto_backing_file value is to have the format driver
set it to whatever is in the image header, and when the backing BDS is
opened based on that, we update it with the filename the backing BDS
actually got.

However, qcow2's qcow2_co_invalidate_cache() implementation breaks this
because it just resets auto_backing_file to whatever is in the image
file without opening a BDS based on it, so we never get
auto_backing_file back to the "refreshed" version, and in the example
above, it would stay "json:{...}".

Then, bs->backing->bs->filename will differ from bs->auto_backing_file,
making bdrv_backing_overridden(bs) return true, which will lead
bdrv_refresh_filename(bs) to generate a json:{} filename for bs, even
though that may not have been necessary.  This is reported in the issue
linked below.

Therefore, skip updating auto_backing_file if nothing has changed in the
image header since we last read it.

Fixes: https://gitlab.com/qemu-project/qemu/-/issues/1117
Signed-off-by: Hanna Reitz <hreitz@redhat.com>
 block/qcow2.c | 21 ++++++++++++++++-----
 1 file changed, 16 insertions(+), 5 deletions(-)

diff --git a/block/qcow2.c b/block/qcow2.c
index c6c6692fb7..a43fee2067 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -1696,16 +1696,27 @@ static int coroutine_fn qcow2_do_open(BlockDriverState 
*bs, QDict *options,
             ret = -EINVAL;
             goto fail;
+        s->image_backing_file = g_malloc(len + 1);
         ret = bdrv_pread(bs->file, header.backing_file_offset, len,
-                         bs->auto_backing_file, 0);
+                         s->image_backing_file, 0);
         if (ret < 0) {
             error_setg_errno(errp, -ret, "Could not read backing file name");
             goto fail;
-        bs->auto_backing_file[len] = '\0';
-        pstrcpy(bs->backing_file, sizeof(bs->backing_file),
-                bs->auto_backing_file);
-        s->image_backing_file = g_strdup(bs->auto_backing_file);
+        s->image_backing_file[len] = '\0';
+        /*
+         * Update only when something has changed.  This function is called by
+         * qcow2_co_invalidate_cache(), and we do not want to reset
+         * auto_backing_file unless necessary.
+         */
+        if (!g_str_equal(s->image_backing_file, bs->backing_file)) {
+            pstrcpy(bs->backing_file, sizeof(bs->backing_file),
+                    s->image_backing_file);
+            pstrcpy(bs->auto_backing_file, sizeof(bs->auto_backing_file),
+                    s->image_backing_file);
+        }

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