From 7e3d98dd31b6de53923bfb0f04b23b42a94f3829 Mon Sep 17 00:00:00 2001 From: Kevin Wolf Date: Fri, 25 Apr 2014 12:36:07 +0200 Subject: [PATCH] Revert "block: another bdrv_append fix" This reverts commit 3a389e7926750cba5c83f662b1941888b2bebc04. The commit was wrong and what it tried to fix just works today without any change. What the commit tried to fix: When creating live snapshots, the new image file is opened with BDRV_O_NO_BACKING because the whole backing chain is already opened. It is then appended to the chain using bdrv_append(). The result of this was that the image had a backing file, but BDRV_O_NO_BACKING was still set. This is obviously inconsistent. There used to be some places in qemu that closed and image and then opened it again, with its old flags (a bdrv_open()/close() sequence involves reopening the whole backing file chain, too). In this case the BDRV_O_NO_BACKING flag meant that the backing chain wasn't reopened and only the top layer was left. (Most, but not all of these places are replaced by bdrv_reopen() today, which doesn't touch the backing files at all.) Other places that looked at bs->open_flags weren't interested in BDRV_O_NO_BACKING, so no breakage there. What it actually did: The commit moved the BDRV_O_NO_BACKING away to the backing file. Because the bdrv_open()/close() sequences only looked at the flags of the top level BlockDriverState and used it for the whole chain, the flag didn't hurt there any more. Obviously, it is still inconsistent because the backing file may have another backing file, but without practical impact. At the same time, it swapped all other flags. This is practically irrelevant as long as live snapshots only allow opening the new layer with the same flags as the old top layer. It still doesn't make any sense, and it is a time bomb that explodes as soon as the flags can differ. bdrv_append_temp_snapshot() is such a case: It adds the new flag BDRV_O_TEMPORARY for the temporary snapshot. The swapping of commit 3a389e79 results in the following nonsensical configuration: bs->open_flags: BDRV_O_TEMPORARY cleared bs->file->open_flags: BDRV_O_TEMPORARY set bs->backing_hd->open_flags: BDRV_O_TEMPORARY set bs->backing_hd->file->open_flags: BDRV_O_TEMPORARY cleared We're still lucky because the format layer ignores the flag and the protocol layer happens to get the right value, but sooner or later this is bound to go wrong... What the right fix would have been: Simply clear the BDRV_O_NO_BACKING flag when the BlockDriverState is appended to an existing backing file chain, because now it does have a backing file. Commit 4ddc07ca already implemented this silently in bdrv_append(), so we don't have to come up with a new fix. Signed-off-by: Kevin Wolf Reviewed-by: Max Reitz --- block.c | 1 - 1 file changed, 1 deletion(-) diff --git a/block.c b/block.c index c094075e4b..9f6f07e75c 100644 --- a/block.c +++ b/block.c @@ -1864,7 +1864,6 @@ static void bdrv_move_feature_fields(BlockDriverState *bs_dest, BlockDriverState *bs_src) { /* move some fields that need to stay attached to the device */ - bs_dest->open_flags = bs_src->open_flags; /* dev info */ bs_dest->dev_ops = bs_src->dev_ops;