From 3db2bd5508c86a1605258bc77c9672d93b5c350e Mon Sep 17 00:00:00 2001 From: Alberto Garcia Date: Wed, 28 Oct 2015 15:43:49 +0200 Subject: commit: reopen overlay_bs before base 'block-commit' needs write access to two different nodes of the chain: - 'base', because that's where the data is written to. - the overlay of 'top', because it needs to update the backing file string to point to 'base' after the operation. Both images have to be opened in read-write mode, and commit_start() takes care of reopening them if necessary. With the current implementation, however, when overlay_bs is reopened in read-write mode it has the side effect of making 'base' read-only again, eventually making 'block-commit' fail. This needs to be fixed in bdrv_reopen(), but until we get to that it can be worked around simply by swapping the order of base and overlay_bs in the reopen queue. In order to reproduce this bug, overlay_bs needs to be initially in read-only mode. That is: the 'top' parameter of 'block-commit' cannot be the active layer nor its immediate backing chain. Cc: qemu-stable@nongnu.org Signed-off-by: Alberto Garcia Reviewed-by: Max Reitz Signed-off-by: Kevin Wolf --- block/commit.c | 8 ++++---- 1 file changed, 4 insertions(+), 4 deletions(-) diff --git a/block/commit.c b/block/commit.c index fdebe87c16..a5d02aa560 100644 --- a/block/commit.c +++ b/block/commit.c @@ -236,14 +236,14 @@ void commit_start(BlockDriverState *bs, BlockDriverState *base, orig_overlay_flags = bdrv_get_flags(overlay_bs); /* convert base & overlay_bs to r/w, if necessary */ - if (!(orig_base_flags & BDRV_O_RDWR)) { - reopen_queue = bdrv_reopen_queue(reopen_queue, base, NULL, - orig_base_flags | BDRV_O_RDWR); - } if (!(orig_overlay_flags & BDRV_O_RDWR)) { reopen_queue = bdrv_reopen_queue(reopen_queue, overlay_bs, NULL, orig_overlay_flags | BDRV_O_RDWR); } + if (!(orig_base_flags & BDRV_O_RDWR)) { + reopen_queue = bdrv_reopen_queue(reopen_queue, base, NULL, + orig_base_flags | BDRV_O_RDWR); + } if (reopen_queue) { bdrv_reopen_multiple(reopen_queue, &local_err); if (local_err != NULL) { -- cgit v1.2.3-55-g7522