From 6ca080453ea403959ccde661030ca16264acc181 Mon Sep 17 00:00:00 2001 From: Daniel Henrique Barboza Date: Wed, 7 Nov 2018 11:09:58 -0200 Subject: block/snapshot.c: eliminate use of ID input in snapshot operations At this moment, QEMU attempts to create/load/delete snapshots by using either an ID (id_str) or a name. The problem is that the code isn't consistent of whether the entered argument is an ID or a name, causing unexpected behaviors. For example, when creating snapshots via savevm , what happens is that "arg" is treated as both name and id_str. In a guest without snapshots, create a single snapshot via savevm: (qemu) savevm 0 (qemu) info snapshots List of snapshots present on all disks: ID TAG VM SIZE DATE VM CLOCK -- 0 741M 2018-07-31 13:39:56 00:41:25.313 A snapshot with name "0" is created. ID is hidden from the user, but the ID is a non-zero integer that starts at "1". Thus, this snapshot has id_str=1, TAG="0". Creating a second snapshot with arg = 1, the first one is deleted: (qemu) savevm 1 (qemu) info snapshots List of snapshots present on all disks: ID TAG VM SIZE DATE VM CLOCK -- 1 741M 2018-07-31 13:42:14 00:41:55.252 What happened? - when creating the second snapshot, a verification is done inside bdrv_all_delete_snapshot to delete any existing snapshots that matches an string argument. Here, the code calls bdrv_all_delete_snapshot("1", ...); - bdrv_all_delete_snapshot calls bdrv_snapshot_find(..., "1") for each BlockDriverState of the guest. And this is where things goes tilting: bdrv_snapshot_find does a search by both id_str and name. It finds out that there is a snapshot that has id_str = 1, stores a reference to the snapshot in the sn_info pointer and then returns match found; - since a match was found, a call to bdrv_snapshot_delete_by_id_or_name() is made. This function ignores the pointer written by bdrv_snapshot_find. Instead, it deletes the snapshot using bdrv_snapshot_delete() calling it first with id_str = 1. If it fails to delete, then it calls it again with name = 1. - after all that, QEMU creates the new snapshot, that has id_str = 1 and name = 1. The user is left wondering that happened with the first snapshot created. Similar bugs can be triggered when using loadvm and delvm. Before contemplating discarding the use of ID input in these operations, I've searched the code of what would be the implications. My findings are: - the RBD and Sheepdog drivers don't care. Both uses the 'name' field as key in their logic, making id_str = name when appropriate. replay-snapshot.c does not make any special use of id_str; - qcow2 uses id_str as an unique identifier but it is automatically calculated, not being influenced by user input. Other than that, there are no distinguish operations made only with id_str; - in blockdev.c, the delete operation uses a match of both id_str AND name. Given that id_str is either a copy of 'name' or auto-generated, we're fine here. This gives motivation to not consider ID as a valid user input in HMP commands - sticking with 'name' input only is more consistent. To accomplish that, the following changes were made in this patch: - bdrv_snapshot_find() does not match for id_str anymore, only 'name'. The function is called in save_snapshot(), load_snapshot(), bdrv_all_delete_snapshot() and bdrv_all_find_snapshot(). This change makes the search function more predictable and does not change the behavior of any underlying code that uses these affected functions, which are related to HMP (which is fine) and the main loop inside vl.c (which doesn't care about it anyways); - bdrv_all_delete_snapshot() does not call bdrv_snapshot_delete_by_id_or_name anymore. Instead, it uses the pointer returned by bdrv_snapshot_find to erase the snapshot with the exact match of id_str an name. This function is called in save_snapshot and hmp_delvm, thus this change produces the intended effect; - documentation changes to reflect the new behavior. I consider this to be an API fix instead of an API change - the user was already creating snapshots using 'name', but now he/she will also enjoy a consistent behavior. Ideally we would get rid of the id_str field entirely, but this would have repercussions on existing snapshots. Another day perhaps. Signed-off-by: Daniel Henrique Barboza Acked-by: Dr. David Alan Gilbert Signed-off-by: Kevin Wolf --- block/snapshot.c | 5 +++-- 1 file changed, 3 insertions(+), 2 deletions(-) (limited to 'block/snapshot.c') diff --git a/block/snapshot.c b/block/snapshot.c index 3218a542df..e371d2243d 100644 --- a/block/snapshot.c +++ b/block/snapshot.c @@ -63,7 +63,7 @@ int bdrv_snapshot_find(BlockDriverState *bs, QEMUSnapshotInfo *sn_info, } for (i = 0; i < nb_sns; i++) { sn = &sn_tab[i]; - if (!strcmp(sn->id_str, name) || !strcmp(sn->name, name)) { + if (!strcmp(sn->name, name)) { *sn_info = *sn; ret = 0; break; @@ -448,7 +448,8 @@ int bdrv_all_delete_snapshot(const char *name, BlockDriverState **first_bad_bs, aio_context_acquire(ctx); if (bdrv_can_snapshot(bs) && bdrv_snapshot_find(bs, snapshot, name) >= 0) { - ret = bdrv_snapshot_delete_by_id_or_name(bs, name, err); + ret = bdrv_snapshot_delete(bs, snapshot->id_str, + snapshot->name, err); } aio_context_release(ctx); if (ret < 0) { -- cgit v1.2.3