On Mon, 7 Sept 2020 at 12:11, Kevin Wolf <kw...@redhat.com> wrote:
>
> From: Max Reitz <mre...@redhat.com>
>
> If the top node's driver does not provide snapshot functionality and we
> want to fall back to a node down the chain, we need to snapshot all
> non-COW children. For simplicity's sake, just do not fall back if there
> is more than one such child. Furthermore, we really only can fall back
> to bs->file and bs->backing, because bdrv_snapshot_goto() has to modify
> the child link (notably, set it to NULL).
>
> Suggested-by: Vladimir Sementsov-Ogievskiy <vsement...@virtuozzo.com>
> Signed-off-by: Max Reitz <mre...@redhat.com>
> Reviewed-by: Andrey Shinkevich <andrey.shinkev...@virtuozzo.com>
> Reviewed-by: Kevin Wolf <kw...@redhat.com>
Hi; Coverity thinks it's found a problem with this code
(CID 1452774):
> @@ -205,39 +258,46 @@ int bdrv_snapshot_goto(BlockDriverState *bs,
> return ret;
> }
>
> - if (bs->file) {
> - BlockDriverState *file;
> - QDict *options = qdict_clone_shallow(bs->options);
> + fallback_ptr = bdrv_snapshot_fallback_ptr(bs);
> + if (fallback_ptr) {
> + QDict *options;
> QDict *file_options;
> Error *local_err = NULL;
> + BlockDriverState *fallback_bs = (*fallback_ptr)->bs;
> + char *subqdict_prefix = g_strdup_printf("%s.",
> (*fallback_ptr)->name);
> +
> + options = qdict_clone_shallow(bs->options);
>
> - file = bs->file->bs;
> /* Prevent it from getting deleted when detached from bs */
> - bdrv_ref(file);
> + bdrv_ref(fallback_bs);
>
> - qdict_extract_subqdict(options, &file_options, "file.");
> + qdict_extract_subqdict(options, &file_options, subqdict_prefix);
> qobject_unref(file_options);
> - qdict_put_str(options, "file", bdrv_get_node_name(file));
> + g_free(subqdict_prefix);
> +
> + qdict_put_str(options, (*fallback_ptr)->name,
> + bdrv_get_node_name(fallback_bs));
>
> if (drv->bdrv_close) {
> drv->bdrv_close(bs);
> }
> - bdrv_unref_child(bs, bs->file);
> - bs->file = NULL;
>
> - ret = bdrv_snapshot_goto(file, snapshot_id, errp);
> + bdrv_unref_child(bs, *fallback_ptr);
> + *fallback_ptr = NULL;
Here we set *fallback_ptr to NULL...
> +
> + ret = bdrv_snapshot_goto(fallback_bs, snapshot_id, errp);
> open_ret = drv->bdrv_open(bs, options, bs->open_flags, &local_err);
> qobject_unref(options);
> if (open_ret < 0) {
> - bdrv_unref(file);
> + bdrv_unref(fallback_bs);
> bs->drv = NULL;
> /* A bdrv_snapshot_goto() error takes precedence */
> error_propagate(errp, local_err);
> return ret < 0 ? ret : open_ret;
> }
>
> - assert(bs->file->bs == file);
> - bdrv_unref(file);
> + assert(fallback_bs == (*fallback_ptr)->bs);
...but here we dereference *fallback_ptr, and Coverity doesn't see
anything that it recognizes as being able to change it.
> + bdrv_unref(fallback_bs);
> return ret;
> }
False positive, or real issue? (If a false positive, a comment
explaining what's going on wouldn't go amiss -- as a human reader
I'm kind of confused about whether there's some kind of hidden
magic going on here.)
thanks
-- PMM
