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Re: [PULL 37/64] block/snapshot: Fix fallback


From: Kevin Wolf
Subject: Re: [PULL 37/64] block/snapshot: Fix fallback
Date: Mon, 3 May 2021 11:40:11 +0200

Am 01.05.2021 um 00:30 hat Peter Maydell geschrieben:
> On Mon, 7 Sept 2020 at 12:11, Kevin Wolf <kwolf@redhat.com> wrote:
> >
> > From: Max Reitz <mreitz@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 <vsementsov@virtuozzo.com>
> > Signed-off-by: Max Reitz <mreitz@redhat.com>
> > Reviewed-by: Andrey Shinkevich <andrey.shinkevich@virtuozzo.com>
> > Reviewed-by: Kevin Wolf <kwolf@redhat.com>
> 
> Hi; Coverity thinks it's found a problem with this code
> (CID 1452774):

Cc: Max as the patch author

> > @@ -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.)

I think it's a false positive because drv->bdrv_open() is supposed to
give it a non-NULL value again. Not sure if we can make the assumption
in every case without checking it, but it feels reasonable to require
that drv->bdrv_open() would return failure otherwise. Max?

Kevin




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