Re: [Qemu-devel] [RFC PATCH 06/41] block: Involve block drivers in permission granting

[Fam Zheng <famz@redhat.com>]

On Tue, 02/14 11:36, Kevin Wolf wrote:
> Am 14.02.2017 um 06:51 hat Fam Zheng geschrieben:
> > On Mon, 02/13 18:22, Kevin Wolf wrote:
> > > +int bdrv_child_try_set_perm(BdrvChild *c, uint64_t perm, uint64_t shared,
> > > +                            Error **errp)
> > > +{
> > > +    int ret;
> > > +
> > > +    ret = bdrv_child_check_perm(c, perm, shared, errp);
> > > +    if (ret < 0) {
> > > +        return ret;
> > > +    }
> > > +
> > > +    bdrv_child_set_perm(c, perm, shared);
> > 
> > This has an issue of TOCTOU, which means image locking cannot fit in easily.
> > Maybe squash them into one callback (.bdrv_try_set_perm) that can return error?
> 
> That doesn't work, it would leave us with broken error handling. If one
> driver in the middle of the update process fails to update the
> permissions, we would end up with half of the nodes having the old
> permissions and half having the new ones.
> 
> I think the file driver needs to lock the file already on check, and
> then we need to add a callback for the failure case so that it gives up
> the lock again. In other words, we might need a transaction with
> prepare/commit/abort here (*sigh*). Hm, or maybe just prepare/abort
> could be enough? Needs some thinking about.

Can perm change be wedged into the reopen transaction? RO flag change there
already requires transactional lock mode update in file driver. I wonder if
the bdrv_reopen call sites can ever have (transactional) perm change
requirements as well..

Fam