Re: [PATCH v2 1/2] migration: rework migrate_set_error() to migrate_report_err()

[Peter Xu]

On Wed, Apr 24, 2024 at 10:50:14PM +0300, Vladimir Sementsov-Ogievskiy wrote:
> On 24.04.24 22:40, Peter Xu wrote:
> > On Wed, Apr 24, 2024 at 08:42:44PM +0300, Vladimir Sementsov-Ogievskiy 
> > wrote:
> > > 1. Most of callers want to free the error after call. Let's help them.
> > > 
> > > 2. Some callers log the error, some not. We also have places where we
> > >     log the stored error. Let's instead simply log every migration
> > >     error.
> > > 
> > > 3. Some callers have to retrieve current migration state only to pass
> > >     it to migrate_set_error(). In the new helper let's get the state
> > >     automatically.
> > > 
> > > Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
> > > ---
> > >   migration/migration.c    | 48 ++++++++++++----------------------------
> > >   migration/migration.h    |  2 +-
> > >   migration/multifd.c      | 18 ++++++---------
> > >   migration/postcopy-ram.c |  3 +--
> > >   migration/savevm.c       | 16 +++++---------
> > >   5 files changed, 28 insertions(+), 59 deletions(-)
> > > 
> > > diff --git a/migration/migration.c b/migration/migration.c
> > > index 696762bc64..806b7b080b 100644
> > > --- a/migration/migration.c
> > > +++ b/migration/migration.c
> > > @@ -285,7 +285,7 @@ void migration_bh_schedule(QEMUBHFunc *cb, void 
> > > *opaque)
> > >   void migration_cancel(const Error *error)
> > >   {
> > >       if (error) {
> > > -        migrate_set_error(current_migration, error);
> > > +        migrate_report_err(error_copy(error));
> > >       }
> > >       if (migrate_dirty_limit()) {
> > >           qmp_cancel_vcpu_dirty_limit(false, -1, NULL);
> > > @@ -779,13 +779,6 @@ process_incoming_migration_co(void *opaque)
> > >       }
> > >       if (ret < 0) {
> > > -        MigrationState *s = migrate_get_current();
> > > -
> > > -        if (migrate_has_error(s)) {
> > > -            WITH_QEMU_LOCK_GUARD(&s->error_mutex) {
> > > -                error_report_err(s->error);
> > > -            }
> > > -        }
> > >           error_report("load of migration failed: %s", strerror(-ret));
> > >           goto fail;
> > >       }
> > > @@ -1402,10 +1395,6 @@ static void migrate_fd_cleanup(MigrationState *s)
> > >                             MIGRATION_STATUS_CANCELLED);
> > >       }
> > > -    if (s->error) {
> > > -        /* It is used on info migrate.  We can't free it */
> > > -        error_report_err(error_copy(s->error));
> > > -    }
> > >       type = migration_has_failed(s) ? MIG_EVENT_PRECOPY_FAILED :
> > >                                        MIG_EVENT_PRECOPY_DONE;
> > >       migration_call_notifiers(s, type, NULL);
> > > @@ -1418,12 +1407,14 @@ static void migrate_fd_cleanup_bh(void *opaque)
> > >       migrate_fd_cleanup(opaque);
> > >   }
> > > -void migrate_set_error(MigrationState *s, const Error *error)
> > > +void migrate_report_err(Error *error)
> > >   {
> > > +    MigrationState *s = migrate_get_current();
> > 
> > Avoid passing in *s looks ok.
> > 
> > >       QEMU_LOCK_GUARD(&s->error_mutex);
> > >       if (!s->error) {
> > >           s->error = error_copy(error);
> > 
> > I think I get your point, but then looks like this error_copy() should be
> > removed but forgotten?
> > 
> > I remember I had an attempt to do similarly (but only transfer the
> > ownership):
> > 
> > https://lore.kernel.org/qemu-devel/20230829214235.69309-3-peterx@redhat.com/
> > 
> > However I gave up later and I forgot why.  One reason could be that I hit a
> > use-after-free, then found that well indeed leaking an Error is much better
> > than debugging a UAF.
> 
> Hmm, yes I saw a leaked Error somewhere, and this patch should fix it. But 
> may be I missed introducing this use-after-free again)

Ah do you mean you fixed a bug?  If so please use a standalone patch for
that and we'll need to copy stable.

I did notice that in this series on patch looks like does more than one
thing.  It'll be helpful too for reviewers when patch can be split
properly.

> 
> > 
> > So maybe we simply keep it like before?  If you like such change, let's
> > just be extremely caucious.
> > 
> > >       }
> > > +    error_report_err(error);
> > 
> > The ideal case to me is we don't trigger an error_report() all over the
> > place.  We're slightly going backward from that regard, IMHO..
> > 
> > Ideally I think we shouldn't dump anything to stderr, but user should
> > always query from qmp.
> > 
> 
> Sounds reasonable to me. I'm just unsure, if keep it like before, how
> should I correctly update logging to stderr in
> process_incoming_migration_co(). Could we just drop error reporting to
> stderr? Or should it be kept as is for the case when exit_on_error is
> true?

I'm not sure I get the question, but I'll comment in patch 2 very soon, so
we can move the discussion there.

Thanks,

-- 
Peter Xu