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

[Vladimir Sementsov-Ogievskiy]

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)

> 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?

--
Best regards,
Vladimir