Re: [PATCH] migration: Don't free the reason after calling migrate_add_blocker

["Daniel P. Berrangé" <berrange@redhat.com>]

On Fri, Oct 24, 2025 at 01:15:40PM +0200, Markus Armbruster wrote:
> Bin Guo <guobin@linux.alibaba.com> writes:
> 
> > Signed-off-by: Bin Guo <guobin@linux.alibaba.com>
> > ---
> >  hw/intc/arm_gicv3_kvm.c | 1 -
> >  target/i386/sev.c       | 1 -
> >  2 files changed, 2 deletions(-)
> >
> > diff --git a/hw/intc/arm_gicv3_kvm.c b/hw/intc/arm_gicv3_kvm.c
> > index 66b0dddfd4..6f311e37ef 100644
> > --- a/hw/intc/arm_gicv3_kvm.c
> > +++ b/hw/intc/arm_gicv3_kvm.c
> > @@ -841,7 +841,6 @@ static void kvm_arm_gicv3_realize(DeviceState *dev, Error **errp)
> >          error_setg(&kvm_nv_migration_blocker,
> >                     "Live migration disabled because KVM nested virt is enabled");
> >          if (migrate_add_blocker(&kvm_nv_migration_blocker, errp)) {
> > -            error_free(kvm_nv_migration_blocker);
> >              return;
> >          }
> >  
> > diff --git a/target/i386/sev.c b/target/i386/sev.c
> > index 1057b8ab2c..fd2dada013 100644
> > --- a/target/i386/sev.c
> > +++ b/target/i386/sev.c
> > @@ -1661,7 +1661,6 @@ sev_snp_launch_finish(SevCommonState *sev_common)
> >      ret = migrate_add_blocker(&sev_mig_blocker, &local_err);
> >      if (local_err) {
> >          error_report_err(local_err);
> > -        error_free(sev_mig_blocker);
> >          exit(1);
> >      }
> >  }
> 
> Does this fix use-after-free bugs?

I don't think so, because when migrate_add_blocker() returns error,
the Error for the blocker will have been propagated into the errp
parameter, and then set to NULL. So these two error_free calls
should be a no-op.

But wow, the migrate_add_blocker API design is unpleasant with its
pair of "Error **" parameters - it is practically designed to
maximise confusion & surprise.

With regards,
Daniel
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