intros. exploit Mem.alloc_parallel_inject; eauto.
intros [? [? [? [? [? [? [? ?]]]]]]].
repeat (econstructor; eauto).
intros b HH.
destruct (ident_eq b b1).
- subst. rewrite H7; discriminate.
- rewrite H8; auto. eapply H1.
eapply Mem.valid_block_alloc_inv in H0; eauto.
destruct H0; auto. subst; contradict n; auto.
Qed.

Lemma extcall_malloc_ok:
extcall_properties extcall_malloc_sem
(mksignature (Tptr :: nil) (Some Tptr) cc_default).