diff --git a/kernel/defs.h b/kernel/defs.h
index 3172cb3..fb2672e 100644
--- a/kernel/defs.h
+++ b/kernel/defs.h
@@ -192,9 +192,10 @@ void            unmappages(pagetable_t, uint64, uint64, int);
 uint64          walkaddr(pagetable_t, uint64);
 int             copyout(pagetable_t, uint64, char *, uint64);
 int             copyin(pagetable_t, char *, uint64, uint64);
-int             copyinstr(pagetable_t pagetable, char *dst, uint64 srcva, uint64 max);
+int             copyinstr(pagetable_t, char *, uint64, uint64);
 char*           map_kstack();
 uint64          kernelpa(uint64);
+void            clearpteu(pagetable_t, uint64);
 
 // plic.c
 void            plicinit(void);
diff --git a/kernel/exec.c b/kernel/exec.c
index b21afbb..c25dfb0 100644
--- a/kernel/exec.c
+++ b/kernel/exec.c
@@ -68,6 +68,7 @@ exec(char *path, char **argv)
   sz = PGROUNDUP(sz);
   if((sz = uvmalloc(pagetable, sz, sz + 2*PGSIZE)) == 0)
     goto bad;
+  clearpteu(pagetable, sz-2*PGSIZE);
   sp = sz;
   stackbase = sp - PGSIZE;
 
diff --git a/kernel/vm.c b/kernel/vm.c
index 96f2ffa..cbea684 100644
--- a/kernel/vm.c
+++ b/kernel/vm.c
@@ -436,3 +436,13 @@ kernelpa(uint64 va) {
   pa = PTE2PA(*pte);
   return pa+off;
 }
+
+void
+clearpteu(pagetable_t pagetable, uint64 va) {
+  pte_t *pte;
+  
+  pte = walk(pagetable, va, 0);
+  if(pte == 0)
+    panic("clearpteu");
+  *pte &= ~PTE_U;
+}